I'm relatively new to assembly programming and was wondering why my code does not print the expected strings. This project is supposed to be a bootloader when finished. I am compiling using the command nasm -f bin boot.asm -o boot.bin. There are no errors during compilation.
boot.asm
bits 16
org 0x7C00
%include "print.asm"
%include "text.asm"
boot:
mov si, boot_string_00
call print
mov si, boot_string_01
call print
times 510 - ($-$$) db 0
dw 0xAA55
print.asm
print:
mov ah, 0x0E
.print_loop:
lodsb
or al, al
je .print_done
int 0x10
jmp .print_loop
.print_done:
cli
ret
text.asm
boot_string_00: db "Placeholder OS Title v0.0.1", 0
boot_string_01: db "Loading Operating system", 0
Expected Output:
PlaceHolder OS Title v0.0.1Loading Operating System
Actual Output:
S
Also, I was wondering how i could implement newlines in assembly so that i could just use '\n' in my strings.
You included stuff at the top of your bootloader, where it will executes first. Instead include extra functions where they aren't in the main path of execution and are only reached by call.
This should work, placing the %include directives where it's safe to put extra function or data, just like if you were writing them all in one file.
boot.asm:
[bits 16]
[org 0x7c00]
boot:
xor ax, ax
mov ds, ax ; set up DS to make sure it matches our ORG
mov si, boot_string_00
call println
mov si, boot_string_01
call println
finish: ; fall into a hlt loop to save power when we're done
hlt
jmp finish
%include "printf.asm" ; not reachable except by call to labels in this file
%include "text.S"
times 510-($-$$) db 0
dw 0xaa55
printf.asm:
print:
mov ah, 0x0E ; call number for int 0x10 screen output
print_loop:
lodsb
test al, al
je print_done
int 0x10
jmp print_loop
print_done:
ret
println:
call print
mov si, line_end
call print
ret
text.S:
boot_string_00: db "Placeholder OS Title v0.0.1", 0
boot_string_01: db "Loading Operating system", 0
line_end: db 0xD, 0xA, 0