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Assembly Language Crash Course 5

Okay, the series is back.

Sorry for the shortness of this one. Just lost motivation. Upvote for longer ones :D

So let's start already.

Section 10: while loop.

Consider the code below

while:
  add eax, 1
  cmp eax, 5
  jl while
  ret

This is a while loop!

So at the end of the code we want we do a cmp and then do a conditional jump back, if that jump is not true we ret.

Section 11: Better print

Consider the code below:

section .text
  global _start
_start:
  mov si, msg
  call printf
printf:
  pusha
  loop:
    mov al, [si]
    cmp al, 0
    jne print_char
    popa
    ret
  print_char:
    mov ah, 0x0e
    int 0x10
    add si, 1
    jmp loop
section .data
  msg db, "Hello World", 0x0a, 0

That was a lot of code! Let's break it down.

ah, si, and al are more registers measuring 16, 8, and 8 bit in size, respectively.

pusha pushes everything to the stack.

popa pops everything from the stack.

int 0x10 is the BIOS interrupt to pint a single character stored in `al to the screen.

And what we are doing is looping until we find a 0, that is why we have the , 0 an=t the end of our string in the data section.

So you push the db to si and call printf.

Anyway.

Feedback, upvote.

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Comments

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Have you assembled your next tutorial??

very corny joke. @DynamicSquid

@Wuru sorry

LOL @DynamicSquid

Lul @DynamicSquid

okay wuru has a alt

no i dont @HahaYes

@HahaYes Link?

wuru do you have two alts? This is pretty suspicous tbh

nah @HahaYes

@Wuru hmmmmm

You don't really use real mode assembly unless you're writing a bootloader

16, 8, and 8

You were intending to say that si and al are both 8 bit registers, right?

Syntax error on line 20 of section 11 - {msg db, "Hello ...} -- comma breaks the code after "db". Also, in attempting to run this program, on various different environments I get:
main.o: In function _start': main.asm:(.text+0x2): relocation truncated to fit: R_386_16 against.data'

In which case, my assumption was that it was attempting to truncate the message which would be 16 bits into si which is only 8: so I tried changing the registers from using si to esi, but I get the error:
timeout: the monitored command dumped core
sh: line 1: 19660 Segmentation fault timeout 10s demo

So far I haven't found a solution.

@danielathome19 run it in QEMU

Randomly saw this in talk :) I'll find the previous and learn from it. Which version of Assembly does this use? I've been wanting to learn ARM Assembly

It uses x86 assembly :D intel syntax. @MesyetiIsTaken