Assembly with %include at the top – Printing Outputs Unexpected Result: just an ” S”

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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

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