lec12 logical instructions

8/2/2019 Lec12 Logical Instructions

1/29


2/29

AND

AND Destination, Source Logical ANDs each bit in the source with the

corresponding bit in the destination (logical

CF and OF both become zero

PF, SF and ZF affected

AF undefined

AND uses any mode except memory to memory andsegment register addressing

3-2


3/29

AND AX,BX

array0110SI 0020DS:[0130]

AND array[SI],AL

AND AX,[DI]

3-3


4/29

AND clears bits of a binary number(masking)

g:xxxx xxxx (Unknown Number)

0000 1111(Mask)

0000 xxxx (Result)

3-4


5/29

An ASCII number can be converted to BCD byusing AND to mask off the leftmost four binarybit positions

Eg:

MOV BX,3236

AND BX,0F0F

3-5


6/29

TEST TEST operand1,operand2

Only Flags are changed

Similar to CMP instruction

(tests single bit unlike CMP)TEST BL,1b

TEST BL,2b

3-6


7/29

OR

OR Destination, Source ORs each bit in the source with the corresponding bit in the

destination (Logical Addition)


PF, SF and ZF affected AF undefined

AND uses any mode except memory to memory and segmentregister addressing

3-7


8/29

MASKING

Eg:

xxxx xxxx (Unknown Number)

0000 1111(Mask)XXXX 1111(Result)

3-8


9/29

XOR

XOR Destination, Source XORs each bit in the source with the corresponding bit inthe destination


PF, SF and ZF affected AF undefined

AND uses any mode except memory to memory and segmentregister addressing

3-9


10/29

MASKINGEg:

xxxx xxxx (Unknown Number)

as

MOV CX,EF45H

XOR CX,1000H ; invert bit 12

XOR CX,CX

3-10


11/29

NOT

NOT Destination Complements each bit in the destination and stores theresult back into the destination

No Flags Affected

NOT AL

NOT BYTEPTR[SI]

3-11


12/29

LEA (Data Movement Instruction)

Loads a 16 -bit register with the offset address of thedata specified by the operand.

.DATA

.CODE

LEA AX,LIST


13/29

Why is LEA instruction when there is OFFSET ?

LEA BX,LIST

MOV BX,OFFSET LIST

The MOV BX,OFFSET LIST instruction is actuallyassembled as a MOVE immediate instruction and is moreefficient.

Microprocessor takes longer to execute the LEA BX,LISTinstruction than the MOV BX,OFFSET LIST


14/29

LEA AX, [DI] LEA AX,LIST[DI]

MOV DI,3412 ; DS:3412 =1234

LEA BX,[DI]

3-14


15/29

Find the sum of elements in an array and store it inmemory location SUM

.model TINY

.dataARRAY DB 00H

BACK:MOV AL,[BX+DI]ADD SUM,ALINC DIDEC CL

3-15

, , , , ,

SUM DB 0.CODE.STARTUPMOV CL,6LEA BX,ARRAYXOR DI,DI

JNZ BACK.EXITEND


16/29


17/29

String: A series of data words (or bytes) thatreside in consecutive memory locations

, ,

LODS,STOS,MOVS,INS,OUTS

3-17


18/29

Direction Flag selects auto-increment or auto-decrement operation for SI and DI during stringinstructions

CLD Clear the D Flag (Auto-increment mode)

STD - Sets the D Flag(Auto-decrement mode)

3-18


19/29

MOVS/MOVSB/MOVSW Transfers a byte, word or double word from data segment

addressed by SI to ExtraSegment location addressed by DI

--pointers incremented as dictated by thedirection flag

The offset of source byte or word in the datasegment must be in SI Register

The offset of destination byte or word in theextra segment must be in DI Register

No Flags Affected3-19


20/29

MOVSB transfers byte from data segment toextra segment

extra segment

3-20


21/29

a) if DF = 0 (CLD)ES:[DI] DS:[SI]

DI DI + 1

CLDMOVSB

+

(b) if DF = 1(STD)

ES:[DI] DS:[SI]

DI DI - 1

SI SI - 1

3-21

STDMOVSB


22/29

a) if DF = 0 (CLD)ES:[DI] DS:[SI]

DI DI + 2

CLDMOVSW

+

(b) if DF = 1(STD)

ES:[DI] DS:[SI]

DI DI - 2

SI SI - 2

3-22

STDMOVSB


23/29

For multiple-byte or multiple-word MOVes, the countto be in CX register

CLD

3-23

REP MOVSBrep causes CX to dec by 1 no flags affectedMOVSB causes SI and DI to inc by 1 no flagsaffected


24/29

.MODEL TINY

.DATA

COUNT DW 04H

SRC_STR DB BITS

DST_STR DB 4 DUP(?)

.CODE

Copy contents of DST_STR to SRC_STR

.

LEA SI,SRC_STR

LEA DI,DST_STR

MOV CX,COUNT

CLD REP MOVSB

3-24

X1: MOV BL,[SI]

MOV [DI],BLINC SIINC DIDEC CXJNZ X1


25/29

The source operand (SI) located in the datasegment may be overridden to another segment

3-25

located in the extra segment

Only instruction which allows memory to memoryoperation


26/29

MOVS byte1,byte2

,

3-26


27/29

LODS

Loads AL or AX with the data stored at the data segmentOffset address indexed by SI register

3-27

g , = =LODS affects no FLAGs

LODSB,LODSW


28/29

LODSW ;AX=DS:[SI], SI=SI+2 LODSB ;AL=DS:[SI],SI=SI+1

MOV SI ,offset MPI LODS MPI ; AL=DS:[SI];

;SI=SI+1 ;if MPI is byte 3-28


29/29

SI = SI 1

LODSW ; AX = DS:[SI];

SI = SI 2

3-29

lec12 logical instructions

Documents