14557_passes of an assembler

8/3/2019 14557_passes of an Assembler

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044264 Software Systems Fall 2006/7 Lect. 8 1

Lecture 8 - Assembler passes,Linking and Loading

2044264 Software Systems Fall 2006/7 Lect. 8

Course Flow

Lecture 1: Intro Lecture 2: Representing code and data Lecture 3: Addressing modes Lecture 4: Procedures Lecture 5: Data Structures Lecture 6: More Procedures & Structures Lecture 7: Interrupts Lecture 8: Assembling, Linking and Loading Lecture 9: Virtual Memory

Lecture 10: File Systems


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Steps in program developmentSource

program

Compiler orassembler

Link the object

modules

Errors?

Errors?

Run executable

Correct the

source program

no

no

CurrentFocus


How does a program becomeexecutable?

High-level program (C, Pascal, C++)

Object code

[.c, .pas, .cpp]

Assembly program [.asm]

[.obj]

Compiler

Assembler

Compiler

Executable program [.exe]

Linker [.lib]Library

Application (algorithm)

Programming

Loader

[.map]Map


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SW program life cycle#include #include #define PRIMES 100long primes=0,value,divisor,prime[PRIMES];void main(void){printf("\nPrime Numbers:\n");for (value=2; primes


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Assembler pass objectives

Pass one: build a symbol table containing thevalues of all labels and symbols defined in theprogram.

Pass two: Use the symbol table to translatethe program


Assembler pass one

Begin pass 1 Init LC=0 Read next line from

source program

Analyze line for

syntaxErrors?

Determine

statement type

End

statement?Label?

Symbol

definition?

Storage

allocation?Instruction?

Comment?Assembler

Directive?

Count errors

yes

no

Pass 2

yes

nonono

no

no

Label

value=LC

yes

Compute

symbol value

specification

yes

Store name and value

in symbol table

Compute

size of

storage

yes

no

Compute

size of

instruction

yes

Increment LC by the

computed size

yes

Perform directive function

yes

no Invalid statement

What is LC?


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Assembler pass two

Begin pass 2 Init LC=0 Read next line from

source and insert in

proper field oflisting line

Determine statement

typeLabel or

symbol?

End

statement?

Assembler

Directive?

Storage

Allocation?Instruction?

Any more

Characters?Write to listing

file

yes

yes

Exit

yes

nonono

Perform directive

function

yes

Translate opcode

to binary

no

yes

no

yes

Output data to

object file

Invalid statement

Evaluate operand

specifiers and

convert address

to binary

Write to object

file

Write binary to

listing fileUpdate LC by

instruction size

Increment LC by

the size of data

no


The job of the linker

Resolve symbol references between

different object files. Creates an executable file out of the

object modules..asm .asm .asm .asm

assembler

.obj .obj .obj .obj

linker

.exe


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The job of the loader

Allocates space in memory for theprogram.

Determines the start address of theprogram.

Has to update all absolute addressing

modes respectively before running theprogram.


Information required for thelinker/loader

The assembler needs to provide the followinginformation: The size of each object module. A list of all the symbols defined in a given module

that can be used by other modules. A list of all symbols used by the given module that

are defined in other modules. A list of all the places in the code that need to be

fixed when the program is being linked/loaded. A machine code

May also provide debugging info: Line#s, etc.


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3 New Assembler Directives

The new Assembler Directives are notsupported by sim: .cset defines the start address of a module.

Relative to this address all other addresses withinthe module will be calculated.

.extern specifies using a symbol defined inanother module.

.entry allows other modules to use a local symbol


Tables generated by theassembler

External Symbol Table EST

For resolving linkage Relocation and Linkage Table RLT

For address fixes


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External symbol table

Each entry consists of 3 fields: Symbol symbol name.

Type 3 symbol types: LD local definition (symbols which appear with .entry)

ER external symbols (symbols which appear with.extern).

SD Symbols which appear with .cset.

Relative location the distance of the symbolfrom .cset. Always 0 for SD symbols.


Relocation and linkage table

A list of all the relative addresses that needto be fixed (position dependent) at link orload time.

Each entry consists of 3 fields: Relative location the distance of the address

that has to be fixed relative to .cset . Flag indication whether we need to add/subtract

from address. Symbol the value that needs to be added to /

subtracted from the address.


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An example:

.cset A

.extern proced

.entry arg

Main: Mov $Main, ..

Calls $1, proced

Arg: .long 0

5

20

40

EST

Symbol Type Rel. Loc

A SD 0

proced ER -

Arg LD 40

RLT

Symbol Flag Rel. Loc

A + 5

A - 20proced + 20

Module A


An example:

.cset B

.extern arg

.entry proced

proced: Arg,

Movb d(r0), r2

d: .long 0

6

16

75

EST

Symbol Type Rel. LocB SD 0

Arg ER -

proced LD 0

RLT


B - 6

Arg + 6

B + 16

Module B


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An example:

EST


B SD 0 50

Arg ER -

proced LD 0 50

RLT


B - 6 56

Arg + 6 56

B + 16 66

Memory map table

A

B

old

0

43

0

78

new

0

43

50

128

Unified ESTEST


A SD 0

proced ER -

Arg LD 40

EST


A SD 0

Arg LD 40

B SD 50

proced LD 50

Unified RLTRLT

Symbol Flag Rel. LocA + 5

A - 20

proced + 20

RLT


A + 5

A - 20

proced + 20

B - 56

Arg + 56

B + 66


The linking process

Link modules A, B and C: Linker prepare memory map according to:

Order of linking (provided by the user). The length of each module (provided by the assembler).

Unify the external symbol table from the differentobjects: Compute all SDs

Check that each ER has a corresponding LD (otherwise this is anerror).

Delete all ERs Update all relative location fields (if necessary).

A

B

C


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The linking process

Unifying relocation and linkage tables: Fix the relative location field in table according to theunified external symbol table.

Fixing in the code: All places with 2 entries in the RLT are fixed accordingly and the

entries are deleted from the RLT (relative reference).

All places with 1 entry in the RLT are fixed in the code according to theRLT and are remained in the RLT.

The fixed code and the new tables are passed to the

loader. Loading : fix the tables according to the start

address where the program will be loaded in memory.


Position dependent symbols

A position dependent symbol appears whenthere is : An absolute reference to a symbol in either

displacement (symbol(rn)), displacement deferred (*symbol(rn)), literal ($symbol), immediate ($symbol) or absolute (*$symbol) addressing modes.

A relative reference to a symbol in either

Relative (symbol). Relative deferred (*symbol).

since the relative PC is used as the base register


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Relocation and linkage tableconstruction rules

Operand has a label

Label is defined in current .csetLabel is defined in current .extern

RLT:LC, +, current cset

Opcode:Value of symbol

RLT:none

Opcode:Relative distance to

address

RLT:LC, +, name ofexternal symbol

Opcode:

Value of symbol istemporarily 0.

RLT:LC, -, current csetLC,+, name ofexternal symbol

Opcode:Current relativedistance.

AbsoluteRelativeAbsoluteRelative


Relocation and linkage tableconstruction rules

Operand is an explicit value

RLT:None.

Opcode:Value of operand.

E.g.Movl $4, R4

RLT:LC, -, current cset

Opcode:Relative distance totarget addressassuming startaddress is 0

E.g.Movl 40, R10

AbsoluteRelative


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Related issues

Symbolic debugging info Object info (symbol overload allowed)

14557_passes of an assembler

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