nasm bootloader, why does where i define ths string matter? - nasm

Question

Im experimenting with writing a bootloader in nasm, at the moment it just prints a string.
[BITS 16]
[org 0x7c00]
myString:
db 'Hello World', 0x00
mov bp, 0x8000
mov sp, bp
mov bx, myString
call printString
jmp $
printString:
pusha
mov ah , 0x0e
printStringA:
mov al , [bx]
cmp al, 0x00
je printStringB
int 0x10
add bx, 0x01
jmp printStringA
printStringB:
popa
ret
times 510 -( $ - $$ ) db 0
dw 0xaa55
that works fine, but if i move the string definition to here:
[BITS 16]
[org 0x7c00]
mov bp, 0x8000
mov sp, bp
myString:
db 'Hello World', 0x00
mov bx, myString
call printString
jmp $
printString:
pusha
mov ah , 0x0e
printStringA:
mov al , [bx]
cmp al, 0x00
je printStringB
int 0x10
add bx, 0x01
jmp printStringA
printStringB:
popa
ret
times 510 -( $ - $$ ) db 0
dw 0xaa55
it prints out garbage, im running this in bochs under windows if that helps.

Answer 1

You are assembling to raw machine code. There are no data and text sections. Everything is interpreted as code, including what you insert using db. Hence both code snippets are wrong.
If you finish with an endless loop (as in your example) or a halt instruction, the data can safely be put after the code as it will never be reached. Otherwise you must arrange for the data to be skipped over.
You also need to set the segment registers correctly at the start.
Here is a corrected version, with early declaration of data:
[BITS 16]
[ORG 0]
;;; Set CS and DS
jmp 0x07c0:start
start:
mov ax, cs
mov ds, ax
;;; set SP
mov bp, 0x8000
mov sp, bp
;;; skip over data
jmp L1
myString:
db 'Hello World', 0x00
L1:
mov bx, myString
...
Note that in your first example, the data was interpreted as code.
db 'Hello World', 0x00
is assembled as
48 65 6c 6c 6f 20 57 6f 72 6c 64 00
and corresponds to:
dec ax
gs insb
insb
outsw
and [bx+0x6f],dl
jc short 0x76
fs
db 0x00
In effect this gets executed before your code. It is pure luck that this fragment doesn't prevent your code from working.