Writing a message using the BIOS

Quick review:

Boot sector loaded by BIOS is 512 bytesThe code in the boot sector of the disk is loaded by the BIOS at 0000:7c00Machine starts in?Real ModeBe aware that the CPU is being interrupted unless you issue the?CLI?assembly command

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.

In real mode, addresses are calculated as segment * 16 + offset. Since offset can be much larger than 16, there are many pairs of segment and offset that point to the same address. For instance, some say that the bootloader is is loaded at 0000:7C00, while others say 07C0:0000. This is in fact the same address: 16 * 0x0000 + 0x7C00 = 16 * 0x07C0 + 0x0000 = 0x7C00.

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. By default, the start of a raw binary is at offset 0, but if you need it you can change the offset to something different and make it work. For instance the following snippet accesses the variable msg with segment 0x7C0.

Asm Example:

;?boot.asm
???mov?ax,?0x07c0
???mov?ds,?ax
?
???mov?si,?msg
ch_loop:lodsb
???or?al,?al?;?zero=end?or?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 is the ORG version. This time, msg is accessed with segment 0. Note that you still need to tell DS what to be as it can hold any value.

[ORG?0x7c00]
?
???xor?ax,?ax?;?make?it?zero
???mov?ds,?ax
?
???mov?si,?msg
ch_loop:lodsb
???or?al,?al??;?zero=end?of?string
???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

Procedures

To save on writing space, the typical 'procedures' are often 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 definitions has to go before it's being called).

%macro?BiosPrint?1
????????????????mov?si,?word?%1
ch_loop:lodsb
???or?al,?al
???jz?done
???mov?ah,?0x0E
???int?0x10
???jmp?ch_loop
done:
%endmacro
?
[ORG?0x7c00]
???xor?ax,?ax
???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 you main code. Like so:

jmp?main
?
%include?"othercode.inc"
?
main:
???;?...?rest?of?code?here

Don't forget the jmp main at the start - otherwise some random other procedure will get called.