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| *C function* | |
| int fact(int n){ | |
| if (n <= 1) return 1; | |
| return n * fact(n-1); | |
| } | |
| -- | |
| *NASM code* | |
| default rel | |
| global _main | |
| section .data | |
| number: dd 5 | |
| section .text | |
| _main: mov eax, 1 | |
| mov edi, [number] | |
| call .fact | |
| .fact: cmp edi, 2 | |
| jl .basecase | |
| push rbx | |
| mov ebx, edi | |
| lea edi, [rbx - 1] | |
| call .fact | |
| imul eax, ebx | |
| pop rbx | |
| .basecase: | |
| ret | |
| -- | |
| *Step-through* | |
| Sample function call: | |
| fact(5); | |
| edi = 5 | |
| 1st pass: | |
| eax = 1 | |
| cmp edi, 2 | |
| islesser? No -> move on | |
| push rbx | |
| mov ebx, edi => ebx = 5 | |
| lea edi, rbx - 1 => edi = 4 | |
| call fact | |
| ebx=5, edi=4 | |
| 2nd pass: | |
| eax=1 | |
| cmp edi, 2 // edi < 2 No, move on | |
| push rbx | |
| mov ebx, edi => ebx = 4 | |
| lea edi, rbx-1 => edi = 3 | |
| call fact | |
| ebx=4, edi=3 | |
| 3rd pass: | |
| eax=1 | |
| cmp edi, 2 // edi < 2 No, move on | |
| push rbx | |
| mov ebx, edi => ebx = 3 | |
| lea edi, rbx-1 => edi = 2 | |
| call fact | |
| ebx=3, edi=2 | |
| 4th pass: | |
| eax=1 | |
| cmp edi, 2 // edi<2 No, move on | |
| push rbx | |
| mov ebx, edi => ebx=2 | |
| lea edi, rbx-1 => edi = 1 | |
| call fact | |
| ebx=2, edi=1 | |
| 5th pass: | |
| eax=1 | |
| cmp edi,2 // edi < 2, Yes | |
| ret to 4th pass with eax=1 | |
| imul eax, ebx 1 * 2 = 2 | |
| pop rbx | |
| ret to 3rd pass with eax=2 | |
| imul eax, ebx => 2 * 3 = 6 | |
| pop rbx | |
| ret to 2nd pass with eax=6 | |
| imul eax, ebx => 6 * 4 = 24 | |
| pop rbx | |
| ret to 1st pass with eax=24 | |
| imul eax, ebx => 24 * 5 = 120 | |
| ret with eax=120 |
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