An SLA realization of the 6502 microprocessor

Item Type text; Thesis-Reproduction (electronic)

Authors Tsuyuki, Kenju

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/557891

http://hdl.handle.net/10150/557891

AN SLA REALI ZATION OF THE 6502 MICROPROCESSOR

by

Ke n j u Tsuyuk i

A T h e s i s S u b m i t t e d to t he F a c u l t y o f t h e

DEPARTMENT OF ELECTRICAL ENGINEERING• ' X .

I n P a r t i a l F u l f i l l m e n t o f t h e R e q u i r e me n t s For t h e Degree of

MASTER OF SCIENCE

; I n t h e G r a d u a t e C o l l e g e

THE UNI VERSI TY OF ARIZONA

1 9 8 1

STATEMENT BY AUTHOR

T h i s t h e s i s has been s u b m i t t e d in p a r t i a l f u l f i l l ment o f r e q u i r e m e n t s f o r an advanced degr ee a t t h e U n i v e r s i t y o f A r i z o n a and i s d e p o s i t e d in t he U n i v e r s i t y L i b r a r y t o be made a v a i l a b l e t o b o r r o w e r s under r u l e s o f t he L i b r a r y *

w i t h o u t s p e c i a l per mi ss i onp p r o v i d e d t h a t a c c u r a t e acknowl edgment of s o u r c e i s made* Re ques t s f o r p e r m i s s i o n f o r ext ended, q u o t a t i o n f rom or r e p r o d u c t i o n o f t h i s m a n u s c r i p t in whol e or i n p a r t may be g r a n t e d by t h e head of t h e maj or d e p a r t me n t or t he Dean o f t h e G r a d u a t e C o l l e g e when in h i s j udgment t he p r oposed use o f t h e m a t e r i a l i s i n t h e i n t e r e s t s of s c h o l a r s h i p * I n a l l o t h e r i n s t a n c e s , h o w e v e r , p e r m i s s i o n must be o b t a i n e d f rom t h e au t hor *

B r i e f q u o t a t i o n s f rom t h i s t h e s i s a r e a l l o w a b l e

S I N G E D ? ^

APPROVAL BY THESIS DIRECTOR

T h i s t h e s i s has been appr oved on t h e da t e shown bel ow

F r e d r i ck^ F r e d r i c k j ^ H i l l P r o f e s s o r of

E l e c t r i c a l E n g i n e e r i n g

Dat ey f j j

ACKNOWLEDGMENTS

I would I i ke t o exp r e s s deep a p p r e c i a t i o n to

P r o f e s s o r F r e d r i c k Jo H I U f o r h i s g u i da nc e and a d v i c e

d u r i n g t he p r e p a r a t i o n o f t h i s t h e s l s o

I a l s o wish to t hank a l l t h os e who p a r t i c i p a t e d

t he ha r dwar e d e s i g n l anguage r e s e a r c h p r o j e c t f o r t h e i r

c ooper a 11 on o

TABLE OF CONTENTS

L I S T OF I LLUSTRATI ONS. o' 0 o o O o 0 O

P age

0 v i i

L I ST OF TABLES 0 0 = 0 0 ° 0 0 0 0 0 0 v i i a

ABSTRACT 0 0 0 .0 0 0 0 ° 0 0 0 0 0 0 5 x

INTRODUCTION. . . 0 0 ° 0 0 0 0 0 1

THE 6502 MICROPROCESSOR 0 0 0 . 0 0 0 0 0 6

Har dwar e O r g a n i z a t i o n . c 0 0 0 0 . 0 o 6I n p u t s and Out p ut s 0 0 0 0 0 0 p 7I n t e r n a l B u s e s . . 0 0 0 0 0 0 10I n t e r n a l R e g i s t e r s 0 0 0 0 0 0 13

I n s t r u c t i o n S e t . . . 0 . 0 0 0 0 0 0 17Fundament a l I n s t r u c t i o n s 0 0 0 0 0 21A d d r e s s i n g Modes . 0 0 0 0 0 0 0 . 24

INTERNAL OPERATIONS ANALYSIS 0 0 0 0 0 0 30

Hy pot heses . . . . 0 0 ' 0 0 0 0 0 31Gener a l Sequence . . 0 0 0 0 0 0 0 32A d d r e s s i n g P a r t s . . 0 0 0 0 0 0 0 . . . 37

I m m e d i a t e . . . 0 0 0 0 0 0 0 . 37A b s o l u t e . . . . • 0 0 0 0 0 0 38Z e r o Page . . . 0 0 0 0 0 0 0 41A c c u m u l a t o r . . . Q 0 0 0 0 0 0 41I m p l i e d . . . . O 0 0 0 0 0 0 42I n d e x e d I n d i r e c t . O 0 0 0 0 0 0 43I n d i r e c t I n d e x e d . 0 0 0 0 0 0 0 45Z e r o Page I n d e x e d . O 0 0 0 0 0 0 0 47I n d i r e c t . ; . . . O 0 0 0 0 0 0 48A b s o l u t e I n d e x e d . O 0 0 0 0 0 0 0 4 9R e l a t i v e . . ' . . 0 0 0 0 0 0 0 51

E x e c u t i o n P a r t s . . . 0 0 0 0 0. 0 0 54Ar i t h m e t i c . . . 0 0 0 0 0 0 0 . 54L o g i c . . . . . 0 0 0 0 0 0 0 * . 62Load 0 . . . . 0 0 0 0 0 0 0 6 3S t o r e . . . . 0 0 0 0 0 0 0 0 64d URi p 0 0 0 . 0 0 0 0 0 0 0 64S e t / C 6 e a r . . . 0 0 0 0 0 0 0 . 66T r a n s f e r 0 . . . 0 0 0 0 Q 0 0 66

i v

V

TABLE OF CONTENTS — C o n t i n u e d

Page

StackO O O O O O O O O O O O O 6 7I n p u t s o o o o o o o o o o o o o o 6 9

I n t e r r u p t I n p u t s o o o o o o o o o 70 SimpOe I n p u t s o o o o o o o o o o 7 3

4 o AHPL DESCRIPTION o o o o o o o o o o o 77

Dec l a r a t i ons o o o o o o o o . o o o o 77D a t a F e t c h C y c i e o o o o o o o o o o 78I n s t r u c t i o n Decodi ng o o o o o o © o © 80OP CODE F e t c h C y c l e o o o o o o o o o 88I n t e r n a l O p e r a t i o n s 0 o © 0 © © © 0 © 93

Scheme Ao o o © © © © © © o o o 93Scheme B© o © o © © © © © © © © 97Scheme C© © © © © © © © © © © © 99S ch em e D © © © © © © © o © © © © 10 0Scheme E© © © © © © © © © © © © 104Scheme F© © © © © © © © © © © © 107E x e c u t i o n s ! O p e r a t i o n s © © © © © © © 109

End of Sequence© © © © © © © © © © © 115Des i gn o f C o m b i n a t i o n a l L o g i c U n i t s © © © © 117

ALU o o o © © 0 0 0 o o o o o 116INC o o o o o o o © o o o . o o 119DEC © O O 0 0 . 0 O O O O ' O 0 0 119

5 © SI MULATION o © © © © © © © © © © © © 120

HPSIM2© © © o o o o o o o o o o o 120M o d i f i c a t i o n s f o r S i m u l a t i o n © © © © © © 121R e s u l t s 0 © 0 0 0 . 0 0 0 0 0 © © © 126

6 © SLA REALIZATION© o © © © © © © © © © © 131

I n t r o d u c t i o n t o a S t o r a g e Log i c A r r a y © © © 131A l g o r i t h m f o r an SLA R e a l i z a t i o n © © © © © 134Ex pec t ed Ou t p u t s o f SLA T r a n s l a t i o n © © © © 138

7© DISCUSSION AND CONCLUSION © © © © © © © © 141

APPENDIX As 6502 INSTRUCTION SET SUMMARY © © © 145

APPENDIX B s SUMMARY OF CYCLE BY CYCLEOPERATION OF INSTRUCTIONS © © © 152

TABLE OF CONTENTS — C o n t i n u e d

APPENDIX

APPENDIX

APPENDIX

APPENDIX

APPENDIX

APPENDIX

LI ST OF

Page

C s INTERNAL OPERATION SEQUENCESOF INSTRUCTIONS 0 . o . o . 0 161

Ds INTERNAL OPERATION SEQUENCESOF INPUTSo o o o o o o o o 210

Es KARNAUGH MAP OF OPERATION CODES o . 216

Fs 6502 AHPL DESCRIPTION o o 218

G? COMBINATIONAL LOGIC UNITDESCRIPTIONSo 0 o o o „ o 226

Hs EXAMPLE OF SIMULATION 0 o o . o 230

REFERENCESo o o o o o o o o o o 252

L IS T OF ILLUSTRATIONS

F i g u r e Page

l o P i n C o n f i g u r a t i o n o f t he 6502 o o o 0 o „ 8

2o 6502 I n t e r n a ! A r c h i t e c t u r e „ 0 =• = . 0 o 0 14

3 o B i t C o n f i g u r a t i o n o f T y p i c a l R e g i s t e r s 0 o o 25

4 o Genera 1 Sequence f o r I n s t r u c t i o n s o o o » o 34

5.o Gener a l Sequence f o r I n t e r r u p t I n pu t s o o = 0 71

6 o F l o w c h a r t f o r I n s t r u c t i o n Decodi ngo 0 o = o 89

7o F l o w c h a r t f o r OP CODE F e t c h C y c l eand Scheme A o o o p o o o © o o © 92

8© F l o w c h a r t f o r Schemes Bj> Cs> and D © © © © © 98

9© F l o w c h a r t f o r Schemes D and E © © © © © © 105

10© F l o w c h a r t f o r Scheme F andE xe cut i on a i O p e r a t i o n s © © © © © © © © 110

11 © F l o w c h a r t f o r Execut i ona8 O p e r a t i o n s © © © © 116

12© . F l o w c h a r t o f 8 - b i t M u l t i p l i e a t i on © © ©. © © 128c

13 © 8 -5-b i t M u l t i p l i c a t i o n Program© © © © © © © 12 9

14© Cl ock ed S t o r a g e Logi c A r r a y © © © © © © © 133

15© Ou t p u t Fl ow o f SLA T r a n s l a t i o n © © © © © © 139

v i i

L IS T OF TABLES

Summary of i n p u t s and O u t p u t s 0 o o o

I n p u t and O u t p u t C o n n e c t i o n s ofR e g i s t e r s and CLU°s o o o o o o 0

Summary o f Ad d r e s s i n g R e g i s t e r s o o o o

S p e c i f i c a t i o n o f t h e A r i t h m e t i c - L o g i c U n i t

C l a s s i f i c a t i o n o f I n s t r u c t i o n s byA d d r e s s i n g Modes and Fea t ur es , , „ „ o

v i i i

ABSTRACT

The f a b r i c a t i o n o f a c o mp l e t e m i c r o p r o c e s s o r on a

s i n g l e VLSI c h i p is now s t a n d a r d p r a c t i ce

CHAPTER 1

INTRODUCTION

I n r e c e n t l y p u b l i s h e d books' and magaz i nes on

comput er s and d i g i t a l sys t ems* t h e t e r m* d e s i g n a u t o ma t i o n

can o f t e n be e nc ount e r ed , , The d e s i g n a u t o ma t i o n as w e l l as

t he t e r m* c o m p u t e r - a s s i s t e d d e s i g n g e n e r a l l y i m p l i e s des i gn

method or a t o o l t h a t can a s s i s t d e s i g n e r s in d e s i g n i n g or

d e v e l o p i n g p r o d u c t s in v a r i o u s f i e l d s such as d i g i t a l

har dwar e sys t ems* comput er s of a l l s i z e s * and i n t e g r a t e d

c i r c u i t s in t he f i e l d o f d i g i t a l syst em d e s i g n The des i gn

a u t o m a t i o n can be a p p l i e d t o a l mo s t a l l a r e a s o f des i gn

p r o c e s s e s because i t i s d e s i r a b l e t o s h o r t e n t h e t u r n a r o u n d

t i me of des i gno .

E s p e c i a l l y * f o r t h e d e s i g n o f v e r y l a r g e s c a l e

i n t e g r a t i o n ( V L S I ) used in d i g i t a l sys t ems* t h e use of t he

r e g u l a r i t y of p a t t e r n s f o r communi ca t i on bet ween a c t i v e

e l e me nt s i s a b s o l u t e l y e s s e n t i a l t o i mpl ement a des i gn

a u t o m a t i o n systemo A l t hough r e s u l t a n t p r o d u c t s r e a l i z e d by

t h e d e s i g n a u t o m a t i o n system t e n d t o have r e d u n t a n t r e g u l a r

p a t t e r n s * t h i s r edundancy can be j u s t i f i e d by t h e

f e a s i b i l i t y of t he d e s i g n a u t o m a t i o n syst em* t h e s h o r t e r

t u r n a r o u n d t i me of d e s i g n * and t h e s i m p l i c i t y o f des i gn

1

pr ocesso

For t h e pur pose of t h e d e s i g n o f d i g i t a l har dwar e

s y s t e ms , har dwar e d e s i g n l anguages have been i n v e n t e d o The

use o f a ha r dwar e d e s i g n l anguage i s one o f t h e i m p o r t a n t

e l e me n t s of a d e s i g n a u t o m a t i o n systems A Ha r d wa r e

Pr ogr ammi ng Language ( AHPL) , e x t e n s i v e l y d e s c r i b e d i n

r e f e r e n c e 1 has been w i d e l y in use t o d e s i g n d i g i t a l

syst ems = Some c o m p i l e r pr ogr ams have been i mpl ement ed in

or der t o a i d t h e d e s i g n of t h e d i g i t a l systems d e s c r i b e d i n

AHPL s i n c e AHPL was i n t r o d u c e d * However , a new m u l t i - s t a g e

c o m p i l e r has been w r i t t e n in c o n n e c t i o n wi th, s e v e r a l new

n o t a t i o n s and c on c e p t s added i n t h e e x i s t i n g AHPL*

T h i s u n i v e r s a l AHPL w i l l be a u s e f u l t o o l f o r no t on l y t h e

d e s i g n of c o n v e n t i o n a l d i g i t a l sys t ems b u t a l s o a VLSI

imp I e m e n t a t i on *

Anot her i m p o r t a n t e l ement of a d e s i g n a u t o ma t i o n

sys t em d e s i gn e d f o r a VLSI i m p l e m e n t a t i o n i s t o use a

r e g u l a r p a t t e r n f o r a b a s i c e l ement s A S t o r a g e Log i c A r r a y

( S L A ) , a d e q u a t e l y d e s c r i b e d in r e f e r e n c e 2 has been exami ned

f o r t he use o f a VLSI i m p l e m e n t a t i o n , and t h e s t u d y of a

c o m p i l e r c a p a b l e o f t r a n s l a t i n g f rom an AHPL d e s c r i p t i o n t o

a VLSI i m p l e m e n t a t i o n in t he f or m of t h e SLA has a l s o been

c a r r i e d out * As a r e s u l t , i t has been pr oved t h a t t h e

c o m p i l e r i s f e a s i b l e and t he use of c l o c k e d D f l i p - f l o p s as

t h e s t o r a g e e l e me n t s in t he SLA is more n a t u r a l *

3

I n t he SLA c o m p i l a t i o n p r o j e c t s an a l g o r i t h m

( s t a g e 3) t o t r a n s l a t e t h e o u t p u t of s t a g e 2 o f t h e new

c o m p i l e r i n t o t he d a t a used f o r an SLA r e a l i z a t i o n must be

devel oped, , I n a d d i t i o n s some d e s i g n exampl es sho u l d be done

so t h a t t he r e s u l t s o f t h e exampl es can be f e d back t o t he

d e s i g n of t he a l g o r i t h m * For t h i s reasons t h e 6502

m i c r o p r o c e s s o r s as an exampl e o f an SLA r e a l i z a t i o n s was

s e l e c t e d *

S i n c e t he a d v e n t o f t h e f i r s t m i c r o p r o c e s s o r in

1971s many m i c r o p r o c e s s o r s have been d e v e l o p e d * I n s p i t e o f

t h e c o m p l i c a t e d o p e r a t i o n s of t h e m i c r o p r o c e s s o r s s t he y ar e

u s u a l l y d e s i g n e d by menas of a random l o g i c t e c h n i q u e * The

6502s an 8 - b i t m i c r o p r o c e s s o r s is a s o p h i s t i c a t e d d i g i t a l

syst em i t s e l f and has many p o w e r f u l i n s t r u c t i o n s *

T h e r e f o r e s t he d e s i g n exampl e of t he 6502 w i l l be a b l e t o

s a t i s f y most of t he r e q u i r e m e n t s on e s t a b l i s h i n g a des i gn

a u t o m a t i o n syst em aimed a t t he SLA r e a l i z a t i o n * The 6502 is

a l s o s u f f i c i e n t l y c o m p l i c a t e d to c o n f i r m t he c o mp l e t e n e s s o f

t he d e s i g n a u t o m a t i o n syst em*

I n i mp l e me n t i n g t he 6502 m i c r o p r o c e s s o r in t he f or m

of t he SLA; t h e f o l l o w i n g t a s k s must be wor ked o u t :

1) D e t a i l e d A n a l y s i s of t he i n t e r n a l o p e r a t i o n s of

t h e 6 5 0 2 .

2) The s y s t e m a t i c d ev e l o pment o f a 6502 AHPL

d e s c r i p t i o n *

3) S i m u l a t i o n of t he 6502 w r i t t e n i n AHPL«

4) T r a n s l a t i o n f rom t h e 6502 AHPL d e s c r i p t i o n t o an

SLA form by e i t h e r manual work or a c o m p u t e r »

Thr ough t h i s paper * t h e f i r s t t h r e e t a s k s a r e d i s c u s s e d w i t h

empha s i s * An i n t r o d u c t i o n to t h e SLA and t h e s t a g e 3

a l g o r i t h m is b r i e f l y me n t i o n e d i n c o n j u n c t i o n w i t h t h e t a s k

o f t h e t r a n s l a t i o n *

C h a p t e r 2 p r e s e n t s a b r i e f d e s c r i p t i o n o f t he

har owar e o r g a n i z a t i o n of t h e 6502 to p r e p a r e t h e backgr ound

f o r t he d i s c u s s i o n i n l a t e r c h a p t e r s * Ch a p t e r 3 a n a l y z e s

t h e i n t e r n a l o p e r a t i o n s of t he 65 02* and a 6502 AHPL

d e s c r i p t i o n is de v e l o pe d s t e p by s t ep in t h e f o l l o w i n g

c h a p t e r * I n Ch a p t e r 5* t he p r e p a r a t i o n f o r s i m u l a t i o n and

t h e r e s u l t s of t he s i m u l a t i o n a r e e x p l a i n e d * C h a p t e r 6

ment i ons an i n t r o d u c t i o n t o an SLA a l ong wi t h an a l g o r i t h m

f o r an SLA r e a l i z a t i o n b r i e f l y * The r e s u l t s o f a des i gn

exampl e a r e d i s c u s s e d in t h e l a s t c h a p t e r .

F i n a l l y * Append i x A p r o v i d e s a 6502 i n s t r u c t i o n s e t

summary* and Append i x B p r e s e n t s a summary of c y c l e by c y c l e

o p e r a t i o n of i n s t r u c t i o n s * Appendi x C l i s t s i n t e r n a l

o p e r a t i o n s of t y p i c a l i n s t r u c t i o n s * w h i l e Appendi x D

d e s c r i b e s i n t e r n a l o p e r a t i o n s of i n p u t s * A Kar naugh map of

o p e r a t i o n codes i s i n c l u d e d in Appendi x E * Ap pend i x F shows

a c o mp l e t e 6502 AHPL d e s c r i p t i o n * w h i l e Appendi x G l i s t s

c o m b i n a t i o n a l l o g i c u n i t d e s c r i p t i o n s ^ The l a s t app end i x *

Append i x Hj> d e mo n s t r a t e s an exampl e o f si mu I at i on o

CHAPTER 2

THE 6502 MICROPROCESSOR

The pur pose of t h i s c h a p t e r i s to g i v e t h e r e a d e r an

o v e r v i e w of t he 6502 so t h a t t h e r e a d e r can p r oc e e d t o t he

f o l l o w i n g c h a p t e r s w i t h an equal knowl edge of t he 6502» I n

o r d e r t o p r e s e n t a c l e a r d e s c r i p t i o n ^ t h i s c h a p t e r i s

d i v i d e d i n t o two s e c t i o n s ; Har dwar e O r g a n i z a t i o n and

I n s t r u c t i o n Se t f o r d e s c r i p t i o n s of a ha r dwar e as p e c t and a

s o f t w a r e a s p e c t ; r e s p e c t i v e l y .

S i n c e t he f i r s t m i c r o p r o c e s s o r a ppear ed on t h e

ma r k e t of d i g i t a l h a r d w a r e sys t ems; many have been

i n t r o d u c e d . The 6502 t h a t i s one o f t he t h i r d g e n e r a t i o n

m i c r o p r o c e s s o r s was i n t r o d u c e d a f t e r t he 6800 m i c r o p r o c e s s o r

had Been put on t he m a r k e t . Because of t he t i m e of

a p p e a r a n c e ; t he 6502 has many s i m i l a r f e a t u r e s t o t he 68.00 .

Har dwar e O r g a n i z a t i o n

The har dwar e o r g a n i z a t i o n and t he i n s t r u c t i o n s e t o f

t h e 6502 ar e ve r y s i m i l a r t o t h o s e of t he 6 8 0 0 . The is due

t o t he f a c t t h a t t h e de s i g n of t he 6502 was based on t he

6 8 0 0 . L i k e many 8 - b i t m i c r o p r o c e s s o r s ; t h e 6502 has a l s o an

8—b i t d a t a bus and a 1 6 - b i t add r e ss bus. T h i s i n d i c a t e s t h e

c a p a b i l i t y o f a d d r e s s i n g 64 Kbyt es ( 8 b i t s ) of memory. I n

a d d i t i o n t o t h e d a t a bus and a dd r e s s bus, t h e r e a r e n i n e

i n p u t s and f o u r o u t p u t s used in t h e 650Zo A i l i n p u t s and

o u t p u t s a r e pu t t o g e t h e r i n t o a 40—p i n d u a l - i n - l i n e package, ,

T h e r e a r e t h r e e unuseo p i n s on t h e packege* The p i n

c o n f i g u r a t i o n i s shown in F i g u r e 1„

I n p u t s and Out put s

The i n p u t s and o u t p u t s c o n s i s t of an 8 - b i t da t a bus,

a 1 6 - b i t add r e s s bu s , I RQ, NMI , RES, ROY, P HI 0 , S „ 0 o, R/.W,

SYNC, P H I 1 , PHI Z , two Vss° s, and Vcco The i n p u t s and

o u t p u t s have t h e i r own d i s t i n c t - oper a t i ons , . T y p i c a l

f e a t u r e s and o p e r a t i o n s of t h e i n p u t s and o u t p u t s w i l l be

b r i e f l y e x p l a i n e d as f o l l o w s :

ADBUST0 : 1 5 3 ■— The 1 6 - b i t o u t p u t bus i s used to

addr e ss l o c a t i o n s a l l o c a t e d in memory and p e r i p h e r a l s and i s

c o m p a t i b l e w i t h TTL„

DBUS CO:73 — The 8 - b i t b i - c i r e c t i onal bus w i t h t h e

t r i - s t a t e o u t p u t i s p r o v i d e d as a medium of t r a n s f e r r i n g

d a t a o For exampl e , t h e m i c r o p r o c e s s o r put s d a t a ont o t he

d a t a bus f o r w r i t e o p e r a t i o n s and g e t s d a t a f r om t he bus f o r

r ead o p e r a t i o n s . '

IRQ — The IRQ i n p u t i s one of t he i n t e r r u p t i n p u t s

p r o v i d e d in t he 6 5 0 Z „ The TTL l e v e l i n p u t r e q u e s t s an

i n t e r r u p t sequence in whi ch t h e Program Co unt e r (PC) and t h e

Pr o c e s s o r S t a t u s R e g i s t e r ( P) w i l l be s t o r e d i n t h e s t a c k .

At t h e end o f t h e se q ue nc e , t h e d a t a f rom t h e l o c a t i o n FFFE

8

1 Vss RES 40

2 ROY PH I 2 ( OUT) 39

3 P H I 1 ( OUT) S. O. 38

4 IRQ PH 1 0 ( IN ) - 37

5 N. C. N.C . 36

6 NMI N.C . 35

7 SYNC R/W 34

8 V cc DBUS £ 0 3 33

9 ADBUSCO] D BUS£13 32

10 ADBUSCU DB US £ 2 3 31

11 AD BUS £ 2 DM 650 2

DBUS £ 3 3 30

12 ADBUSC33 DBUS£ 4 3 29

13 ADBUS £4 3 DBUS £ 5 3 28

14 - AD BUS £ 5 3 DBUS£63 27

15 ADBUS £ 6 3 DBUS£ 73 26

l b ADBUS£73 ADBUS£153 25

17 ADBUS £ 8 3 ADBUS£143 24

18 AD BUS £ 9 3 ADBUS£133 23

19 AD BUS £ 10 3 ADBUS£123 - 22

20 ADBUSU13 Vss — 21

N. C. * No Connect i on.

F i g u r e 1 . Pin C o n f i g u r a t i o n of t he 6502

9

and t he d a t a f rom t h e l o c a t i o n FFFF w i l l be l oa de d i n t o t he

pr ogr am c o u n t e r low ana t h e program c o u n t e r h i g h ,

r e s p e c t i v e l y f o r t h e s t a r t i n g l o c a t i o n o f an i n t e r r u p t

r o u t i h e o Th i s i s a l e v e l s e n s i t i v e i nput o

NMI ■— The NMI i n p u t i s a l s o used f o r an i n t e r r u p t .

The o n l y d i f f e r e n c e f rom IRQ i s t he use of t he v e c t o r

l o c a t i o n s FFFA and FFFB, a n o n - ma s k a b l e i n t e r r u p t sequence ,

and an edge s e n s i t i v e i n p u t .

RES — The RES i n p u t is used to r e s e t or s t a r t t h e

6502 f rom a power down c o n d i t i o n . As in t he i n t e r r u p t

sequence , t h e s t a r t i n g l o c a t i o n f o r program c o n t r o l w i l l be

l oaded f rom t h e v e c t o r l o c a t i o n s FFFC and FFFD.

ROY — By t h e RDY i n p u t , a l l c y c l e s e x c e p t w r i t e

c y c l e s can be h a l t e d . T h i s f e a t u r e a l l o ws t he

m i c r o p r o c e s s o r t o be c o n n e c t e d w i t h a low speed memory.

PH 10 — T h i s PHIO i n p u t i s used t o g e n e r a t e a two

phase n o n - o v e r l a p p i n g c l o c k . An i n t e r n a l c l o c k g e n e r a t o r i s

s u p p l i e d i n s i d e t he 6 5 0 2 .

S. O. The o v e r f l o w b i t i n t he p r o c e s s o r s t a t u s

r e g i s t e r can be se t by a n e g a t i v e go i ng edge on t h e Set

O v e r f l o w F l a g ( S . O . ) i n p u t .

R/W — The R/W o u t p u t i s used to d i s t i n g u i s h bet ween

t hos e c y c l e s in which t h e m i c r o p r o c e s s o r i s do i ng a read

o p e r a t i o n and t hose c y c l e s in whi ch t he p r o c e s s o r i s doi ng

a w r i t e o p e r a t i o n . T h i s o u t p u t can a l s o be used t o c o n t r o l

10

r e a a and w r i t e op er at i ens of memory modul es and p e r i p h e r a l

d e v i c e s .

SYNC — The SYNC o u t p u t i s p r o v i d e d t o i d e n t i f y

t hos e c y c l e s i n whi ch t h e m i c r o p r o c e s s o r i s do i ng an OP CODE

f e t c h .

P H I 1 £ PHI 2 — The phase 1 c l o c k and phase 2 c l o c k

o f t h e two phase n o n - o v e r l a p p i n g c l o c k g e n e r a t e d in t h e 6502

ar e o u t p u t on PHI1 and PHI 2 ; r e s p e c t i v e l y t o be used by

o t h e r d e v i c e s .

By now; a l l o f t h e i n p u t s and o u t p u t s have been

b r i e f l y d e s c r i b e d exc ept f o r Vss and Vcc t h a t a r e used f o r

power s u p p l y . The i n p u t s and o u t p u t s a r e summar i zed in

T a b l e 1 f o r c l a r i t y . D e t a i l e d i n f o r m a t i o n c o n c e r n i n g t he

i n t e r n a l o p e r a t i o n s of IRQ.? NMI jp and RES and t h e e x a c t

t i m i n g of t he i n p u t s and o u t p u t s sh o u l d be r e f e r r e d t o in

r e f e r e n c e 3 f o r f u r t h e r u n d e r s t a n d i n g .

I n t e r n a I Buses

The d a t a bus and addr ess bus are p r o v i d e d in t he

6502 so t h a t t h e m i c r o p r o c e s s o r can communi cat e w i t h o u t s i d e

d e v i c e s . On t h e o t h e r hand* i n t e r n a l buses a r e empl oyed i n

or oer t h a t d a t a can move i n s i d e t h e m i c r o p r o c e s s o r . Ther e

ar e t h r e e i n t e r n a l buses* namel y t he I n t e r n a l Address Bus

( I ADBUS) * t h e I n t e r n a l Da t a Bus ( I DBUS) * and t h e Argument

Bus (ARGBUS) . These buses w i l l be s i m p l y e x p l a i n e d in

c o n n e c t i o n w i t h t he h a r d wa r e o r g a n i z a t i o n .

T a o l e 1 . Summar y o f I n p u t s and O u t p u t s

S i gna l Name I n p u t / O u t p u t F e a t u r e s

ADBUSLO:15 ] Ou t p u t TI L Comp a t i bl e«

DBUS tO : 7 ] I n p u t / O u t p u t B i d i r e c t i o n a l Bus w i t h T r i - s t a t e O u t p u t .

T rqI n p u t L e v e l S e n s i t i v e .

NMII n p u t Edge Sens i t i v e .

RESI n p u t Leve l S e n s i t i v e .

RDY I n p u t TTL Comp a t i b l e .

PHIO I n p u t TI L C o m p a t i b l e .

S . 0 . I n p u t TTL C o m p a t i b l e .

R/W Ou t p u t TTL Comp at i b l e .

SYNC Out put TTL C o m p a t i b l e .

PHI 1 Outp ut TTL Compa t i b 1e .

PHIZ Ou t p u t TTL C o m p a t i b l e .

12

IADBUS CO s15 ] — The i n t e r n a l addr ess bus c o n s i s t s o f

two 8 - b i t bases t IAD BUSC Os 73 and IADBUSC8 s1 5 3 0 The bus is

c o n n e c t e d t o t h e ADBUS t h r o u g h t he addr ess b u f f e r s so t h a t

i n t e r n a l i y g e n e r a t e d a d dr e sses can be s e n t to t he 'ADBUS„

F u r t h e r m o r e , t h e bus i s used t o r o u t e d a t a t o t h e I n c r e m e n t /

Decr ement R e g i s t e r ( I N R ) or t h e I n c r e m e n t e r / D e c r e m e n t e r

( INC / DEC) o The d a t a w i l l be used i n l a t e r o p e r a t i o n s , , The

o u t g o i n g d a t a on t h e IADBUS w i l l come f r om i n t e r n a l

r e g i s t e r s such as t h e Da t a La t c h ( DL) , t he T empor a r y

R e g i s t e r ( TR) , I NR, PC, t h e ALU R e g i s t e r ( ALUREG) , and t he

St a c k P o i n t e r (SP) t o make up 1 6 - b i t addresses put onto t he

ADBUSo Onl y t h e IADBUS[8 5 15 3 can be c o n ne c t e d t o t he

ARGBUS, so t he da t a on t he IADBUSC 85 153 can be an argument

of t h e ALU f o r a r i t h m e t i c o p e r a t i o n s o

IDBUSCO 8 7 3 — U n l i k e t h e IADBUS, t he i n t e r n a l da t a

bus i s c on nec t ed t o a l mos t a i l i n t e r n a l r e g i s t e r s * The

IDBUS is m a i n l y used as a medium of t r a n s f e r r i n g d a t a

bet ween t he r e g i s t e r s * The d a t a on t he IDBUS can be l oaded

ont o t he DBUS t h r o u g h t h e d a t a bus b u f f e r * The d a t a coming

f rom o u t s i d e must be l a t c h e d i n t o t h e d a t a l a t c h f i r s t *

ARGBUS CO s7 3 — The 8 - b i t argument bus i s p r e pa r e d

f o r r o u t i n g t he d a t a f rom one of t he s o u r c e s , t h e I ndex X

R e g i s t e r ( X ) , t he I n d e x Y R e g i s t e r ( Y ) , and t h e IADBUSC8s153

to t h e ALU» The ARGBUS i s one o f t he f o u r a r gument s of t he

ALU* , .

13

These buses a r e c l o s e l y con nec t ed to t h e i n t e r n a l

r e g i s t e r s and c o m b i n a t i o n a l l o g i c u n i t s ( C L U ' s ) f whi ch w i l l

be e x p l a i n e d ne x t o A p i c t o r i a l r e p r e s e n t a i o n o f t he

i n t e r n a l buses can be seen in F i g u r e 2o

I n t e r n a l R e g i s t e r s

The i n t e r n a l r e g i s t e r s as we l l as t h e i n t e r n a l buses

a r e q u i t e s i g n i f i c a n t to t he h a r d wa r e o r g a n i z a t i o n * El even

r e g i s t e r s have been u t i l i z e d to c a r r y out t he i n t e r n a l

o p e r a t i o n s of a l l i n s t r u c t i o n s and i n p u t s i g n a l s *

AS a l r e a d y n o t i c e d , t he p i c t o r i a l v i ew o f 6502

i n t e r n a l a r c h i t e c t u r e is shown i n F i g u r e 2* . However , t h i s

f i g u r e i s no t c o m p l e t e l y t h e same as those a v a i l a b l e in

manuals and books on t h e 6502« T h i s is because t h ose

p i c t u r e s do no t r e f l e c t r e a l d e s i g n and some m o d i f i c a t i o n s

had to be done f o r t h e 6502 AH PL d e s c r i p t i o n t o be d i s c u s s e d

in t h i s paper * Even among t h o s e p i c t u r e s , t h e r e ar e s t i l l

some d i f f e r e n c e s * A ma j or d i f f e r e n c e bet ween F i g u r e 2 and

t h e o t h e r s is t h a t t he t e mp o r a r y r e g i s t e r i s d e p i c t e d in

F i g u r e 2* An o t h e r i s t h e e x p l i c i t l y i n d i c a t e d ARGBUS *

I n d i v i d u a l r e g i s t e r s w i l l be e x p l a i n e d i n b r i e f as f o l l o w s :

DL — The d a t a l a t c h i s used to l a t c h i ncomi ng da t a

on t h e DBUS f o r d a t a f e t c h e s * The DL w i l l a l s o load t he

d a t a ont o e i t h e r t h e IDBUS f o r t r a n s f e r s or t h e DBUS f o r

w r i t e o p e r a t i o n s * The d a t a f e t c h e s w i l l occur depend i ng

upon t he i n s t r u c t i o n b e i n g p e r f o r me d by t h e p r o c e s s o r * The

14

cu0)3CQCDCO0)k735

r-0 1_i1§d

PCL | PCH

__INR ___INC^DEC

J T

TR

DL

T T

C

InterruptLogic

ControlLogic

Instruction Decoder

RESIRQM I

PHIOPHI1PHI 2R/WSYNCRDYS. O.

IR

’L r

DATABUSBuffern

DBUS[0i7] DBUS

Figure 2. 6502 Internal Architecture

15

d a t a s t o r e d in DL w i l l be used e i t h e r as h a l f p o r t i o n s o f

a dd r e s s e s or i n l a t e r o p e r a t i o n s , ,

TR — The t e m p o r a r y r e g i s t e r is p r o v i d e d to save

d a t a t h a t w i l l be used as h a l f p a r t s of a d d r e s s e s in l a t e r

oper a t t ons

IR — The d a t a f e t c h e d f rom o u t s i d e d e v i c e s in t he

OP CODE f e t c h c y c l e w i l l be s t o r e d in t he i n s t r u c t i o n

r e g i s t e r , , The d a t a w i l l i n t u r n be s e n t t o t h e i n s t r u c t i o n

dec oder t o g e n e r a t e a p p r o p r i a t e c o n t r o l s i g n a l s *

P — The pr ocessor s t a t u s r e g i s t e r is p r ov i ded to

i n d i c a t e the c u r r e n t s t a t u s of t he pr ocessor * The P

r e g i s t e r is f u r t h e r p a r t i t i o n e d i n t o the Car r y ( C ) , Zero

( Z )* I n t e r r u p t D i s a b l e ( I ) » Decimal Mode ( D)» Break Command

( b ) p Over f l ow ( 0 ) j> and N e g a t i v e ( N) f l a g s to be used by

branch i n s t r u c t i o n s , a r i t h m e t i c and l ogi c o p e r a t i o n s , etc *

INR — The i n c r e me n t / de cr eme nt r e g i s t e r s t o r e s data

f rom e i t h e r INC/DEC or IADBUS* Data st or ed in INR w i l l

u s u a l l y be used f or addr ess i ng*

PC The pr ogr am c o u n t e r i s made up o f t h e program

c o u n t e r low and pr ogr am c ou n t e r h i g h * L i k e I NR, Dat a s t o r e d

i n PC w i l l be used f o r a d d r e s s i n g *

AC The a c c u m u l a t o r , as t he name i m p l i e s ,

a c c u mu l a t e s most of t he computed d a t a f rom t he ALU* The

c o n t e n t s o f t he a c c u m u l a t o r can a l s o be an ar gument o f t h e

ALU f o r a r i t h m e t i c and l o g i c o p e r a t i o n s *

16

ALUREG — The ALU r e g i s t e r is a t e mp o r a r y s t o r a g e

f o r t he d a t a comi ng out of t h e A l U o The ALUREG w i l l be

r e f r e s h e d e v e r y c y c l e o T h e r e f o r e , ALUREG can not s t o r e d a t a

f o r more t han one cycl e, , The c o n t e n t s of t he r e g i s t e r can

be h a l f p a r t s o f addr esses put ont o t he I A D B U S „

SP “ = The s t a c k p o i n t e r is used t o p o i n t t o and save

t h e l o c a t i o n c u r r e n t l y used in t h e s t ack o I n o r d e r t o s e t

t h e s t a c k p o i n t e r , t he d a t a on t h e IDBUS can be s t o r e d in

SP.

X — The i ndex X r e g i s t e r i s p r e p a r e d f o r t he

a d d r e s s i n g modes a s s o c i a t e d wi t h t he X r e g i s t e r . Thi s

r e g i s t e r can be used as a t e mp o r a r y s t o r a g e f rom

pr ogr ammer s* v i e w p o i n t . The d a t a in t he X r e g i s t e r can be

l oaded ont o e i t h e r t h e IDBUS or ARGBUS,

Y — The i ndex Y r e g i s t e r i s p r e p a r e d f o r t he

a d d r e s s i n g modes a s s o c i a t e d w i t h t h e Y r e g i s t e r . Th i s

r e g i s t e r can be used as a t e m p o r a r y s t o r a g e as w e l l , The

d a t a in t he Y r e g i s t e r can be l oaded ont o e i t h e r t he IDBUS

or ARGBUS,

The buses and r e g i s t e r s p r o v i d e d i n s i d e t h e 6502

have been d e s c r i b e d , y e t a d e s c r i p t i o n of t h e c o m b i n a t i o n a l

l o g i c u n i t s , t h e ALU and INC/DEC is not p r e s e n t e d . The I N C /

DEC w i l l not need an e x p l a n a t i o n a t a i l because i t i s a mere

i n c r e m e n t e r / d e c r e m e n t e r u n i t t h a t w i l l i n c r e me n t or

dec r e ment addr e ss d a t a on t he IADBUS, However , t h e ALU w i l l

- 17

r e q u i r e a' c omp r e h e n s i v e s p e c i f i c a t i o n o I n Ch a p t e r 3s> t he

ALU w i l l be d i s c u s s e d in c o n j u n c t i o n w i t h a r i t h m e t i c and

l o g i c oper a t ionso

D e t a i l e d i n f o r m a t i o n about t he i n p u t and o u t p u t

c o n n e c t i o n s o f t he r e g i s t e r s and C LU0 s i s p r o v i d e d in T a b l e

2 o T a b l e 2 w i l l compensat e f o r t he l i m i t a t i o n o f t he

i l l u s t r a t i o n o f F i g u r e 2» Thus* e x a c t c o n n e c t i o n s sh o u l d be

l ooked i n t o in T a b l e 2» I n f a c t ; T a b l e 2 has been updat ed

as t he AHPL d e s c r i p t i o n f o r t h e 6502 has been m o d i f i e d .

I n t h e ne x t c h a p t e r , t he i m p l i c a t i o n of t h e s e c o n n e c t i o n s

w i l l be c l a r i f i e d , and F i g u r e 2 and Tab l e 2 w i l l o f t e n be

r e f e r r e d t o f o r f u r t h e r d i s c u s s i o n .

I n s t r u c t i o n Set

The 6502 m i c r o p r o c e s s o r has a p o we r f u l i n s t r u c t i o n

s e t , whi ch i s v e r y s i m i l a r to t h e 6800 m i c r o p r o c e s s o r ' s

i n s t r u c t i o n s e t . Because o f t h e c omp l e t e n e s s and

s i m i l a r i t y , t he user can w r i t e e f f i c i e n t pr ogr ams and

t r a n s l a t e a program f o r t h e 6800 i n t o a pr ogr am f o r t h e 6502

e a s i l y . The 6502 i n s t r u c t i o n s e t c o n s i s t s o f 151 o p e r a t i o n

codes (OP CODE' S) , and t h e s e OP CODE”s can be c l a s s i f i e d

i n t o two c a t e g o r i e s , i n s t r u c t i o n and a d d r e s s i n g mode, 56

k i n d s o f i n s t r u c t i o n s and 11 a d d r e s s i n g modes a r e be i ng

used, A b r i e f d e s c r i p t i o n of t h e i n s t r u c t i o n s and

a d d r e s s i n g modes w i l l be g i v e n bel ow f o r t he d i s c u s s i o n in

Ch a pt e r 3,

18

T a b l e 2 • I n p u t and O u t p u t C o n n e c t i ns o f R e g i s t e r s and CL U ' s

Reg i s t e r or CLU I np u t / O u t p u t C o n n e c t i o n s and T r a n s f e r s

I npu t Da t a L a t c h (DL )

I n p u t DL

19

T a b l e 2 . — C o n t i n u e c

R e g i s t e r or CLU I n p u t / O u t p u t Co n n e c t i o n s and T r a n s f e r s

A r i thme t i c Log i c U n i t (ALU)

I n p u t( Ar gment )

ALU : IDBUS ALU : ARGBUS ALU : OP ALU : PS

Out put ALUREG

20

Ta o l e 2 . - » Con t i nue d

R e g i s t e r or CLU I n p u t / O u t p u t Co n n e c t i o n s and T r a n s f e r s

I n cr ement er / De c r e me n t e r

( I N C / DEC ) ;

I n p u t( Ar gme nt )

INC i IADBUS DEC : IADBUS

Ou t p u t I N R [ 8 : 1 5 3 < =INC [ 8 : 1 5 3 ( I ADBUS)

I NR[ 8 : 1 5 3 < =DECC8S153( I AD B US )

INR

Fundament a l I n s t r u c t i o n s

For a more c o n c i s e d e s c r i p t i o n , t he 56 i n s t r u c t i o n s

can be f u r t h e r d i v i d e d i n t o n i n e f u n d a me n t a l i n s t r u c t i o n s o

They a r e a r i t h m e t i c , l o g i c , l o a d , s t o r e , b r a n c h , j ump,

s e t / c i e a r , t r a n s f e r , and s t a c k i n s t r u c t i o n s . The

i n s t r u c t i o n s w i l l be d e s c r i b e d in t erms o f t h e f un da me n t a l

i n s t r u c t i ons =.

A r i t h m e t i c I n s t r u c t i o n — The a r i t h m e t i c i n s t r u c t i o n

uses t h e ALU, so t h a t t h e d a t a s t o r e d in AC and DL w i l l be

e i t h e r added or s u b t r a c t e d , , The c a p a b i l i t i e s o f a d d i t i o n

and s u b t r a c t i o n a r e p r o v i d e d in t he ALU f o r such

i n s t r u c t i o n s as ADC, CMP, CPX, CPY, DEC, DEX, DEY, I NC, I NX,

I NY, and SBC o Some f l a g s of t he P r e g i s t e r w i l l be

r e f r e s h e d as a r e s u l t of an a r i t h m e t i c o p e r a t i o n , ,

Logi c I n s t r u c t i o n — The l o g i c i n s t r u c t i o n a l s o uses

t he ALU t o p e r f o r m a l o g i c o p e r a t i o n on t he d a t a i n AC and

DL o The f u n c t i o n s of AND, OR, e x c l u s i v e OR, s h i f t , and

r o t a t i o n a r e f u r n i s h e d in t h e ALU as we I I „ The l o g i c

i n s t r u c t i o n a l so a f f e c t s t h e P r e g i s t e r , , The i n s t r u c t i o n s

c l a s s i f i e d i n t o t h e l o g i c i n s t r u c t i o n ar e AND, ASL, B I T ,

EOR, L S R , ORA, ROL, and RORo Not e t h a t t h e s h i f t and

r o t a t i o n f u n c t i o n s a r e c l a s s i f i e d i n t o t h i s i n s t r u c t i o n f o r

t h e s i m p l e d i s c u s s i o n in Ch a p t e r 3 , a l t h o u g h t h e f u n c t i o n s

ar e no t l o g i c f u n c t i o n So

Load I n s t r u c t i o n — The l oa d i n s t r u c t i o n imp I i e s

22

t h a t d a t a t r a p p e d by DL w i l l be s t o r e d in t h e X r e g i s t e r * Y

r e g i s t e r , or a c c u m u l a t o r . I n a d d i t i o n , t he z e r o and

n e g a t i v e f l a g s of t he P r e g i s t e r may be changed dependi ng on

t h e c o n t e n t s of t he d a t a . The i n s t r u c t i o n s p l a c e d under t h e

l oad i n s t r u c t i o n a r e LDA, LDX, and LDY.

S t o r e I n s t r u c t i o n — The p r o c e s s o r w i l l e x e c u t e t he

s t o r e i n s t r u c t i o n by l o a d i n g d a t a s t o r e d in t h e X r e g i s t e r ,

Y r e g i s t e r , or a c c u mu l a t o r ont o t he DBUS. The STA, STX, and

STY i n s t r u c t i o n s a r e c a t e g o r i z e d i n t o t he s t o r e i n s t r u c t i o n .

Branch I n s t r u c t i o n — The pr o c e s s or w i l l br anch t o

t he l o c a t i o n where t he n e x t OP CODE f e t c h w i l l be e xe cut ed

i f a br anch c o n d i t i o n is s a t i s f i e d . The br anc h i n s t r u c t i o n

makes use of t he c a r r y , z e r o , o v e r f l o w , and n e g a t i v e f l a g s

f o r a br anch c o n d i t i o n . The br anc h l o c a t i o n , i s c a l c u l a t e d

i n t he ALU. The i n s t r u c t i o n s c l a s s i f i e d under t h e br anch

i n s t r u c t i o n a r e BCC, BCS, BEQ, BMI , BNE, BPL, BVC, and BVS.

Jump I n s t r u c t i o n — By t he jump i n s t r u c t i o n , the

p r o c e s s o r w i l l jump u n c o n d i t i o n a l l y to t h e l o c a t i o n where

t he n e x t OPCODE f e t c h w i l l be d o n e . U n l i k e t h e branch

i n s t r u c t i o n , t h i s i n s t r u c t i o n i n c l u d e s r a t h e r c o m p l i c a t e d

o p e r a t i o n s f o r w r i t i n g da t a in t h e s t a c k and r e t r i e v i n g d a t a

f rom t h e s t a c k . The i n s t r u c t i o n s put under t h e jump

i n s t r u c t i o n a r e BRK, JMP, JSR, R T I , and RTS. Onl y t he JMP

i n s t r u c t i o n does not r e q u i r e t h e use of t h e s t a c k .

S e t / C l e a r I n s t r u c t i o n — The s e t / c l e a r i n s t r u c t i o n

23

i s f u r n i s h e d t o mod i f y t h e P r e g i s t e r o By t h i s i n s t r u c t i o n ,

t h e c a r r y , dec i mal mode, and i n t e r r u p t d i s a b l e f l a g s can be

s e t or c l e a r e d , w h i l e t h e o v e r f l o w f l a g can o n l y be c I e a r e d o

The o v e r f l o w f l a g w i l l be s e t in case t h a t an o v e r f l o w

c o n d i t i o n occur s as a r e s u l t of an a r i t h m e t i c o p e r a t i o n o

The o t h e r f l a g s a r e not a c c e s s i b l e t o t he programmer 0 The

i n s t r u c t i o n s , c l a s s i f i e d i n t o t he s e t / c l ear i n s t r u c t i o n a r e

CLC, CLD, C L I , CLV, SEC, SED, and SEXo

T r a n s f e r I n s t r u c t i o n — The t r a n s f e r i n s t r u c t i o n i s

p r o v i d e d t o t r a n s f e r d a t a f r om one o f t he f o u r r e g i s t e r s ,

t he X r e g i s t e r , Y r e g i s t e r , s t a c k p o i n t e r , and a c c u mu l a t o r

i n t o a no t h e r r e g i s t e r of t h e s e « Onl y s i x c o m b i n a t i o n s o f

sou r c e and d e s t i n a t i o n r e g i s t e r s a r e imp I ement edo The

i n s t r u c t i o n s p l a c e d under t h e t r a n s f e r i n s t r u c t i o n a r e TAX,

TAY, TSX, TXA, TXS, and TYAo A l l t he i n s t r u c t i o n s e x c e p t

TXS a f f e c t t he z e r o and n e g a t i v e f l a g s .

S t a c k I n s t r u c t i o n - - The s t a c k i n s t r u c t i o n uses t h e

s t a c k p o i n t e r i n o r d e r t o pu t d a t a in t he s t a c k or r e t r i e v e

d a t a f rom t h e s t a c k » Th i s i n s t r u c t i o n i s s i m i l a r to t he

l oad and s t o r e i n s t r u c t i o n s but uses t he s t a c k p o i n t e r

i n s t e a d of t he program c o u n t e r f o r add r essi n g o However , t he

s t a c k i n s t r u c t i o n u t i l i z e s PC once t h e c u r r e n t l y used s t a c k

addr ess has been s e t i n PCo The i n s t r u c t i o n s c l a s s i f i e d

under t he s t a c k i n s t r u c t i o n a r e PH A, PH P, PLA, and PLPo

The r e a d e r s h o u l d r e f e r t o r e f e r e n c e 4 f o r f u r t h e r

i n f o r m a t i o n because t h e above d e s c r i p t i o n i s n o t i n t e n d e d to

p r e s e n t a c o mpl e t e e x p l a n a t i o n o f t h e i n s t r u c t i o n s e t .

A l t h o u g h each f u n d a me n t a l i n s t r u c t i o n does no t d e s c r i b e

e x a c t l y t he e x e c u t i o n p a r t o f t he i n t e r n a l o p e r a t i o n f o r

each i n s t r u c t i o n , i t w i l l he l p t o u n d e r s t a n d d e t a i l s of t he

i n s t r u c t i o n s e t o A 6502 i n s t r u c t i o n s e t summary has been

p r o v i d e d in Appendi x A f o r r e a d e r s " c o n v e n i e n c e and w i l l

i t s e l f e x p l a i n t he e x e c u t i o n p a r t of each i n s t r u c t i o n more

a c c u r a t e l y . . T y p i c a l i n t e r n a l r e g i s t e r s a r e i l l u s t r a t e d in

F i g u r e 3 t o i n d i c a t e t h e b i t c o f i g u r a t i o n ado pt e d in t he

6502 AHPL d e s c r i p t i o n *

Addr ess i ng Modes

As s t a t e d b e f o r e , t h e 6502 was i n t r o d u c e d a f t e r t he‘ ' t

6 800 * As a ' r e s u l t of t he l a t e a p p e a r a n c e , v e r s a t i l e

a d d r e s s i n g modes and e f f i c i e n t i n s t r u c t i o n c y c l e s were

i mp l ement ed* Programs such as a program t h a t m a n i p u l a t e s

t a b l e s and ar r ays can be w r i t t e n v e r y e f f e c t i v e l y by means of

t h e s e a d d r e s s i n g modes* M o r e o v e r , t he m i c r o p r o c e s s o r can

e x e c u t e i n s t r u c t i o n s f a s t e r because o f s h o r t e r e x e c u t i o n

t i me * S i nc e a f u l l d e s c r i p t i o n of t he a d d r e s s i n g modes has

been l e f t t o t he n e x t c h a p t e r , each a d d r e s s i n g mode w i l l be

s i m p l y d e s c r i b e d in ac c or da n c e w i t h t he pur pose of t h i s

c h a p t e r *

I mmed i a t e ~ The i n s t r u c t i o n s w i t h t he i mmedi at e

a d d r e s s i n g mode c o n t a i n two byt es (8 b i t s ) , an OP CODE and a

25

AC Ac c u mu l a t o r

I ndex R e g i s t e r X

I n de x R e g i s t e r Y

7 8 15

PC( Hi gh) PC(Low ) Program Count er

MSB LSB

SP S t a c k P o i n t e r

0 7

N V B D I Z c

P

Pr o c e s s o r St atusReg i s t e r

F i g u r e B i t C o n f i g u r a t i o n o f T y p i c a l R e g i s t e r s

26

c o n s t a n t v a l u e » The OP CODE s p e c i f i e s t he i n t e r n a l

o p e r a t i o n and a d d r e s s i n g mode, and t he c o n s t a n t v a l u e is

used in a program, , T h i s mode a l l o w s t he programmer t o

s p e c i f y v a l u e s n e c e s s a r y f o r t he program, . The OP CODE and

v a l u e a r e f e t c h e d f rom c o n s e c u t i v e J o c a t i o n s o f program

memo r y „

Aoso I u t e — The i n s t r u c t i o n s us i ng the a b s o l u t e

a d d r e s s i n g mode use t h r e e b y t e s „ The f i r s t b y t e c o n t a i n s an

OP CODE f o r s p e c i f y i n g t h e i n t e r n a l o p e r a t i o n and a d d r e s s i n g

modeo The second b y t e i n c l u d e s t he low o r d e r b y t e of an

e f f e c t i v e a d d r e s s , w h i l e t h e t h i r d b y t e i n c l u d e s t h e h i gh

o r d e r b y t e o f t h e e f f e c t i v e addr e s s „ Thus, a f u l l 1 6 - b i t

addr ess can be s p e c i f i e d by t h i s mod e o The e f f e c t i v e

a d d r e s s , a f u l l 1 6 - b i t a d d r e s s , w i l l be used to address a

l o c a t i o n a t whi ch d a t a w i l l be r e t r i e v e d and s t o r e d *

Zero Page — The i n s t r u c t i o n s w i t h t he z e r o page

a d d r e s s i n g mode a r e made up of two b y t e s „ The f i r s t by t e i s

an OP CODE as b e f o r e , t h e second b y t e c o n t a i n s an e f f e c t i v e

addr ess in page z e r o o f memory * The m i c r o p r o c e s s o r

g e n e r a t e s a l l 0 ' s used f o r t he h i gh or der by t e of a f u l l

1 6 - b i t address * T h i s mode w i l l a l l o w t h e programmer t o

s h o r t e n bot h memory space ano e x e c u t i o n t i m e * Onl y

l o c a t i o n s in page z e r o can be used f o r r ead and w r i t e

opeat i o n s „

A c c u m u l a t o r — The i n s t r u c t i o n s h a v i n g t h e

27

a c c u m u l a t o r a d d r e s s i n g mode i n c l u d e on l y an OP CODE t h a t

i n d i c a t e s t he i n t e r n a l o p e r a t i o n and a d d r e s s i n g modeo T h i s

mode can be c o n s i d e r e d t o be t he same as t h e i m p l i e d mode

d e s c r i b e d bel ow because bot h t h e i n s t r u c t i o n s c o n t a i n on l y

an OP CODEo However * t he a c c u mu l a t o r a d d r e s s i n g mode is

c a t e g o r i z e d d i s t i n c t l y because o n l y t he a c c u m u l a t o r o f t h e

r e g i s t e r s a c c e s s i b l e t o t he programmer is u t i l i z e d f o r t he

i n t e r n a l o p e r a t i o n s of t h e i n s t r u c t i o n s u s i n g t h i s mode.

I m p l i e d — As me n t i o n e d a b o v e ; t he i n s t r u c t i o n s w i t h

t he i m p l i e d a d d r e s s i n g mode use o n l y an OP CODE f o r

s p e c i f y i n g t he i n t e r n a l o p e r a t i o n and a d d r e s s i n g m o d e o Some

i n s t r u c t i o n s u s i n g t h e mode r e q u i r e more t ha n 2 eye I es *

E x p l a n a t i o n s o f t h e s e i n s t r u c t i o n s ar e beyond t h e scope of

t h i s c h a p t e r o

I n de x e d I n d i r e c t — - The i n s t r u c t i o n s u s i n g t h e

i ndexed i n d i r e c t a d d r e s s i n g mode c o n t a i n an OP CODE and a

base addr ess i n page z e r o „ F i r s t ; t h e base a d d r e s s i s added

t o t he d a t a s t o r e d in t h e X r e g i s t e r . The r e s u l t a n t by t e

becomes a n o t h e r addr ess i n page z e r o t h a t c o n t a i n s t h e low

o r d e r b y t e o f an e f f e c t i v e a d d r e s s ; w h i l e i t s c o n s e c u t i v e

addr ess c o n t a i n s t h e h i gh o r d e r b y t e o f t h e e f f e c t i v e

a d d r e s s . The e f f e c t i v e addr ess w i l l be used f o r read and

wr i t e o p e r a t i o n s .

I n d i r e c t I n d e x e d — The i n s t r u c t i o n s w i t h t h e

i n d i r e c t i ndexed a d d r e s s i n g mode c o n t a i n an OP CODE and an

i n d i r e c t a dd r e ss in page z e r o * The i n d i r e c t ad dr ess

c o n t a i n s a base a d d r e s s low* and t he nex t c o n s e c u t i v e

addr e ss o f t h e i n d i r e c t address c o n t a i n s a base address

hi gho Whi l e t h e base addr ess h i gh i s be i ng f e t c h e d * t he

base addr ess low i s added t o t he d a t a s t o r e d i n t h e Y

r e g i s t e r , , The r e s u l t a n t b y t e becomes t h e low o r d e r b y t e o f

an e f f e c t i v e a d d r e s s * I f a c a r r y i s g e n e r a t e d f rom t he

a d d i t i o n * t h e c a r r y * as b i n a r y one* i s added t o t h e base

addr e ss h i gh t o i n c r e m e n t t he addr ess by 1 , The r e s u l t a n t

b y t e becomes t h e h i gh o r d e r by t e of t he e f f e c t i v e a d d r e s s *

The base addr e ss h i gh w i l l be used as t he h i gh o r d e r by t e i f

no c a r r y *

Zero Page I n d e x e d The i n s t r u c t i o n s w i t h t he z e r o

page i ndexed a d d r e s s i n g mode i n c l u d e an DP CODE and a base

address low * Whi l e d a t a i s b e i ng f e t c h e d f rom t he [ l ocat i on

s p e c i f i e d by t h e base addr ess low i n page z e r o * t he base

addr ess low is added t o t he da t a s t o r e d in e i t h e r t h e X

r e g i s t e r or Y r e g i s t e r in o r d e r t o ge t an e f f e c t i v e addr ess

i n page z e r o * . The d a t a f e t c h e d is no t used in l a t e r

o p e r a t i o n s * No page c r o s s i n g is a l l o w e d in t h i s mode*

I n d i r e c t — The i n s t r u c t i o n s w i t h t h e i n d i r e c t

a d d r e s s i n g mode c o n t a i n t h r e e by t es l i k e t hose w i t h the

a b s o l u t e a d d r e s s i n g mode* The second b y t e c o n t a i n s t h e low

o r d e r b y t e of an i n d i r e c t a d d r e s s * w h i l e t he t h i r d by t e

c o n t a i n s t he h i gh o r d e r b y t e o f t he i n d i r e c t a d d r e s s * Once

29

t h e i n d i r e c t addr ess has been s e t upj> t h e low o r de r b y t e and

h i gh o r d e r b y t e of an e f f e c t i v e addr ess ar e f e t c h e d f rom t h e

l o c a t i o n s s p e c i f i e d by t h e i n d i r e c t address and i t s

c o n s e c u t v i e a d d r e s s , r e s p e c t i v e l y , , The e f f e c t i v e address

wi I I be used f o r t he n e x t OP CODE f e t c h o

A b s o l u t e I n de x e d — The i n s t r u c t i o n s us i ng t h e

a b s o l u t e i ndexed a d d r e s s i n g mode i n c l u d e t h r e e b y t e s , an

OP CODE, a base address low, and a base a d d r e s s hi gho The

f a c i l i t y to add t h e base addr ess low to e i t h e r t h e X

r e g i s t e r or Y r e g i s t e r i s p r o v i d e d t o compose t h e low o r de r

by t e of an e f f e c t i v e addresso To c a r r y out a page c r o s s i n g ,

t he t e c h n i q u e to add a c a r r y to t he base a dd r e ss h i gh is

used o

R e l a t i v e — The i n s t r u c t i o n s w i t h t he r e l a t i v e

a d d r e s s i n g mode c o n s i s t o f an OP CODE and an o f f s e t o An

e f f e c t i v e a dd r e ss is c a l c u l a t e d by means of t h e address of

t h e l o c a t i o n where t h e n e x t OP CODE f e t c h w i l l be pe r f o r me d

i f a br anch is not t a k e n and t h e o f f s e t . The page c r o s s i n g

can occur in t h i s modeo

F u r t h e r i n f o r m a t i o n r e g a r d i n g t h e a d d r e s s i n g modes

i s e x t e n s i v e l y d e s c r i b e d in r e f e r e n c e 4«

CHAPTER 3

I NTERNAL OPERATIONS ANALYSI S

I n t h e p r e c e d i n g c h a p t e r » a b r i e f d e s c r i p t i o n of t he

6502 m i c r o p r o c e s s o r has been p r e s e n t e d , , Whi l e t h e

d e s c r i p t i o n wi . i l g i v e t he r e a d e r a b a s i c k nowl edge of t h e

6502.P i t w i l l not be enough f o r t h e d e v e l o pme n t o f a 6502

AH PL d e s c r i p t i o n , . S u f f i c i e n t i n f o r m a t i o n c o n c e r n i n g t h e

e x t e r n a l b e h a v i o r o f t h e 6502 must be o b t a i n e d so t h a t t he

i n t e r n a l o p e r a t i o n s of t h e 6502 can be a n a l y z e d f o r t h e

devel opment , . The i n f o r m a t i o n can be f ound i n r e f e r e n c e s 3

and 4 o A summary o f c y c l e by c y c l e o p e r a t i o n o f t h e 6502

has been pr oduced f rom t h e i n f o r m a t i o n a v a i l a b l e i n t he

above r e f e r e n c e s and has been a t t a c h e d i n Appendi x B„

Appendi x B w i l l be h e l p f u l in a n a l y z i n g t h e i n t e r n a l

o p e r a t i o n s of a l l i n s t r u c t i o n s o

The pur pose o f t h i s c h a p t e r i s t o a n a l y z e t he

i n t e r n a l o p e r a t i o n s o f a i l i n s t r u c t i o n s and i n p u t s in o r d e r

t h a t a 6502 AHPL d e s c r i p t i o n can be e f f i c i e n t l y and

s y s t e m a t i c a l l y dev e l opedo Ho we v e r s> i t w i l l no t be an easy

t a s k t o a n a l y z e t h e i n t e r n a l o p e r a t i o n s o f a l l i n s t r u c t i o n s

i n d i v i d u a l l y because t h e i n s t r u c t i o n s e t has as many as 151

OP CODE0 s o S i n c e t h e i n t e r n a l o p e r a t i o n o f each i n s t r u c t i o n

30

31

c o n s i s t s o f an a d d r e s s i n g p a r t and an e x e c u t i o n p a r t y t he

i n t e r n a l o p e r a t i o n s of a l l i n s t r u c t i o n s can be co mp a c t ! y

a n a l y z e d i n t er ms o f t h e a d d r e s s i n g p a r t s and t h e e x e c u t i o n

p a r t s o f t he i n t e r n a l o p e r a t i o n s o.

Hy pot heses

B e f o r e p r o c e e d i n g w i t h t he a n a l y s i s o f t h e i n t e r n a l

o p e r a t i o n s # s e v e r a l hypot heses w i l l be d e f i n e d , i n o r der to

make t he 6502 AHPL d e s c r i p t i o n c l o s e r t o t h e a c t u a l ha r d wa r e

o r g a n i z a t i o n of t he 6502o These hy p o t h e s e s must be t a k e n

i n t o acc ount i n a n a l y z i n g t h e i n t e r n a l o p e r a t i o n s # because

t h e AHPL d e s c r i p t i o n w i l l be based d i r e c t l y on t h e r e s u l t s

o f t h i s a n a l y s i s o The hy p o t h e s e s w i l l be d e f i n e d as

f o l l o w s :

The Co n n e c t i o n s t o ALU *=— The ARGBUS must a l ways be

co n n e c t e d t o argument 1 of t he ALU# w h i l e t h e IDBUS must be

c o n n e c t e d t o a r gument 2« T h i s i s because o f a v o i d i n g t he

n e c e s s i t y of a d d i t i o n a l i n t e r n a l buseso The o t h e r two

ar gument s a r e c on nec t ed t o t he P r e g i s t e r and i n s t r u c t i o n

decodero The c o n n e c t i o n bet ween t h e ARGBUS and ar gument 2

and t he c o n n e c t i o n bet ween t he IDBUS and ar gument 1 ar e n o t

P e r m i s s i b I e o

ALUREG — The ALUREG can n o t s t o r e d a t a f o r more

t hanone c y c l e 0 T h i s means t h a t ALUREG can not be used as a

t e m p o r a r y r e g i s t e r u n l e s s t h e da t a w i l l be s t o r e d in ALUREG

aga i n d u r i n g t he n e x t eye I eo Onl y t h e d a t a f rom t h e ALU can

be s t o r e d in ALUREGo

The Use o f TR Owing t o t h e h y p o t h e s i s o f t he

c o n n e c t i o n s t o t he ALUs> t h e use o f t h e t e mp o r a r y r e g i s t e r i s

assumed t o save d a t a used f o r a d d r e s s i n g in l a t e r

op e r a t e o n s „

I ADBUS[8 s153 t o ALU — The d a t a on t he IADBUSC8S15]

i s a l l o w e d t o be t r a n s m i t t e d t o t he ALU over t h e ARGBUS f o r

an a r i t h m e t i c o p e r a t i o n * The c o n n e c t i o n be t ween t h e

IADBUSC OS 73 and ARGBUS i s not a l l o w e d .

These h y p o t h e s e s a r e a l s o d e f i n e d i n F i g u r e 2 and

T a b l e 2 . S i nce t he o t h e r c o n n e c t i o n s and o p e r a t i o n a l

f e a t u r e s no t i n c l u d e d i n t h e h y p o t h e s e s can be e a s i l y

d e r i v e d f rom t h e i n f o r m a t i o n a v a i l a b l e in r e f e r e n c e s 3, 4j>

and 5s> t h e s e c o n n e c t i o n s and o p e r a t i o n a l f e a t u r e s were

d e f i n e d i n advance w i t h o u t q u e s t i o n . By add i ng t he

hypot heses^ t he c o n n e c t i o n s bet ween t h e i n t e r n a l buses*

i n t e r n a l r e g i s t e r s * and CLU°s have been c o m p l e t e l y d e f i n e d .

Now* t h e i n t e r n a l o p e r a t i o n s can be a n a l y z e d by means of t h e

c o m p l e t e l y d e f i n e d i n t e r n a l a r c h i t e c t u r e .

Genera I Sequence

As a l l i n s t r u c t i o n s have common o p e r a t i o n c y c l e s

such as t he OP CODE f e t c h c y c l e and t h e f o l l o w i n g d a t a f e t c h

c y c l e * i t i s p o s s i b l e to b u i l d a g e n e r a l sequence o f t h e

i n t e r n a l o p e r a t i o n s o f a f l - i n s t r u c t ions. . An i n t e r n a l

o p e r a t i o n sequence o f each i n t e r n a l o p e r a t i o n can be w r i t t e n

33

by u s i n g t h e g e n e r a l sequence as a b a s i c formo The g e n e r a l

sequence os g i v e n in F i g u r e 4 o To p r e p a r e f o r t h e l a t e r

d i s c u s s i o n on t he a d d r e s s i n g p a r t s and e x e c u t i o n p a r t s * each

c y c l e o f t h e g e n e r a l sequence w i l l be e x p l a i n e d as f o l l o w s :

TO — The TO c y c l e i s t h e OP CODE f e t c h c y c l e and i s

common t o t he i n t e r n a l o p e r a t i o n s of a l l i n s t r u c t 5o n s 0 To

f e t c h an OP CODE f rom pr ogr am memory* t he f o l l o w i n g

o p e r a t i o n s must occ ur : F i r s t * t h e p r oc e s s or p u t s a 1 6 - b i t

addr e ss on t he IADBUS* and p l a c e s l o g i c 1 on t h e SYNC and

R/W o u t p u t l i n e s t o i n d i c a t e t h e e x e c u t i o n of t h e OP CODE

f e t c h c y c l e and a r ead o p e r a t i o n , , Then* t h e OP CODE on t he

DBUS is t r a p p e d i n t o I R 0 Du r i n g t he c y c l e * t h e address on

t he IADBUS is t r a n s f e r r e d i n t o INC ° The addr ess i n c r e me n t e d

in INC i s s t o r e d in INR f o r l a t e r o p e r a t i o n s * I n t e r n a l

r e g i s t e r s t h a t ar e t o p r o v i d e an addr ess on t h e IADBUS w i l l

be d e t e r m i n e d by t he p r e c e d i n g i n s t r u c t i o n * The PC i s

s e l e c t e d i n t he TO c y c l e w i t h o u t any meani ngs*

The above o p e a t i ons ar e c o n c i s e l y d e s c r i b e d in AHPL

i n F i g u r e 4* I n i n t e r n a l o p e r a t i o n sequences* owi ng t o t h e

naming c o n v e n t i o n o f t h e HPSIM2 s i m u l a t o r C63* c l o s e l y

r e l a t e d symbol names a r e adopt ed f o r t he a c t u a l i n p u t and

o u t p u t * For i n s t a n c e * t h e R/W o u t p u t s i s named RW h e r e *

I n a d d i t i o n * t he IADBUS i s i m p l i c i t l y c o n nec t ed t o t h e

ADBUS * The above c o n s i d e r a t i o n i s because t he i n t e r n a l

o p e r a t i o n sequences d e r i v e d f rom t h e g e n e r a l sequence can be

34

TO:

T l :

T n - 1 :

Tn:

Tn+1:

TOCOVLP] :

T1COVLP3:

1 ADBUS « PC; IR

35

d i r e c t l y u t i l i z e d f o r t he d ev e l o pment o f t he 65 0 2 AHPL

d e s c r i p t i o n o

I I — The T1 c y c l e i s t h e d a t a f e t c h c y c l e f o l l o w i n g

t he OP CODE f e t c h and i s a l s o common t o t he i n t e r n a l

o p e r a t i o n s o f a l l i n s t r u c t i o n s o F i r s t ? t h e add r e s s s t o r e d

i n INR i s p l a c e d on t h e IADBUS f o r a d d r e s s i n g o At t h e same

t i me? t h e p r o c e s s o r put s l o g i c 0 on t h e SYNC l i n e and l o g i c

1 on t he R/.W l i n e t o i n d i c a t e t he e x e c u t i o n o f a non-OP CODE

f e t c h c y c l e and a r e a d o p e r a t i o n , , Next? as i n t h e TO cyc l e ?

t h e addr ess on t he IADBUS i s i n c r e m e n t e d in INC and t hen

s t o r e d in I N R 0 Be f o r e d e l e t e d by t he new a dd r e s s f rom INC?

t h e addr ess p r e v i o u s l y s t o r e d i n INR must be t r a n s f e r r e d

i n t o PC t o be used in l a t e r op er a t i ons o

Tn- 1? Tn? and Tn-H — These c y c l e s s t a r t a t t h e T2

c y c l e and end i m m e d i a t e l y b e f o r e t he TOCOVLP3 eye 1e 0 The

number of t h e c y c l e s v a r i e s d e p end i ng upon each i n s t r u c t i o n ?

For example? t h e i n s t r u c t i o n s u s i n g t he i mme d i a t e a d d r e s s i n g

mode do not need such c y c l e s ? w h i l e t he i n s t r u c t i o n s

empl oyi ng" t h e i n d i r e c t i n d e x e d a d d r e s s i n g mode r e q u i r e a t

l e a s t t h r e e c y c l e s ? These c y c l e s a r e p r o v i d e d t o s p e c i f y

o p e r a t i o n s f o r an a d d r e s s i n g p a r t and e x e c u t i o n p a r t ? Some

i n s t r u c t i o n s such as t h e JMP i n s t r u c t i o n and b r a n c h

i n s t r u c t i o n s do not r e q u i r e any o p e r a t i o n s f o r t h e

e x e c u t i o n p a r t s of t h e i r i n t e r n a l o p e r a t i o n s ?

T0E0VLP3 — The T0E0VLP3 c y c l e i s t h e OP CODE

f e t c h c y c l e of t he n e x t i n s t r u c t i o n and I s e x a c t l y t h e same

as t h e TO eye 1 eo However * t o d i s t i n g u i s h t he OP CODE f e t c h

c y c l e of t he n e x t I n s t r u c t i o n f rom t h e OP CODE f e t c h c y c l e

of t h e c u r r e n t i n s t r u c t i o n ? t h e space f o r s p e c i f y i n g

o p e r a t i o n s f o r t h e a d d r e s s i n g p a r t and e x e c u t i o n p a r t

d e f i n e d by t h e c u r r e n t i n s t r u c t i o n has been p r o v i d e d in t he

TOEOVLP3 eye 1eo W i t h r e s p e c t to t h e a d d r e s s i n g p a r t in t h e

T0C0VLP3 c y c l e ? r e g i s t e r s to s u p p l y a . 1 6 - b i t f u l l addr ess t o

t h e 1 ADBUS a r e d e t e r m i n e d by t h e c u r r e n t i n s t r u c t i o n o Some

o p e r a t i o n s f o r t he e x e c u t i o n p a r t can o v e r l a p t h e TOE0VLP3

c y c l e i f n e c e s s a r y o However? t h e c o n f l i c t on t h e i n t e r n a l

b u s e s ? i n t e r n a l r e g i s t e r s ? and CL.lHs must be avo i de do The

IDBUS i s u s u a l l y a v a i l a b l e f o r t h e e x e c u t i o n p a r t d u r i n g

t h i s cyc l eo

T1C0VLP3 — The T1E0VLP3 c y c l e has t h e same

o p e r a t i o n s as t h e T l c y c l e has o The T1E0VLP3 c y c l e can a l s o

be o v e r l a p p e d by t he e x e c u t i o n p a r t o f t h e p r e c e d i n g

i n s t r u c t i o n ? as in t he T0EDVLP3 eye Ieo

Si nce i t is assumed t h a t t he OP CODE f e t c h e d d u r i n g

t h e TO c y c l e i s t he f i r s t i n s t r u c t i o n and no i n s t r u c t i o n

pr e c e d e s t h a t i n s t r u c t i o n ? t he TO and T l c y c l e s c ou l d not be

o v e r l a p p e d by an e x e c u t i o n par to T h i s is t he r eas on t h a t

o n l y t he T0EOVLP3 and T1E0VLP3 c y c l e s can accommodate

o p e r a t i o n s f o r an e x e c u t i o n p a r t o The T0E0VLP3 c y c l e can

s p e c i f y r e g i s t e r s used f o r a d d r e s s i n g as w e l l o The

37

a s s umpt i on me n t i o n e d above i s a l s o a p p l i e d t o t h e summary of

c y c l e by c y c l e o p e r a t i o n of a l l i n s t r u c t i o n s i n Appendi x B

and t he i n t e r n a l o p e r a t i o n sequences o f t y p i c a l i n s t r u c t i o n s

i n Appendi x C 6

Address 8 ng P ar t s

Because t h e r e a r e s t i l l more t han 35 d i f f e r e n t

a d d r e s s i n g p a r t s of t he i n t e r n a l o p e r a t i o n s o f a l l

i n s t r u c t i o n s ; t he a d d r e s s i n g p a r t s w i l l be d i s c u s s e d in

t e r ms o f t h e a d d r e s s i n g t e c h n i q u e s used in t he 6502 f o r a

more c o n c i s e d e s c r i p t i o n 0 The a d d r e s s i n g t e c h n i q u e s in

a c c o r d a n c e w i t h t h e a d d r e s s i n g modes me n t i o n e d i n Ch a p t e r 2

w i l l cover t h e most o f t h e a d d r e s s i n g p a r t s s u f f i c i e n t l y ;

and a r e i n t e n d e d t o a n a l y z e o n l y t h e o p e r a t i o n s used f o r t h e

a d d r e s s i n g p a r t s o I n advance o f a n a l y s i s of any

i n s t r u c t i o n ; t he summary o f c y c l e by c y c l e o p e r a t i o n o f t he .

i n s t r u c t i o n s p r o v i d e d in Appendi x B sho u l d be s t u d i e d t o be

a b l e t o do c y c l e by c y c l e a n a l y s i s , .

The a d d r e s s i n g p a r t in t he TO; T l ; and T1COVLP3

c y c l e s i s common t o a l l i ns t r u c t i ons o S i n c e t h e d i s c u s s i o n

on t h e o p e r a t i o n s in t he s e c y c l e s has a l r e a d y been

p r e s e n t e d ; o n l y t he o p e r a t i o n s i n t h e T2 c y c l e t h r o u g h t h e

TO EOVLP 3 c y c l e w i l l be d i s c u s s e d as f o l l o ws , ,

I mmed i a t e

I mmed i a t e a d d r e s s i n g does not r e q u i r e any a d d i t i o n a l

38

c y c l e s . Thusj> t n e p r o c e s s o r t a k e s 2 c y c l e s t o p e r f o r m

I mme d i a t e a d d r e s s i n g . S i n c e t h e i n s t r u c t i o n s u s i n g t he

i mmed i a t e a d d r e s s i n g mode c o n s i s t o f two bytesj> an OP CODE

and a c o n s t a n t va l ues INR must be used as an a d d r e s s i n g

r e g i s t e r t h a t p r o v i d e s a X6^bi t addr ess on t he I ADBUS, The

INR has t h e i n c r e me n t e d address p o i n t i n g t o t h e l o c a t i o n ,

s t o r i n g t he OP. CODE f o l l o w i n g t h e c o n s t a n t v a l u e . The

r e a d e r sh o u l d r e f e r t o t h e g e n e r a l sequence i n F i g u r e 4 f o r

i mmedi a t e a d d r e s s i n g and i g n o r e t he T n - l s Tns and Tn - 1

c y c l e s . Not e t h a t INR w i l l be s e l e c t e d among t h e a d d r e s s i n g

r e g i s t e r s d u r i n g t he TOCOVLP] c y c l e ,

A b s o l u t e

A b s o l u t e a d d r e s s i n g r e q u e s t s a t l e a s t one a d d i t i o n a l

c y c l e . The i n s t r u c t i o n s u t i l i z i n g t h e a b s o l u t e a d d r e s s i n g

mode have an OP CODE* t h e low o r d e r b y t e of ah e f f e c t i v e

addr ess * and t he h i gh o r d e r by t e of t h e e f f e c t i v e a d d r e s s .

D u r i n g t h e TX c y c l e * t h e low o r d e r b y t e is f e t c h e d and

T 2 : IADBUS = I NR I DL

addr ess i n c r e m e n t e d in t he T1 c y c l e °

B e f o r e m o d i f i e d by t he h i gh o r d e r by t e» t h e low

o r d e r b y t e must be t r a n s f e r r e d f r om DL i n t o TR o v e r t he

IDBUS d u r i n g t he T2 eye I eo Then? t he low o r d e r b y t e in TR

and h i g h o r d e r b y t e in DL a r e c a t e n a t e d t o make up a 1 6 - b i t

e f f e c t i v e addr ess i n t h e T3 c y c l e * The d a t a t o be t r e a t e d

J n l a t e r o p e r a t i o n s w i l l be f e t c h e d f rom t h e l o c a t i o n

s p e c i f i e d by t h i s a d d r e s s * S i n c e t h e address f o r t he n e x t

DP CODE f e t c h has been pr oduced and s t o r e d i n PC i n t h e T2

and T3 cy c l e s ? PC is used as an a d d r e s s i n g r e g i s t e r d u r i n g

t he T0C0VLP3 eye ! e*

The above o p e r a t i o n s a r e a l l common to t he

a d d r e s s i n g p a r t s of t he i n t e r n a l o p e r a t i o n s o f t h e

i n s t r u c t i o n s us i ng t he a b s o l u t e a d d r e s s i n g mode e x c e p t t h e

JSR and JMP i n s t r u c t i o n s * The JHP i n s t r u c t i o n uses DL and

TR as a d d r e s s i n g r e g i s t e r s d u r i n g t h e T DC OVLP 3 * The r e a d e r

can i g n o r e t h e R/W and SYNC o u t p u t s u n l e s s o t h e r w i s e

i n d i c a t e d * The d e l i b e r a t e use o f PC as a d d r e s s i n g r e g i s t e r

i s a r r a n g e d f o r t he e f f e c t i v e s y n t h e s i s o f t he i n t e r n a l

o p e r a t i o n s i n r e l a t i o n to a b s o l u t e i ndexed a d d r e s s i n g

d i s c u s s e d l a t e r *

The a d d r e s s i n g o f t h e JSR i n s t r u c t i o n r e q u i r e s 4

e x t r a c y c l e s ? t h a t i s t h e T2 c y c l e t h r o u g h t he T5 c y c l e *

Because of t h e o p e r a t i o n t o s t o r e a r e t u r n i n g addr ess in t he

s t ack ? A JSR a d d r e s s i n g sequence w i l l d i f f e r r e m a r k a b l y f rom

t he t y p i c a l a b s o l u t e a d d r e s s i n g S e q u e n c e «

B e f o r e t h e TZ c y c l e , DL s t o r e s t he low or der byt e o f

an e f f e c t i v e a d d r e s s , and INR keeps the n e x t c o n s e c u t i v e

a d d r e s s , as an t h e g e n e r a l $ e q u e n c e 0 Then, d u r i n g t h e TZ

c y c l e , t he low o r d e r b y t e is t r a n s f e r r e d i n t o TR, and t he

addr e ss as moved i n t o PC„ The DL and INR w i l l become f r e e

IADBUS = \ 0 , 0 , 0 , 0 , 0 , 0 , 0 , 1 \ , S P | DL

41

a r e t u r n i n g a d d r e s s 0 I n a d d i t i o n ; t h e a dd r e s s i n PC os used

t o f e t c h t he h i gh o r d e r b y t e o f t h e e f f e c t i v e addr ess d u r i n g

t he T5 eye Deo The l o c a t i o n t h a t c o n t a i n s t he n e x t OP CODE

is a dd r e s s e d by DL and TR t h a t s t o r e t he h i gh o r d e r b y t e and

low o r d e r b y t e * r e s p e c t i v e l y .

Ze r o Page

Zer o page a d d r e s s i n g r e q u i r e s a t l e a s t t h e T2 c y c l e .

The i n s t r u c t i o n s u s i n g t h e z e r o page a d d r e s s i n g mode c o n s i s t

of an OP CODE and an e f f e c t i v e a dd r e s s in page z e r o ; which

ar e f e t c h e d in t h e TO and T l c y c l e s . Now; by u s i n g t h e

e f f e c t i v e addr ess in DL; t he p r o c e s s o r can a dd r e s s a

l o c a t i o n i n page z e r o where e i t h e r a r ead o p e r a t i o n or w r i t e

T 2 o I AD BUS = \ 0 ; 0 ; 0 ; 0 ; 0 ; 0 ; 0 ; 0 \ ; D L | SYNC « . \ 0 \ |RW s \ 0 \ | PC

42

mode have o n l y an OP CODEo The g e n e r a l sequence sho u l d be

r e f e r r e d t o f o r t h i s addr e s s i ng; ,

I m p l i e d

As f a r as t he PHAj> PHPp PLAj, PLP» RT I f RTSp and BRK

i n s t r u c t i o n s a r e no t concer ned^ t h e i m p l i e d a d d r e s s i n g i s

c o m p l e t e l y t he same as a c c u m u l a t o r a d d r e s i n g * No

e x p l a n a t i o n w i l l be n e c e s s a r y 0 The RTI i n s t r u c t i o n w i l l be

e x p l a i n e d on b e h a l f o f t h e above i n s t r u c t i o n s as f o 1 1ows §

As in t h e case o f t h e JSR i n s t r u c t i o n p t h e RTI

a d d r e s s i n g empl oys t h e s t a c k p o i n t e r o An 8 - b i t b i n a r y one

f o r page one i s f i r s t g e n e r a t e d by t h e p r o c e s s o r and t hen

T 2: IADBUS = X Op Op Op Op Op Op Op 1 \ p S P 5 DL

43

d i r e c t l y t r a n s f e r r e d i n t o INRCOs7 3 o Th i s i s f o r a v o i d i n g a

page c r o s s i n g o The p r e v i o u s c o n t e n t s o f t h e P r e g i s t e r a r e

r e s t o r e d f rom t h e i n c r e m e n t e d s t a c k addr ess i n t h e T3 e y c l e 0

I n t h e T3 and T4 eyclesj> t he s t a c k addr ess i s

i n c r e m e n t e d t w i c e t o ge t back a 1 6 - b i t r e t u r n i n g addresso

The low o r d e r b y t e o f t he r e t u r n i n g address i s f i r s t f e t c h e d

and s t o r e d i n .01 d u r i n g t h e T4 e y e I e D Du r i n g t h e T5 c y c l e *

w h i l e t h e h i gh o r d e r b y t e i s b e i n g f e t c h e d * t h e low o r d e r

b y t e i s moved i n t o TR0 F i n a l l y * an e f f e c t i v e addr ess f rom

0L and TR* whi ch is a l s o a r e t u r n i n g a dd r e ss i s p u t on t he

IADBUS t o p e r f o r m t h e n e x t OP CODE f e t c h 0 The above

sequence does not i n c l u d e any o p e r a t i o n s f o r t h e e x e c u t i o n

p a r t d e s c r i b e d above = A c o mp l e t e i n t e r n a l o p e r a t i o n

sequence o f RTI i s a t t a c h e d in Appendi x Co

I n d e x e d I n d i r e c t

I nde x e d i n d i r e c t a d d r e s s i n g r e q u i r e s t h e T2 c y c l e

t h r o u g h t he T5 e y c l e 0 1 Fi r s t * p l a c i n g a base a d d r e ss low

S t o r e d in DL and e i g h t 0 ° s f o r page z e r o on t h e IADBUS* t h e

p r o c e s s o r f e t c h e s d a t a d u r i n g t he T2 c y c l e o I n t h e same

c y c l e * t he i n c r e me n t e d addr ess in INR is t r a n s f e r r e d i n t o PC

t o be saved u n t i l i t i s needed o Then* INREO s73 s t o r e s o n l y

t h e h i g h o r d e r b y t e on t he IADBUS t h a t i n d i c a t e s page z e r o o

The low o r d e r b y t e on t he IADBUS f r om DL i s p l a c e d on t h e

ARGBUS f o r an a r i t h m e t i c o p e r a t i on o S i nc e t he add r e ss on

t h e IADBUS is not a d d r e s s i n g an e x p e c t e d memory space in

page z e r o ? t h e d a t a f e t c h e d is s i m p l y d i sca r de d , ,

To ge t t h e e x p e c t e d a d d r e s s , t he 8 - b i t da t a on t he

ARGBUS and t he c o n t e n t s o f t h e X r e g i s t e r must be added in

t h e ALUo Four a r gument s a r e r e q u i r e d in t he ALU„ Argument

12 s IADBUS “ 0 , 0 , 0 , 0 , 0 , 0 , 0 \ , D L a DL

45

e f f e c t i v e addr ess i s f e t c h e d f rom t he add r e ssed l o c a t i o n and

t h e n s t o r e d in D i o The addr ess on t h e I A D B U S i s i n c r e me n t e d

i n XNCp ' b u t . o n l y t h e low o r de r b y t e i s t r a n s f e r r e d i n t o

I N R C 8 s i 5 ] o The above o p e r a t i o n is d e v i s e d t o a v o i d not on l y

a page c r o s s i n g but a l s o t he n e c e s s i t y o f an 8 - b i t

i n c r e m e n t e r / d e c r e m e n t e r u n i t on t h e A H P l d e s c r i p t i o n

d e v e l o p e d in t he n e x t c h a p t e r o The h i gh o r d e r b y t e o f t h e e f f

a c t i v e add r e ss is f e t c h e d f rom t h e c o n s e c u t i v e l o c a t i o n

a d d r e s s e d by INR in t he T4 eye Ie 0 The low o r d e r b y t e must

be t r a n s f e r r e d i n t o TR b e f o r e d e l e t e d , , F i n a l l y * DL and TR

p l a c e t h e e f f e c t i v e addr ess on t he I ADBUS d u r i n g t h e T5

c y c l e ° The address of t h e l o c a t i o n where t h e n e x t OP CODE

f e t c h w i l l be p e r f o r me d comes f r om PC k e e p i n g t h e addr ess

s i n c e t h e T2 c y c l e o

I n d i r e c t I n d e x e d

I n d i r e c t i n d e x e d a d d r e s s i n g uses t h e Y r e g i s t e r as

an argument of t he ALU f o r addr ess c a l c u l a t i o n , ) The

c a t e n a t i o n of e i g h t 0 ? s and an i n d i r e c t a d d r e s s low i n DL

pr oduces a 1 6 - b i t i n d i r e c t addr ess i n page z e r o to be used

t o f e t c h a base addr ess low d u r i n g t he T2 eye I e

46

Wh i l e a base addr e ss -h igh i s b e i n g f e t c h e d f rom t he

l o c a t i o n s p e c i f i e d by t he i n c r e m e n t e d i n d i r e c t addr ess

s t o r e d i n I NR* t he base addr ess flow i n DL must be added t o

J 2 i I ADBUS = \ { b O j > 0 » O j > 0 4 0 j > b # 6 \ j > D L ? DL

47

F i r s t s t he iow o r d e r b y t e on t he IADBUS f rom ALUREG

must be t r a n s f e r r e d i n t o I NRC8S153 f o r . t he n e x t c y e i e o The

d a t a f e t c h e d i n t h i s c y c l e i s d i s c a r d e d because o f an

i n c o r r e c t a d d r e s s . Then* f o r a n o t h e r a r i t h m e t i c o p e r a t i o n s

t h e base a dd r e ss h i gh c u r r e n t l y s t o r e d in DL i s p u t on t h e

IDBUS and an 8 - b i t b i n a r y one is p l a c e d on t he ARGBUSo By

t h e s e o p e r a t i o n s s t h e base a dd r e ss h i gh can be i n c r e me n t e d

by 1 in o r d e r to r e f l e c t a page c r o s s i n g . I n t h e n e x t

c y c l e s ALUREG and I NRC8S15] pr oduce a c o r r e c t address t o

s p e c i f y a l o c a t i o n where r e a d and w r i t e o p e r a t i o n s can be

done. The PC as an a d d r e s s i n g r e g i s t e r p l a c e s t h e addr ess

f o r t h e ne x t OP CODE f e t c h on t h e IADBUS d u r i n g t he T0C0VLP3

c y c l e .

Zer o Page I nde x e d

Zer o page i ndexed a d d r e s s i n g empl oys t h e ALU f o r

e f f e c t i v e addr e ss c a l c u l a t i o n . The above i s t h e o n l y

d i f f e r e n c e f r om z e r o page a d d r e s s i n g . Du r i n g t h e T2 cyc l e s

T2s IADBUS = XOsOsOsOs OsOsOsOVsDL l DL < * DBUS5SYNC = \OX 5 RW = X1XJ I N R £ 0 S 7 3

48

t h e p r o c e s s o r p l a c e s an 8 - b l t b i n a r y ze r o and t h e base

a dd r e ss low k e p t I n DL on t h e IADBUS to f e t c h d a t a »

However ; t h e d a t a l a t c h e d I n t o DL I s c o m p l e t e l y v o i d because

t he a dd r e ss I s port i t i n g t o a f a l s e I o c a t i on o The p r i ma r y

aim o f t h i s c y c l e i s t o e x e c u t e t h e e f f e c t i v e addr ess

c a l c u l a t i o n I n t he ALUo Thus; t h e low o r d e r b y t e on t he

IADBUS f rom DL is c o n n e c t e d to a r gument 1 t h r o u g h t h e

ARGBUS; and t he c o n t e n t s o f e i t h e r t h e X r e g i s t e r or Y

r e g i s t e r a r e s e n t t o ar gument 2 over t he XDBUSo

The r e s u l t a n t 8 - b i t d a t a f rom t h e ALU os s t o r e d in

ALUREGo I n t h e same c y c l e ; onl y t he XADBUSEOs 73 i nd i c a t i n g

page z e r o i s t r a n s f e r r e d i n t o XNRCOs73 The add r e ss f o r t h e

n e x t OP CODE f e t c h in XNR is saved in PC u n t i l I i t i s

n e c e s s a r y o I n t he n e x t c y c l e ; XNR CO $73 and ALUREG f orm an

e f f e c t i v e a d d r e s s . The above sequence i n c l u d e s two

a d d i t i o n a l c y c l e s f o r t he i n s t r u c t i o n s t h a t p e r f o r m a l o g i c

o p e r a t i o n . The e f f e c t i v e addr ess d u r i n g t he T 4 and T5

c y c l e s i s p r o v i d e d by XNR in whi ch t h e e f f e c t i v e addr ess on

t h e XADBUS was s t o r e d d u r i n g t h e T3 c y c l e . No page c r o s s i n g

i s a l l o w e d ; as in z e r o page a d d r e s s i n g .

I n d i r e c t

Onl y a c e r t a i n OP CODE can f o r c e t h e p r o c e s s o r t o

p e r f o r m i n d i r e c t a d d r e s s i n g . The i n s t r u c t i o n u s i n g t he

i n d i r e c t a d d r e s s i n g mode c o n t a i n s an OP CODE; an i n d i r e c t

addr ess low; and an