some more code that won't work

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jatos

Joined: 04 Nov 2006
Posts: 20

jatos 05 Oct 2007, 12:26

Hi

I got some more code that fails to work correctly. This code worked when it was 16 bit code, and I now I have converted it to 32bit code it doesn't work

Code:

org 0x1600
use32

app_start:

func_set_registers:
        MOV ax, 0xB800
        MOV es, ax
        LEA ebp, [msg_1]
        MOV si, 0
               MOV ax, 0x0000
              MOV ds, ax
        MOV ebx, 0x0000
        MOV ax, 0x0000
        MOV cx, 0

func_print_string:
        MOV al, [ds:ebp]
        CMP al, 0
        JE app_end
        MOV [es:ebx], al    

        INC ebp
        ADD ebx, 2
        JMP func_print_string

jmp app_end

app_data:

        msg_1 db "Hello World!", 0 

app_end:
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        NOP
        jmp app_end

I can't actually see anything wrong with the program. the boot loader is loading the code at 1600h (its the fat 12 boot level downloaded http://flatassembler.net/examples.php).

While I am it: is there anything it might be advisable for me to read, apart from the IA-32 dev manuals, which I have already read.

_________________
Jamie

05 Oct 2007, 12:26

Artlav

Joined: 23 Dec 2004
Posts: 188
Location: Moscow, Russia

Artlav 05 Oct 2007, 13:37

My best guess is that 32bit code requires 32bit (protected) mode, while you run it in 16bit (real) mode.
use32 makes fasm compile it as 32bit code, yet the CPU still decode it as 16bit code, so that's what is being run:

Code:

app_start:

func_set_registers: 
mov      eax,0xc08eb800
lea   bp,[ds:di]
push  ss
add       [ds:bx+si],al
mov        esi,0xb8660000
add   [ds:bx+si],al
mov        ds,ax
mov    bx,0x0000
add        [ds:bx+si],al
mov        eax,0xb9660000
add   [ds:bx+si],al

func_print_string:
mov  al,[ds:di]
cmp   al,0
je      app_end
mov  [es:bp+di],al
inc        bp
add       bx,2
jmp     func_print_string

;...

app_end:

I can advice you to read some assembler programming tutorials, along with making sense of some examples. You can't learn it by reading dev manuals - it will be like learning foreign language by reading it's dictionary.

05 Oct 2007, 13:37

Dex4u

Joined: 08 Feb 2005
Posts: 1601
Location: web

Dex4u 05 Oct 2007, 16:28

In pmode you would use something like this:

Code:

org 0x1600use32app_start:func_set_registers:        mov esi,msg_1            mov edi,0xB8000        mov ah,' 'func_print_string:        mov al, byte[ds:esi]        cmp al,0        JE app_end        mov word[es:edi],ax            inc esi        add edi,2        JMP func_print_stringjmp app_endapp_data:        msg_1 db "Hello World!", 0 app_end:        NOP        NOP        NOP        NOP        NOP        NOP        NOP        NOP        NOP        NOP        jmp app_end

The above code assumes data descriptor are 0 based.

You should also look up lodsb and stosw etc.

05 Oct 2007, 16:28

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