1. Jeremy R. Johnson Anatole D. Ruslanov William M. Mongan
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
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Systems Architecture

2. September 4, 1997
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.
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Systems Architecture

3. September 4, 1997
Compilation Process
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Systems Architecture

4. September 4, 1997
Compilation Process
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5. Below Your Program Example from a Unix system
September 4, 1997
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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Systems Architecture

6. Producing an Executable Program
September 4, 1997
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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Systems Architecture

7. Source Files void main() { int n,s; printf("Enter upper limit: ");
September 4, 1997
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; }
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Systems Architecture

8. Assembly Code for MIPS (count.s)
September 4, 1997
Assembly Code for MIPS (count.s)
#.file 1 "count.c"
.option pic2
.section text
.text
.align 2
.globl count
.ent count
count:
.LFB1:
.frame $fp,48,$ # vars= 16, regs= 2/0, args= 0, extra= 1 6
.mask 0x ,-8
.fmask 0x ,0
subu $sp,$sp,48
.LCFI0:
sd $fp,40($sp)
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Systems Architecture

9. .LCFI1: lw $3,20($fp) sd $28,32($sp) .LCFI2: move $fp,$sp .LCFI3:
September 4, 1997
.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, # 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
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Systems Architecture

10. Executable Program for MIPS (a.out)
September 4, 1997
Executable Program for MIPS (a.out)
f45 4c c
e e
b b0 b
c c8 c
• • • • • • • • • • • • • • • • • • • • •
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Systems Architecture

11. 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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Systems Architecture

12. 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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13. Object Code Code for sum Assembler Linker 0x401040 <sum>: 0x55
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
0x <sum>: 0x55
0x89
0xe5
0x8b
0x45
0x0c
0x03
0x08
0xec
0x5d
0xc3
Total of 13 bytes
Each instruction 1, 2, or 3 bytes
Starts at address 0x401040
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14. Disassembling Object Code
Disassembled
<_sum>:
0: push %ebp
1: 89 e mov %esp,%ebp
3: 8b 45 0c mov 0xc(%ebp),%eax
6: add 0x8(%ebp),%eax
9: 89 ec mov %ebp,%esp
b: 5d pop %ebp
c: c ret
d: 8d lea 0x0(%esi),%esi
Disassembler
objdump -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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Systems Architecture

15. Alternate Disassembly
Disassembled
Object
0x <sum>: push %ebp
0x <sum+1>: mov %esp,%ebp
0x <sum+3>: mov 0xc(%ebp),%eax
0x <sum+6>: add 0x8(%ebp),%eax
0x <sum+9>: mov %ebp,%esp
0x40104b <sum+11>: pop %ebp
0x40104c <sum+12>: ret
0x40104d <sum+13>: lea 0x0(%esi),%esi
0x401040:
0x55
0x89
0xe5
0x8b
0x45
0x0c
0x03
0x08
0xec
0x5d
0xc3
Within gdb Debugger
gdb p
disassemble sum
Disassemble procedure
x/13b sum
Examine the 13 bytes starting at sum
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Systems Architecture

16. What Can be Disassembled?
% objdump -d WINWORD.EXE
WINWORD.EXE: file format pei-i386
No symbols in "WINWORD.EXE".
Disassembly of section .text:
<.text>:
: push %ebp
: 8b ec mov %esp,%ebp
: 6a ff push $0xffffffff
: push $0x
a: dc 4c 30 push $0x304cdc91
Anything that can be interpreted as executable code
Disassembler examines bytes and reconstructs assembly source
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Systems Architecture