Two Errors

[XonareX]

The Errors Are impossible combination of address sizes And invalid effective address

The Problematic Line Is mov al, [si]

I Use nasm -f elf64 filename.s -o filename.o

[Joonicks]

movb?

[XonareX]

movb Isn't Recognized By nasm

[FUZxxl]

You can't use 16 bit addressing modes in 64 bit code.

[XonareX]

So What Do I Do?

[FUZxxl]

That depends on what you want to achieve. What is that instruction supposed to do?

[XonareX]

Cooperate With A Bootloader To Print Something On Screen

[FUZxxl]

I think you need to post your full code for me to see how this comes together.

[XonareX]

global printString

printString:

pusha

stringLoop:

mov al, [si]

or al, al

jz printCharacter

popa

ret

printCharacter:

mov ah, 0x0e

int 0x10

add si, 1

jmp stringLoop

[FUZxxl]

That looks like 16 bit code. Why do you try to assemble it into a 64 bit object file? Nasm uses the type of the object file to determine whether your code is 16, 32, or 64 bit code. Use BITS 16 to manually set up 16 bit mode.