Lec 4 Systems Architecture 1

Systems Architecture

Lecture 4: Compilers, Assemblers, Linkers & Loaders

Jeremy R. Johnson

Anatole D. Ruslanov

William M. Mongan

Some material drawn from CMU CSAPP Slides: Kesden and Puschel

Lec 4 Systems Architecture 2

Introduction

• Objective: To introduce the role of compilers, assemblers, linkers and loaders. To see what is underneath a C program: assembly language, machine language, and executable.

Lec 4 Systems Architecture 3

Compilation Process

Objectfile

Sourcefile Assembler

LinkerAssembler

AssemblerProgramlibrary

Objectfile

Objectfile

Sourcefile

Sourcefile

Executablefile

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Below Your Program

Example from a Unix system

• Source Files: count.c and main.c• Corresponding assembly code: count.s and main.s• Corresponding machine code (object code): count.o and

main.o• Library functions: libc.a• Executable file: a.out

• format for a.out and object code: ELF (Executable and Linking Format)

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Producing an Executable Program

Example from a Unix system (SGI Challenge running IRIX 6.5)

• Compiler: count.c and main.c count.s and main.s– gcc -S count.c main.c

• Assembler: count.s and main.s count.o and main.o– gcc -c count.s main.s– as count.s -o count.o

• Linker/Loader: count.o main.o libc.a a.out– gcc main.o count.o– ld main.o count.o -lc (additional libraries are required)

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Source Files

void main()

{

int n,s;

printf("Enter upper limit: ");

scanf("%d",&n);

s = count(n);

printf("Sum of i from 1 to %d = %d\n",n,s);

}

int count(int n)

{

int i,s;

s = 0;

for (i=1;i<=n;i++)

s = s + i;

return s;

}

Lec 4 Systems Architecture 7

Assembly Code for MIPS (count.s)

#.file 1 "count.c"

.option pic2

.section .text

.text

.align 2

.globl count

.ent count

count:

.LFB1:

.frame $fp,48,$31 # vars= 16, regs= 2/0, args= 0, extra= 1

6

.mask 0x50000000,-8

.fmask 0x00000000,0

subu $sp,$sp,48

.LCFI0:

sd $fp,40($sp)

Lec 4 Systems Architecture 8

.LCFI1:

sd $28,32($sp)

.LCFI2:

move $fp,$sp

.LCFI3:

.set noat

lui $1,%hi(%neg(%gp_rel(count)))

addiu $1,$1,%lo(%neg(%gp_rel(count)))

daddu $gp,$1,$25

.set at

sw $4,16($fp)

sw $0,24($fp)

li $2,1 # 0x1

sw $2,20($fp)

.L3:

lw $2,20($fp)

lw $3,16($fp)

slt $2,$3,$2

beq $2,$0,.L6

b .L4

L6:

lw $2,24($fp)

lw $3,20($fp)

addu $2,$2,$3

sw $2,24($fp)

.L5:

lw $2,20($fp)

addu $3,$2,1

sw $3,20($fp)

b .L3

.L4:

lw $3,24($fp)

move $2,$3

b .L2

.L2:

move $sp,$fp

ld $fp,40($sp)

ld $28,32($sp)

addu $sp,$sp,48

j $31

.LFE1:

.end count

Lec 4 Systems Architecture 9

Executable Program for MIPS (a.out)0000000 7f45 4c46 0102 0100 0000 0000 0000 0000

0000020 0002 0008 0000 0001 1000 1060 0000 0034

0000040 0000 6c94 2000 0024 0034 0020 0007 0028

0000060 0023 0022 0000 0006 0000 0034 1000 0034

0000100 1000 0034 0000 00e0 0000 00e0 0000 0004

0000120 0000 0004 0000 0003 0000 0114 1000 0114

0000140 1000 0114 0000 0015 0000 0015 0000 0004

0000160 0000 0001 7000 0002 0000 0130 1000 0130

0000200 1000 0130 0000 0080 0000 0080 0000 0004

0000220 0000 0008 7000 0000 0000 01b0 1000 01b0

0000240 1000 01b0 0000 0018 0000 0018 0000 0004

0000260 0000 0004 0000 0002 0000 01c8 1000 01c8

0000300 1000 01c8 0000 0108 0000 0108 0000 0004

0000320 0000 0004 0000 0001 0000 0000 1000 0000

0000340 1000 0000 0000 3000 0000 3000 0000 0005

• • • • • • • • • • • • • • • • • • • • •

Assembly Characteristics: Data Types

• “Integer” data of 1, 2, or 4 bytes– Data values– Addresses (untyped pointers)

• Floating point data of 4, 8, or 10 bytes

• No aggregate types such as arrays or structures– Just contiguously allocated bytes in memory

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Assembly Characteristics: Operations

• Perform arithmetic function on register or memory data

• Transfer data between memory and register– Load data from memory into register– Store register data into memory

• Transfer control– Unconditional jumps to/from procedures– Conditional branches

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Code for sum0x401040 <sum>:

0x550x890xe50x8b0x450x0c0x030x450x080x890xec0x5d0xc3

Object Code• Assembler

– Translates .s into .o– Binary encoding of each instruction– Nearly-complete image of executable code– Missing linkages between code in

different files

• Linker– Resolves references between files– Combines with static run-time libraries

• E.g., code for malloc, printf

– Some libraries are dynamically linked• Linking occurs when program begins

execution

• Total of 13 bytes• Each instruction

1, 2, or 3 bytes• Starts at address 0x401040

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Disassembled00401040 <_sum>: 0: 55 push %ebp 1: 89 e5 mov %esp,%ebp 3: 8b 45 0c mov 0xc(%ebp),%eax 6: 03 45 08 add 0x8(%ebp),%eax 9: 89 ec mov %ebp,%esp b: 5d pop %ebp c: c3 ret d: 8d 76 00 lea 0x0(%esi),%esi

Disassembling Object Code

• Disassemblerobjdump -d p– Useful tool for examining object code– Analyzes bit pattern of series of instructions– Produces approximate rendition of assembly code– Can be run on either a.out (complete executable) or .o file

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Disassembled0x401040 <sum>: push %ebp0x401041 <sum+1>: mov %esp,%ebp0x401043 <sum+3>: mov 0xc(%ebp),%eax0x401046 <sum+6>: add 0x8(%ebp),%eax0x401049 <sum+9>: mov %ebp,%esp0x40104b <sum+11>: pop %ebp0x40104c <sum+12>: ret 0x40104d <sum+13>: lea 0x0(%esi),%esi

Alternate Disassembly

• Within gdb Debuggergdb p

disassemble sum– Disassemble procedure

x/13b sum– Examine the 13 bytes starting at sum

Object0x401040:

0x550x890xe50x8b0x450x0c0x030x450x080x890xec0x5d0xc3

Lec 4 Systems Architecture 14

What Can be Disassembled?

• Anything that can be interpreted as executable code• Disassembler examines bytes and reconstructs assembly

source

% objdump -d WINWORD.EXE

WINWORD.EXE: file format pei-i386

No symbols in "WINWORD.EXE".Disassembly of section .text:

30001000 <.text>:30001000: 55 push %ebp30001001: 8b ec mov %esp,%ebp30001003: 6a ff push $0xffffffff30001005: 68 90 10 00 30 push $0x300010903000100a: 68 91 dc 4c 30 push $0x304cdc91

Lec 4 Systems Architecture 15