CPE23 KU

Program Linking

System Software

Emergency 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.

CPE23 KU Agenda

• Recap of our past: The SIC/XE Civilization

• Program Linking Procedures • Solution #1

• Solution #2

• Linking Loader • Principles

• Overview of Post-Assembly-Time

CPE23 KU 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 support Linking Loader

• Allows “fully multiple-file” programming Control Section

• M-Record • Relocation Bit Mask

Relocating Loader (SIC or SIC/XE)

• BSL, for example Absolute Loader

CPE23 KU What is Program Linking?

OBJ

OBJ

OBJ

Linker Load Module Load

Module

CPE23 KU Reasons for Program Linking (1)

MAIN (symbols) Address (in Hex)

START 000000

dump 00007E

mem 0000BE

MATH (symbols) Address (in Hex)

sin 00001E

cos 00003E

tan 00005E

asin 00007E

acos 00009E

atan 0000BE

… …

Can we differentiate • MAIN.dump and MATH.asin, or • MAIN.mem and MATH.atan by 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.

CPE23 KU 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.”

CPE23 KU Types of Linking

• Linking Loader • Link, Load later (Linkage Editor)

Linking Loader

Addr Value

01000 0

01003 0

01006 0

01009 0

OBJ OBJ OBJ

Linker

OBJ OBJ OBJ

Load_Module.exe* Load_Module.exe*

Loader

* Or “755”, you geek.

CPE23 KU 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=SPROGA+LPROGA

LPROGB SPROGC=SPROGB+LPROGB LPROGC

CPE23 KU 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

CPE23 KU 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 Symbol Address Length

PROGA 00000 00063

LISTA 00040

ENDA 00054

PROGB 00063 0007F

LISTB 000C3

ENDB 000D3

PROGC 000E2 00051

LISTC 00112

ENDC 00124

CPE23 KU 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 ...

CPE23 KU 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

CPE23 KU Method #1: Pass 2/2: Shift T

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

ESTAB Symbol Address Length

PROGA 00000 00063

LISTA 00040

ENDA 00054

PROGB 00063 0007F

LISTB 000C3

ENDB 000D3

PROGC 000E2 00051

LISTC 00112

ENDC 00124

T,000020,0A,...

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

CS PROGA: Offset = 0

CPE23 KU Method #1: Pass 2/2: Shift T (2)

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

ESTAB Symbol Address Length

PROGA 00000 00063

LISTA 00040

ENDA 00054

PROGB 00063 0007F

LISTB 000C3

ENDB 000D3

PROGC 000E2 00051

LISTC 00112

ENDC 00124

T,000036,0B,...

+63

CS PROGB: Offset = 0x63

T,000099,0B,...

CPE23 KU Method #1: Pass 2/2: Shift T (3)

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

ESTAB Symbol Address Length

PROGA 00000 00063

LISTA 00040

ENDA 00054

PROGB 00063 0007F

LISTB 000C3

ENDB 000D3

PROGC 000E2 00051

LISTC 00112

ENDC 00124

T,000018,0C,...

+E2

CS PROGC: Offset = 0x63+0x7F

T,0000FA,0C,...

CPE23 KU Method #1: Pass 2/2: Change M

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

• Don’t forget to add offset. M,000024,05,+LISTB

M,000054,06,+LISTC

M,000057,06,+ENDC

CS PROGA: Offset = 0

M,000024,05,+0000C3

M,000054,06,+000112

M,000057,06,+000124

+0 Substitute

ESTAB Symbol Address Length

PROGA 00000 00063

LISTA 00040

ENDA 00054

PROGB 00063 0007F

LISTB 000C3

ENDB 000D3

PROGC 000E2 00051

LISTC 00112

ENDC 00124

CPE23 KU Method #1: Pass 2/2: Change M (2)

• Don’t forget to add offset.

M,000037,05,+LISTA

M,00003E,05,+ENDA

M,00003E,05,-LISTA

CS PROGB: Offset = 0x63

M,00009A,05,+000040

M,0000A1,05,+000054

M,0000A1,05,-000040

+63 Substitute

ESTAB Symbol Address Length

PROGA 00000 00063

LISTA 00040

ENDA 00054

PROGB 00063 0007F

LISTB 000C3

ENDB 000D3

PROGC 000E2 00051

LISTC 00112

ENDC 00124

CPE23 KU Method #1: Pass 2/2: Change M (3)

• Offsets add up!

M,000019,05,+LISTA

M,00001D,05,+LISTB

M,000021,05,+ENDA

CS PROGC: Offset = 0x63+0x7F

M,0000FB,05,+000040

M,0000FF,05,+0000C3

M,000103,05,+000054

+E2 Substitute

ESTAB Symbol Address Length

PROGA 00000 00063

LISTA 00040

ENDA 00054

PROGB 00063 0007F

LISTB 000C3

ENDB 000D3

PROGC 000E2 00051

LISTC 00112

ENDC 00124

CPE23 KU 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,+0000C3

M,000054,06,+000112

M,000057,06,+000124

M,00009A,05,+000040

M,0000A1,05,+000054

M,0000A1,05,-000040

M,0000FB,05,+000040

M,0000FF,05,+0000C3

M,000103,05,+000054

T,000020,0A,...

T,000099,0B,...

T,0000FA,0C,...

CPE23 KU 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.

CPE23 KU 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)

CPE23 KU 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,+LISTB

M,000054,06,+LISTC

M,000057,06,+ENDC

M,000057,06,-LISTC

M,000037,05,+LISTA

M,00003E,05,+ENDA

M,00003E,05,-LISTA

M,000070,06,+ENDA

M,000019,05,+LISTA

M,00001D,05,+LISTB

M,000021,05,+ENDA

M,000021,05,-LISTA

CPE23 KU Method #2: Pass 1: Make the M-Table

• Make M-table! Location Length Expression

00024 05 +LISTB

00054 06 +LISTC

00057 06 +ENDC - LISTC

0009A 05 +LISTA

000A1 05 +ENDA - LISTA

000D3 06 +ENDA

000FB 05 +LISTA

000FF 05 +LISTB

00103 05 +ENDA - LISTA

M,000024,05,+LISTB

M,000054,06,+LISTC

M,000057,06,+ENDC

M,000057,06,-LISTC

M,000037,05,+LISTA

M,00003E,05,+ENDA

M,00003E,05,-LISTA

M,000070,06,+ENDA

M,000019,05,+LISTA

M,00001D,05,+LISTB

M,000021,05,+ENDA

M,000021,05,-LISTA

Start = 0 Length = 0x63

Start = 0x63 Length = 0x7F

Start = 0xE2 Length = 0x51

CPE23 KU Method #2: Pass 1.5: Evaluate M-Table

Eval . Result Type (Abs/ Rel )

000C3 R

000112 R

000012 A

00040 R

00014 A

000054 R

00040 R

000C3 R

00014 A

Location Length Expression

00024 05 +LISTB

00054 06 +LISTC

00057 06 +ENDC - LISTC

0009A 05 +LISTA

000A1 05 +ENDA - LISTA

000D3 06 +ENDA

000FB 05 +LISTA

000FF 05 +LISTB

00103 05 +ENDA - LISTA

ESTAB Symbol Address

PROGA 00000

LISTA 00040

ENDA 00054

PROGB 00063

LISTB 000C3

ENDB 000D3

PROGC 000E2

LISTC 00112

ENDC 00124

ESTAB Old M-Table

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

Result

CPE23 KU 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.

CPE23 KU Method #2: Pass 2: Edit T records.

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

+000C3 T,000020,0A,03201D 771000C7 ...

T,000054,0F,000014 ...

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

etc.

Location Length

00024 05

00054 06

00057 06

0009A 05

000A1 05

000D3 06

000FB 05

000FF 05

00103 05

Eval . Result Type

000C3 R

000112 R

000012 A

00040 R

00014 A

000054 R

00040 R

000C3 R

00014 A

CPE23 KU Method #2: Dump the M Records

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

Loader to relocate.

• Keep only Location and Length.

Location Length

00024 05

00054 06

00057 06

0009A 05

000A1 05

000D3 06

000FB 05

000FF 05

00103 05

Eval . Result Type

000C3 R

000112 R

000012 A

00040 R

00014 A

000054 R

00040 R

000C3 R

00014 A

M,000024,05

M,000054,06

M,00009A,05

M,0000D3,06

M,0000FB,05

M,0000FF,05

CPE23 KU Method #2: Finished

• Just dump the modified T records.

• Add the M lines

• The rest is no different. T ...

T ...

M,000024,05

M,000054,06

M ...

M ...

M ...

H,LDMODL,000000,000133

E,000020

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

CPE23 KU 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