100 to 1... in x86-64 ASM

100.asm

DEFAULT REL
%define SYSCALL_WRITE 0x2000004
%define SYSCALL_EXIT  0x2000001

section .data

section .bss

section .text
  global start

start:
  mov rax, 100            ; starting with 100
dec_loop:
  call display_str        ; display our current value
  dec rax                 ; decrement
  cmp rax,0
  jg dec_loop             ; loop the display
  mov rax, SYSCALL_EXIT   
  mov rdi, 0
  syscall                 ; Terminate


display_str:
  push rax                ; store the value to the stack
  mov rbx, 0              ; init our character count
  push 0xA                ; push a new line
  inc rbx                 ; count the new line.
reg_to_disp:
  inc rbx                 ; we're adding a character, increment
  push rbx                ; store temporarily
  mov rbx, 10             ; We're writing out in base 10
  mov rdx, 0              ; set to 0, otherwise div will segfault (?!)
  div rbx                 ; divide rax, store remainder in rdx
  pop rbx                 ; retrieve our char count.
  or rdx, 0x30            ; OR 0x30 to our remainder, so it's the ASCII digit
  push rdx                ; push it to the stack
  cmp rax, 0
  jg reg_to_disp          ; if rax isn't 0, loop because we have more conversion
  push rsp
  mov rax, rbx            ; move char count to rax
  mov rbx, 8              ; set rbx to wordsize
  mul rbx                 ; multiply by wordsize to get bitcount
  mov rbx, rax            ; move the result back to rbx.
  mov rdi, 1              ; print to stdout
  mov rsi, [rsp]          ; point rsi to the stack pointer location
  mov rdx, rbx            ; set rdx to the number of bits to print
  mov rax, SYSCALL_WRITE  ; we're writing out
  syscall                 ; DO IT
  add rsp, rbx            ; reset our stack pointer
  add rsp, 8              ; one more word for the rsp pointer pushed.
  pop rax                 ; retrieve rax from the stack
  ret