pftgu_nasm

1 ; Chapter 4 "Power Function" 2 ; 3 ; Computes the value of 2^3 + 5^2, exits with value. 4 ; Demonstrates how functions work. 5 ; 6 ; $ ./go.sh ch04.1_power_function 7 ; Exited with code: 33 8 ; 9 10 section .data 11 12 section .text 13 14 ; Start - Call power twice, return result 15 ; --------------------------------------------------------- 16 global _start 17 _start: 18 push dword 3 ; second arg, push immediate value onto stack 19 push dword 2 ; first arg, pushed last! 20 call power 21 22 add esp, 8 ; move stack pointer back to "erase" args 23 push eax ; save current answer on stack 24 25 push dword 2 ; second arg 26 push dword 5 ; first arg 27 call power 28 29 add esp, 8 ; move stack pointer back to "erase" args 30 pop ebx ; put previous saved answer into ebx 31 add ebx, eax ; add current answer to previous 32 33 ; exit! ebx contains answer and is "returned" as exit code 34 mov eax, 1 ; linux syscall 'exit' 35 int 0x80 36 37 ; power function 38 ; --------------------------------------------------------- 39 ; input: 40 ; arg1 -> ebx is the base number 41 ; arg2 -> ecx is the exponent to raise base by 42 ; output: 43 ; answer in eax 44 ; 45 ; Note: We're not testing this function externally yet, 46 ; but I *think* I have properly translated this 47 ; global function directive (elf specific?) to NASM: 48 global power:function 49 power: 50 ; We move the stack pointer FORWARD to make room for 4 bytes 51 ; of space for a "local variable" in traditional C function 52 ; style. We point the base pointer register, ebp, to the 53 ; the current stack position so we can use it to reference 54 ; relative parts of the stack for "private use" in our 55 ; function. Again, this is just a style or convention. 56 push ebp ; save previous base pointer 57 mov ebp, esp ; set it to stack pointer 58 sub esp, 4 ; move pointer FORWARD to make room 59 60 mov ebx, [ebp + 8] ; first arg - base number 61 mov ecx, [ebp + 12] ; second arg - exponent 62 mov [ebp - 4], ebx ; store base number as start of total 63 64 power_loop_start: 65 cmp ecx, 1 ; once power is 1, we're done! 66 je end_power 67 mov eax, [ebp - 4] ; put total in eax 68 imul eax, ebx ; multiply total by base 69 mov [ebp - 4], eax ; store total 70 dec ecx ; reduce power by 1 71 jmp power_loop_start 72 73 end_power: 74 mov eax, [ebp - 4] ; put total in eax to return 75 mov esp, ebp ; restore stack pointer 76 pop ebp ; restore base pointer (we pushed it) 77 ret