C compiler
make
and run...
./0cc '<C code>'
make test
test1.c
int main() {
return 42;
}Compile this to test1 & Try objdump
$ gcc test1.c -o test1
$ objdump -d -M intel ./test1
./test1: ファイル形式 mach-o-x86-64
セクション .text の逆アセンブル:
0000000100000fa5 <_main>:
100000fa5: 55 push rbp
100000fa6: 48 89 e5 mov rbp,rsp
100000fa9: b8 2a 00 00 00 mov eax,0x2a
100000fae: 5d pop rbp
100000faf: c3 ret
test1.c is equal to test2.s(Assembly file)
.intel_syntax noprefix
.global _main
_main:
mov rax, 42
ret
Try simple function.
test3.c
int plus(int x, int y) {
return x + y;
}
int main() {
return plus(20, 22);
}Objdump this
$ gcc test3.c -o test3
$ objdump -d -M intel ./test3
./test3: ファイル形式 mach-o-x86-64
セクション .text の逆アセンブル:
0000000100000f4f <_plus>:
100000f4f: 55 push rbp
100000f50: 48 89 e5 mov rbp,rsp
100000f53: 89 7d fc mov DWORD PTR [rbp-0x4],edi
100000f56: 89 75 f8 mov DWORD PTR [rbp-0x8],esi
100000f59: 8b 55 fc mov edx,DWORD PTR [rbp-0x4]
100000f5c: 8b 45 f8 mov eax,DWORD PTR [rbp-0x8]
100000f5f: 01 d0 add eax,edx
100000f61: 5d pop rbp
100000f62: c3 ret
0000000100000f63 <_main>:
100000f63: 55 push rbp
100000f64: 48 89 e5 mov rbp,rsp
100000f67: be 16 00 00 00 mov esi,0x16
100000f6c: bf 14 00 00 00 mov edi,0x14
100000f71: e8 d9 ff ff ff call 100000f4f <_plus>
100000f76: 5d pop rbp
100000f77: c3 ret
Write assembly equal to test3.c
.intel_syntax noprefix
.global plus, _main
plus:
add rsi, rdi
mov rax, rsi
ret
_main:
mov rdi, 20
mov rsi, 22
call plus
ret
rax, rsi, rdi, rsp, rbp: 64-bit register names
If a register name has r prefix, it means 64-bit register.
On the other hand, if one has e prefix, it means 32-bit register.
If a register name is like r?x(? filled with a, b, c or d), it is called General Purpose Register.
raxisAccumlatorrbxisBase index: used for arraysrcxisCounter: used for loops and stringsrdxis used toextend the precision of the accumlator
Registers named rsi or rdi are called Index Registers:
rsiis specially calledSource Index for string operationsrdiis specially calledDestination Index for string operations
When to compile a C function to an assembly, the first argument should be passed on to rdi, and the second argument to rsi.
rspisStack pointer for the stack top addressrbpisStack base pointer to hold the current stack frame
References:
0cc.c
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv) {
if (argc != 2) {
fprintf(stderr, "Wrong number of arguments.\n");
return 1;
}
printf(".intel_syntax noprefix\n");
printf(".global _main\n");
printf("_main:\n");
printf(" mov rax, %d\n", atoi(argv[1]));
printf(" ret\n");
return 0;
}
Compile it & Pass any integer as an argument.
$ gcc 0cc.c -o 0cc
$ ./0cc 42 > tmp.s
$ cat tmp.s
.intel_syntax noprefix
.global _main
_main:
mov rax, 42
ret
$ gcc tmp.s -o tmp
$ ./tmp
$ echo $?
42
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