OsFaqWiki - Baby Step 2

Quick review:

boot sector loaded by BIOS is 512 bytes
the code in the boot sector of the disk is loaded by the BIOS at 0000:7c00
machine starts in real mode
be aware that the CPU is being interrupted unless you CLI

Many (but not all) BIOS interrupts expect DS to be filled with a real-mode segment value. This is why many BIOS interrupts won't work in protected mode. So if you want to use int 10h/ah=0eh to print to the screen, then you need to make sure that your seg:offset for the characters to print is correct.

It doesn't matter if you use 0000:7c00 or 07c0:0000, but if you use ORG, you need to be aware of what's happening.

AsmExample:

   mov ax, 0x07c0
   mov ds, ax

   mov si, msg
ch_loop:lodsb
   or al,al ;zero=end of str
   jz hang   ;get out
   mov ah,0x0E
   int 0x10
   jmp ch_loop

hang:
   jmp hang

msg   db 'Welcome to Macintosh', 13, 10, 0
   times 510-($-$$) db 0
   db 0x55
   db 0xAA

Here's the ORG version. Note that you still need to tell DS what to be

[ORG 0x7c00]

   xor ax, ax  ;make it zero
   mov ds, ax

   mov si, msg
ch_loop:lodsb
   or al,al ;zero=end of str
   jz hang   ;get out
   mov ah,0x0E
   int 0x10
   jmp ch_loop

hang:
   jmp hang

msg   db 'Welcome to Macintosh', 13, 10, 0

   times 510-($-$$) db 0
   db 0x55
   db 0xAA

New topic: Typically 'procedures' are separated from the code using CALL/RET like the following

[ORG 0x7c00]
   xor ax, ax  ;make it zero
   mov ds, ax

   mov si, msg
   call bios_print

hang:
   jmp hang



msg   db 'Welcome to Macintosh', 13, 10, 0


bios_print:
   lodsb
   or al,al ;zero=end of str
   jz done   ;get out
   mov ah,0x0E
   int 0x10
   jmp bios_print
done:
   ret

   times 510-($-$$) db 0
   db 0x55
   db 0xAA

For some inexplicable reason, loading SI then jumping to the procedure always bugged me. Fortunately for psychos like me NASM's macros let you pretend that you are passing a parameter (macro definition has to go before it's being called).

%macro BiosPrint 1
                mov si, word %1
ch_loop:lodsb
   or al,al ;zero=end of str
   jz done   ;get out
   mov ah,0x0E
   int 0x10
   jmp ch_loop
done:
%endmacro

[ORG 0x7c00]

   xor ax, ax  ;make it zero
   mov ds, ax

   BiosPrint msg

hang:
   jmp hang

msg   db 'Welcome to Macintosh', 13, 10, 0

   times 510-($-$$) db 0
   db 0x55
   db 0xAA

And in case your code is becoming long and unreadable, you can break it up into different files, then include the files at the beginning of your main code. Like so:

   jmp main

   %include "othercode.inc"

main:
   ;... rest of your code here

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