main.asm

bits 64

section .text

global _start

_start:
        push   NUM_FORKS
        .loop:
                mov    rax,  SYS_FORK     ; Moving '57' into 'rax', syscall for 'stub_fork()'.
                syscall                   ; Calling the value in 'rax'.

                mov    rax,  SYS_WRITE    ; Moving '1' into 'rax', syscall for 'write()'.
                mov    rdi,  STDOUT       ; Moving '1' into 'rdi', the second argument in a syscall.
                mov    rsi,  message      ; Moving our message into 'rsi', the third argument in a syscall.
                mov    rdx,  messagel     ; Moving the message length into the fourth argument of the 'write()' call.
                syscall                   ; Calling the value in 'rax'.

                lea    rax,  [time]       ; Loading the effective address of 'time' in the '.data' section.
                mov    DWORD [rax],     2 ; Moving '2', our seconds argument to the upper portion of 'rax'.
                mov    BYTE  [eax + 4], 0 ; Moving '0', our nano-seconds argument to the lower portion of the 'ax' regsiter.
                mov    rax,  SYS_SLEEP    ; Moving our sleep syscall to 'rax' to be called with a syscall instruction.
                mov    rdi,  time         ; Moving our time structure to 'rdi' to be passed as the second argument in our syscall.
                xor    esi,  esi          ; Setting the lower portion of the 'si' register to '0' to pass '0' as the third argument.
                syscall                   ; Calling the value in 'rax'.

                pop    rcx
                dec    rcx                ; Decrementing our loop counter.
                push   rcx
                jnz    .loop              ; If the counter isn't zero, jump to '.loop'.

                call   _exit              ; Exiting the program once our counter holds a value of '0'.

_exit:
        mov    rax, SYS_EXIT              ; Moving '60' into 'rax', syscall for 'exit()'
        xor    rdi, rdi                   ; Setting the return value to '0'.
        syscall                           ; Calling the value in 'rax'.

section .data

SYS_EXIT   equ 0x3C                       ; 0x3C = 60    
SYS_FORK   equ 0x39                       ; 0x39 = 57
SYS_WRITE  equ 0x1                        ; 0x1  = 1
SYS_SLEEP  equ 0x23                       ; 0x23 = 35
STDOUT     equ 0x1                        ; 0x1  = 1

NUM_FORKS  equ 10

time       dq 0, 0

message    db "23 instructions executed.", 0xA, 0xD
messagel   equ $ - message