Write Your Own PRINT Function in Assembly | Beginner Tutorial | x86-64 Assembly Programming

Published: 01 November 2025
on channel: Mxy

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Emulator Link: https://x64.halb.it

00:00 Step 1: Put string into RAM
00:50 Step 2: Understanding Syscall Write
04:00 Understanding Arguments in Assembly
06:27 Testing Program - Success!
09:45 Making it more Dynamic
12:30 Testing Again
13:00 Finding a Bug in strlen()
14:31 Caller Saved vs Callee Saved
16:00 Using Push/Pop with the Stack
CODE TO COPY:

/* Code Start */
_strlen:
mov rax, rdi

loop:
mov bl, [rax]
cmp bl, 0
je _strlen_exit
inc rcx
inc rax
jmp loop
_strlen_exit:
mov rax, rcx
ret
/* Code End */

Note: In the video, we add one line to clear RCX during initialization (not during the loop, or your strings will all evaluate to 0 length!)

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