program linking

CPE23 KUProgram Linking

System SoftwareEmergency

Resolution Lecture #1

CPE23 KU

Why This Presentation?• Due to unforeseen circumstances,

the entire course was abruptly terminated.

• Class terminated means no lectures. No lectures means no materials for exam, which, unfortunately, still exists.

• This file fills the gap by adding material to course, even without lecture.

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Agenda• Recap of our past: The SIC/XE

Civilization• Program Linking Procedures• Solution #1• Solution #2

• Linking Loader• Principles

• Overview of Post-Assembly-Time

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The SIC/XE Civilization [BECK]

• Allows “library sharing” at runtime• Very important, but we can’t talk right

now…

Dynamic

Linking • Lets us create “.exe” for faster

loading• Allows us to NOT program linker in

SIC or SIC/XE

Linkage

Editor• Enables Linking supportLinking

Loader• Allows “fully multiple-file”

programmingControl Section

• M-Record• Relocation Bit Mask

Relocating Loader (SIC or SIC/XE)

• BSL, for exampleAbsolute Loader

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What is Program Linking?OBJOBJOBJ

Linker

LoadModu

le

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Reasons for Program Linking (1)MAIN (symbols)

Address (in Hex)

START 000000dump 00007Emem 0000BE

MATH (symbols)

Address (in Hex)

sin 00001Ecos 00003Etan 00005Easin 00007Eacos 00009Eatan 0000BE… …

Can we differentiate• MAIN.dump and

MATH.asin, or• MAIN.mem and

MATH.atanby byte number?

How can we jump from an instruction in MAIN to, say, MATH.sin, when they “share” the same “address numbers”?

You can’t answer? Good. That’s what’s supposed to happen.

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Reasons for Program Linking (2)

• So why not just “include”?• Answer: It will work, but

assembly will be slowed down. Libraries like “math.h” are usually pre-compiled (.o, .lib, .pyc, …), very large, and has only some headers to help you “reach” the functions inside. Don’t be fooled by their appearances.

MAIN

MATH

#include MATH

NOTE: In real-world, “headers” usually contain only declarations. So there can already be math.h and math.dll separately. This is actually linking, but still gives illusion of “including.”

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Types of Linking• Linking Loader • Link, Load later

(Linkage Editor)

Linking Loader

Addr Value01000 001003 001006 001009 0

OBJ

OBJ

OBJ

Linker

OBJ

OBJ

OBJ

Load_Module.exe*

Loader* Or “755”, you geek.

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Understanding Program Offsets

• Files/CSs are joined end-to-head, like this diagram.

• Each file/CS has its own Length.

• First part starts at zero, or S1=0

• Subsequent parts start at {previous part’s start} + {previous part’s length}, or Sn=Sn-1+Ln-1 for positive int n

PROGA

PROGB

PROGC

SPROGA=0

LPROGA

SPROGB=SPROG

A+LPROGA

LPROGB

SPROGC=SPROG

B+LPROGB

LPROGC

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Linking Method #1: “Naïve” Linking [CCD1]• Simple• Easy to implement• Use 2-pass style

• Fast (to run: O(2n), n: code lines)• Drawbacks:• Produces lots of M records• May or may not work with complex

expressions

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Method #1: Pass 1/2• Scan H and D records only• Keep results in ESTAB

R,LISTB ,ENDB ,LISTC ,ENDC

E,000020

H,PROGA ,000000,000063

D,LISTA ,000040,ENDA ,000054

... T RECORDS ...

... M RECORDS ...

R,LISTA ,ENDA ,LISTC ,ENDC

E

H,PROGB ,000000,00007F

D,LISTB ,000060,ENDB ,000070

... T RECORDS ...

... M RECORDS ...

R,LISTA ,ENDA ,LISTB ,ENDB

E

H,PROGC ,000000,000051

D,LISTC ,000030,ENDC ,000042

... T RECORDS ...

... M RECORDS ...

Current Offset: 0 63 E2

ESTAB SymbolAddress LengthPROGA 00000 00063LISTA 00040ENDA 00054PROGB 00063 0007FLISTB 000C3ENDB 000D3PROGC 000E2 00051LISTC 00112ENDC 00124

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Method #1: Pass 2/2• Time to modify our

code.• Unify H&E• Delete D&R• No ext. symbols means

no D/R• Shift T• Use ESTAB for length

detect• Substitute into & Shift M• Use ESTAB

R,LISTB ,ENDB ,LISTC ,ENDC

E,000020

H,PROGA ,000000,000063

D,LISTA ,000040,ENDA ,000054

... T RECORDS ...

... M RECORDS ...

R,LISTA ,ENDA ,LISTC ,ENDC

E

H,PROGB ,000000,00007F

D,LISTB ,000060,ENDB ,000070

... T RECORDS ...

... M RECORDS ...

R,LISTA ,ENDA ,LISTB ,ENDB

E

H,PROGC ,000000,000051

D,LISTC ,000030,ENDC ,000042

... T RECORDS ...

... M RECORDS ...

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Method #1: Pass 2/2: Unify H & E

• H• Sum length,

put at top• Program start

= 0• Absolute

programs are usually not linkable…

• E• Keep execution

start address of Main CS

• Put at end of program

H,PROGA ,000000,000063

H,PROGB ,000000,00007F

H,PROGC ,000000,000051

E,000020

E

E

H,LDMODL,000000,000133

E,000020

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Method #1: Pass 2/2: Shift T

• To make program codes co-exist peacefully, we need to shift addresses.


T,000020,0A,...

+0T,000020,0A,...

CS PROGA: Offset = 0

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Method #1: Pass 2/2: Shift T (2)



T,000036,0B,...

+63

CS PROGB: Offset = 0x63

T,000099,0B,...

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Method #1: Pass 2/2: Shift T (3)



T,000018,0C,...

+E2

CS PROGC: Offset = 0x63+0x7F

T,0000FA,0C,...

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Method #1: Pass 2/2: Change M

• Just use ESTAB. (No expressions allowed for now)

• Don’t forget to add offset.M,000024,05,+LISTBM,000054,06,+LISTCM,000057,06,+ENDC

CS PROGA: Offset = 0

M,000024,05,+0000C3M,000054,06,+000112M,000057,06,+000124

+0 Substitute


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Method #1: Pass 2/2: Change M (2)

• Don’t forget to add offset.M,000037,05,+LISTA

M,00003E,05,+ENDAM,00003E,05,-LISTA

CS PROGB: Offset = 0x63

M,00009A,05,+000040M,0000A1,05,+000054M,0000A1,05,-000040

+63 Substitute


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Method #1: Pass 2/2: Change M (3)

• Offsets add up!M,000019,05,+LISTAM,00001D,05,+LISTBM,000021,05,+ENDA

CS PROGC: Offset = 0x63+0x7F

M,0000FB,05,+000040M,0000FF,05,+0000C3M,000103,05,+000054

+E2 Substitute


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Method #1: Ready for Loading

• We put the H and E we processed earlier…

• And the shifted T records…

• Then the M records…

• We call this a load module.

H,LDMODL,000000,000133

E,000020

M,000024,05,+0000C3M,000054,06,+000112M,000057,06,+000124

M,00009A,05,+000040M,0000A1,05,+000054M,0000A1,05,-000040

M,0000FB,05,+000040M,0000FF,05,+0000C3M,000103,05,+000054

T,000020,0A,...

T,000099,0B,...

T,0000FA,0C,...

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Method #1: Notes• Just one note: In pass-2, you just run

it through and use string processing to determine line types and act automatically. You don’t and shouldn’t need to separate the code lines into types like I demonstrated.

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Method #2: Introduction [CCD2]• Likeness with Method #1• 2-pass• Utilizes ESTAB

• Difference against Method #1• More compact list of M records, which means faster Loading• Unified M record offset: No need to add/subtract by symbol

addresses• However, you may need to implement Expression (-A+B+C-

D+E…)• Creates M-Table, so this will use more memory in

implementation• And uses more time: O(2n+ms) (m: M-table size, s: # of

Symbols)

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Method #2: Pass 1: M-Table

• While reading in Pass 1, we scan ALL M records to make M-table.

• M-table should “summarize” the expression. Cut those loose ends.

• For better illustration, we will add one more M-record per CS.

M,000024,05,+LISTBM,000054,06,+LISTCM,000057,06,+ENDCM,000057,06,-LISTCM,000037,05,+LISTAM,00003E,05,+ENDAM,00003E,05,-LISTAM,000070,06,+ENDAM,000019,05,+LISTAM,00001D,05,+LISTBM,000021,05,+ENDAM,000021,05,-LISTA

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Method #2: Pass 1: Make the M-Table

• Make M-table!LocationLengthExpression

00024 05 +LISTB00054 06 +LISTC00057 06 +ENDC-LISTC0009A 05 +LISTA000A1 05 +ENDA-LISTA000D3 06 +ENDA000FB 05 +LISTA000FF 05 +LISTB00103 05 +ENDA-LISTA

M,000024,05,+LISTBM,000054,06,+LISTCM,000057,06,+ENDCM,000057,06,-LISTCM,000037,05,+LISTAM,00003E,05,+ENDAM,00003E,05,-LISTAM,000070,06,+ENDAM,000019,05,+LISTAM,00001D,05,+LISTBM,000021,05,+ENDAM,000021,05,-LISTA

Start = 0Length = 0x63Start = 0x63Length = 0x7F

Start = 0xE2Length = 0x51

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Method #2: Pass 1.5: Evaluate M-Table

Eval. ResultType (Abs/Rel)000C3 R000112 R000012 A00040 R00014 A000054 R00040 R000C3 R00014 A

LocationLengthExpression00024 05 +LISTB00054 06 +LISTC00057 06 +ENDC-LISTC0009A 05 +LISTA000A1 05 +ENDA-LISTA000D3 06 +ENDA000FB 05 +LISTA000FF 05 +LISTB00103 05 +ENDA-LISTA

ESTAB SymbolAddressPROGA 00000LISTA 00040ENDA 00054PROGB 00063LISTB 000C3ENDB 000D3PROGC 000E2LISTC 00112ENDC 00124

ESTAB Old M-Table

TIP: If (Number of +’s == Number of –’s) then Absolute, else Relative.

Result

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Method #2: Pass 2• Same thing as Method #1• Process H and E records normally

• However, …• You must add the M values right into the

code• Since M records are already processed,

we can just dump only Relative rows in.

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Method #2: Pass 2: Edit T records.

T,000020,0A,03201D 77100004 ...

+000C3T,000020,0A,03201D 771000C7 ...

T,000054,0F,000014 ...

+000112T,000054,0F,000126 ...

etc.

LocationLength00024 0500054 0600057 060009A 05000A1 05000D3 06000FB 05000FF 0500103 05

Eval. ResultType000C3 R000112 R000012 A00040 R00014 A000054 R00040 R000C3 R00014 A

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Method #2: Dump the M Records

• For R types only:• Dump “empty” M record

to tell Loader to relocate.• Keep only Location and

Length.

LocationLength00024 0500054 0600057 060009A 05000A1 05000D3 06000FB 05000FF 0500103 05

Eval. ResultType000C3 R000112 R000012 A00040 R00014 A000054 R00040 R000C3 R00014 A

M,000024,05M,000054,06

M,00009A,05M,0000D3,06

M,0000FB,05M,0000FF,05

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Method #2: Finished• Just dump the

modified T records.

• Add the M lines

• The rest is no different.

T ...

T ...

M,000024,05M,000054,06M ...

M ...

M ...

H,LDMODL,000000,000133

E,000020

T,000020,0A,03201D 771000C7...T,000054,0F,000126 ...T ...

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Further Reading• [BECK] L.L. Beck. "Loaders and Linkers" in System

Software: An Introduction to Systems Programming, 3rd ed. Addison-Wesley, 1997, ch 3. (ISBN: 0201423006)

• [CCD1] C. Chatdokmaiprai. (2004, August 25). Load Module & Linker (Solution #1) [Online]. Available: http://cpe.ku.ac.th/~ccd/204331/lmdfmt1.pdf

• [CCD2] ____________. (2004, August 25). Load Module & Linker (Solution #2) [Online]. Available: http://cpe.ku.ac.th/~ccd/204331/lmdfmt2.pdf

• [CSSH] C.S. Shieh. (?). Linking Loader for SIC/XE Machine [Online]. Available: http://bit.kuas.edu.tw/~csshieh/teach/93A/sp/note/sp07.ppt

http://cpe.ku.ac.th/~ccd/204331/lmdfmt1.pdf




http://bit.kuas.edu.tw/~csshieh/teach/93A/sp/note/sp07.ppt



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