Improved countdown program in x86 NASM

Question

A while back, I posted a question involving code of the same function. This post contains improved code.

From the last post, I have improved size, my work with conventions, and my comments. I was also able to minimalism memory interaction to speed things up/make things more efficient.

%define KERNEL_CALL int 80h
%define STDOUT 1
%define SYS_WRITE 4
%define EXIT 1
%define NEWLINE 0xA
%define ASCII_NUM 30h
; @params: AX - a number that is converted to ASCII, then printed
; @return: no return
%macro put_num 0
 push ax; push ax onto the stack(for restoring value)
 add ax, ASCII_NUM; numerical to ascii
 push ax; push ax onto the stack(for reference by ESP)
 mov edx, 1; message length
 mov ecx, esp; the number from the stack pointer
 mov ebx, STDOUT
 mov eax, SYS_WRITE
 KERNEL_CALL; logs the number to the console
 pop ax; getting it out of the way
 pop ax; restoring the original value of ax
%endmacro
section .text
 global _start; for gnu linker
_start:
 push '10'; push '10' to the stack for reference by ESP
 mov edx, 2; length
 mov ecx, esp; message(number)
 mov ebx, STDOUT
 mov eax, SYS_WRITE
 KERNEL_CALL; logs '10' to the console
 mov ax, 9; initialize ax
countdown: 
 put_num; log the current number to stdout
.subtract:
 sub ax, 1; subtract 1
 cmp ax, 0; compare ax with 0...
 je .blast_off; if equal, jump to .blast_off
 jmp countdown; if not equal, go back to countdown
.blast_off:
 mov edx, len; message length
 mov ecx, msg; message
 mov ebx, STDOUT
 mov eax, SYS_WRITE
 KERNEL_CALL; logs 'Blast off!' to the console
 xor ebx, ebx
 mov eax, EXIT
 KERNEL_CALL; exit
section .data
 msg db "Blast off!", NEWLINE
 len equ $ - msg

Output (I know - the formatting still needs work):

10987654321Blast off!

Assembled: nasm -f elf bo7_working.s

Linked:: ld -m elf_i386 -s -o bo bo7_working.o

Some concerns:

• After I learned about macros, I started to use them a lot. Am I using them in place of headers (and is this bad), or is how I am using them fine?
• Am I over-using/misusing %defines (more specifically, KERNEL_CALL)?
• Do I have too many or too few comments?
• Are my comments unclear?
• Are there any unnecessary lines? I'm a little worried about the popping at the end of put_num...
• Am I following convention, or are there more things I should do?
• Is my code understandable?

Answer 1

•After I learned about macros, I started to use them a lot. Am I using them in place of headers (and is this bad), or is how I am using them fine?

Looks OK to me.

•Am I over-using/misusing %defines (more specifically, KERNEL_CALL)?

I wouln't use KERNAL_CALL in stead of int 80h. The latter is shorter and the value involved will not change any soon in the future.

•Do I have too many or too few comments?

I find commenting on .subtract: a bit stating the obvious.

•Are my comments unclear?

No, they are very readable.

•Are there any unnecessary lines? I'm a little worried about the popping at the end of put_num...

The popping at the end of put_num is correct (pushing/popping 2 words in a row nicely keeps the stack aligned at dwords) but a few lines seem unnecessary to me. You do not need to compare AX with 0 since subtracting already defines the flags. You could also shave off an extra jump.

sub ax, 1; subtract 1
cmp ax, 0; compare ax with 0...
je .blast_off; if equal, jump to .blast_off
jmp countdown; if not equal, go back to countdown
.blast_off:

Better code:

sub ax,1
jnz countdown
.blast_off:

•Am I following convention, or are there more things I should do?

The ordering of the sections of the program is a matter of taste. But I see most people put the .data section before the .text section.

•Is my code understandable?

Very understandable.