More efficient way to output an integer in pure assembly

Question

I'm looking to output an integer using pure assembly. I'm using nasm on a 64-bit linux machine. At the moment I'm looking for a way to output integers to debug a compiler, but I want to use the same code for writing an OS, which is also the reason I don't simply use printf(). After much searching and frustration I have come up with this code

    SECTION .data
var:    db  "      ",10,0

    SECTION .text
global main
global _printc
global _printi

main:
    mov rax, 90
    push    rax
    call    _printi

    xor rbx, rbx
    mov rax, 1
    int 0x80

_printi:
    pushf
    push    rax
    push    rbx
    push    rcx
    push    rdx

    mov rax, [rsp+48]
    mov rcx, 4
.start:
    dec rcx
    xor rdx, rdx
    mov rbx, 10
    div rbx
    add rdx, 48
    mov [var+rcx], dl
    cmp rax, 0
    jne .start

    mov rax, [var]
    push    rax
    call    _printc
    pop rax

    pop rdx
    pop rcx
    pop rbx
    pop rax
    popf
    ret

_printc:
    push    rax
    push    rbx
    push    rcx
    push    rdx

    mov rax, [rsp+40]
    mov [var], rax
    mov rax, 4
    mov rbx, 1
    mov rcx, var
    mov rdx, 4
    int 0x80

    pop rdx
    pop rcx
    pop rbx
    pop rax
    ret

Note that I'll be replacing 0x80 calls with BIOS calls when porting to OS development.

My question is how to optimize, or even prettify, this code further. My first thought would be to replace pushing all the registers individually, but there isn't any 64-bit pusha instruction...

Hex version: stackoverflow.com/questions/3853730/…

Ciro Santilli OurBigBook.com
– Ciro Santilli OurBigBook.com

2015-09-29 21:01:25 +00:00
Commented Sep 29, 2015 at 21:01 — Ciro Santilli OurBigBook.com
– Ciro Santilli OurBigBook.com, Commented Sep 29, 2015 at 21:01

Guffa · Accepted Answer · 2010-11-07 11:49:26Z

2

Here are some possible changes to the routine:

_printi:
    pushf
    push    rax
    push    rbx
    push    rcx
    push    rdx

    mov rax, [rsp+48]
    mov rcx, 4
    mov rbx, 10 ; --moved outside the loop
.start:
    dec rcx
    xor rdx, rdx
    div rbx
    add rdx, 48
    mov [var+rcx], dl
    cmp rax, 0
    jne .start

    ; mov rax, [var] -- not used
    ; push    rax -- not used
    call    _printc
    ; pop rax -- not used

    pop rdx
    pop rcx
    pop rbx
    pop rax
    popf
    ret

I also noted some limitations in the algorithm. If the number is larger than 9999, the code will continue to put digits outside of the allocated space, overwriting some other data. The routine is not fully reusable, i.e. if you print 123, then 9 it will come out as 129.

answered Nov 7, 2010 at 11:49

Guffa

703k111 gold badges760 silver badges1k bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Guffa Over a year ago

@eZanmoto: Regarding the pushing and poping, consider what the code does. You put a value in rax that is neither used by _printc not later in the code. The _printc routine does itself preserve rax, so you wouldn't need to push and pop it even if it was needed later. If _printc is only used by _printi, you can inline it's code, saving you a call and a set of pushes and pops.

Sean Kelleher Over a year ago

Sorry for my ignorance, but I still don't follow... I'm pushing the data in the variable var so that it can be passed as a parameter in the _printc routine. I know _printc is only used by _printi at the moment, but I intend to use it in a library of functions while building my compiler and OS. But you're right, in pertaining to my original question of a more efficient way of outputting an integer it's much better to just use _printc inline, thanks for the answer, and sorry for the ambiguity in the question.

Guffa Over a year ago

@eZanmoto: I see... you get the first part of the string, passes it as a parameter to _printc, which puts it back where it was, overwriting the characters with the same characters... It was just so pointless that I didn't see it. :)

Sean Kelleher Over a year ago

Yeah, sorry... I've never coded in assembly before so I don't know any idioms or the correct way to do anything... Like I've done lots of disassembly, but actually writing it is a different beast... My way of thinking for this is that I'll be including it in other assembly files using the %include directive, so instead of having to remember the variable name I figured I can put the value or character I want to output on the stack and call _printc or _printi, depending on what functionality I want... And again it'll be easy to replace the 0x80 linux call with a BIOS call...

Collectives™ on Stack Overflow

More efficient way to output an integer in pure assembly

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related