Sometimes to understand code , you need to look into assembly language. I write mostly using C. So sometimes to better understand the program i converte the program into assembly. The syntax changes from machine to machine.
Here is the program that i will use to convert to assembly using two different machine. One is ARM based and the other is x86_64.
#include <stdio.h>
struct CarDetail{
int CarNumber;
char CarOptions;
};
int main() {
struct CarDetail maruti_alto;
maruti_alto.CarNumber = 123;
maruti_alto.CarOptions = 'L';
printf("Maruti Alto Details\nNumber = \t%d\nOptions = \t%c",maruti_alto.CarNumber,maruti_alto.CarOptions);
return 0;
}ARM
If i convert the above program using ARM based machine using command
gcc main.c -S -o main.s .arch armv6
.file "main.c"
.text
.section .rodata
.align 2
.LC0:
.ascii "Maruti Alto Details\012Number = \011%d\012Options ="
.ascii " \011%c\000"
.text
.align 2
.global main
.arch armv6
.syntax unified
.arm
.fpu vfp
.type main, %function
main:
@ args = 0, pretend = 0, frame = 8
@ frame_needed = 1, uses_anonymous_args = 0
push {fp, lr}
add fp, sp, #4
sub sp, sp, #8
mov r3, #123
str r3, [fp, #-12]
mov r3, #76
strb r3, [fp, #-8]
ldr r3, [fp, #-12]
ldrb r2, [fp, #-8] @ zero_extendqisi2
mov r1, r3
ldr r0, .L3
bl printf
mov r3, #0
mov r0, r3
sub sp, fp, #4
@ sp needed
pop {fp, pc}
.L4:
.align 2
.L3:
.word .LC0
.size main, .-main
.ident "GCC: (Raspbian 8.3.0-6+rpi1) 8.3.0"
.section .note.GNU-stack,"",%progbits
If we want to remove relative addressing modes and make it very simple you can use -fomit-frame-pointer option
gcc main.c -S -fomit-frame-pointer -o main-omit-fp.s .arch armv6
.file "main.c"
.text
.section .rodata
.align 2
.LC0:
.ascii "Maruti Alto Details\012Number = \011%d\012Options ="
.ascii " \011%c\000"
.text
.align 2
.global main
.arch armv6
.syntax unified
.arm
.fpu vfp
.type main, %function
main:
@ args = 0, pretend = 0, frame = 8
@ frame_needed = 0, uses_anonymous_args = 0
str lr, [sp, #-4]!
sub sp, sp, #12
mov r3, #123
str r3, [sp]
mov r3, #76
strb r3, [sp, #4]
ldr r3, [sp]
ldrb r2, [sp, #4] @ zero_extendqisi2
mov r1, r3
ldr r0, .L3
bl printf
mov r3, #0
mov r0, r3
add sp, sp, #12
@ sp needed
ldr pc, [sp], #4
.L4:
.align 2
.L3:
.word .LC0
.size main, .-main
.ident "GCC: (Raspbian 8.3.0-6+rpi1) 8.3.0"
.section .note.GNU-stack,"",%progbits
X86_64
gcc main.c -S -o main.s .file "main.c"
.text
.section .rodata
.align 8
.LC0:
.string "Maruti Alto Details\nNumber = \t%d\nOptions = \t%c"
.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
movl $123, -8(%rbp)
movb $76, -4(%rbp)
movzbl -4(%rbp), %eax
movsbl %al, %edx
movl -8(%rbp), %eax
movl %eax, %esi
leaq .LC0(%rip), %rdi
movl $0, %eax
call printf@PLT
movl $0, %eax
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (Debian 8.3.0-6) 8.3.0"
.section .note.GNU-stack,"",@progbits
gcc main.c -S -fomit-frame-pointer -o main-omit-fp.s .file "main.c"
.text
.section .rodata
.align 8
.LC0:
.string "Maruti Alto Details\nNumber = \t%d\nOptions = \t%c"
.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
subq $24, %rsp
.cfi_def_cfa_offset 32
movl $123, 8(%rsp)
movb $76, 12(%rsp)
movzbl 12(%rsp), %eax
movsbl %al, %edx
movl 8(%rsp), %eax
movl %eax, %esi
leaq .LC0(%rip), %rdi
movl $0, %eax
call printf@PLT
movl $0, %eax
addq $24, %rsp
.cfi_def_cfa_offset 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (Debian 8.3.0-6) 8.3.0"
.section .note.GNU-stack,"",@progbits
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