Problem on second use of print text macro

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oscar

Joined: 01 Feb 2006
Posts: 5

oscar 11 Feb 2006, 13:08

Hi,

I am trying to write a macro to print a line of text to teh screen (In bios mode) but i seem to only be able to work once then if i call it again it doesn't print anything. It compiles fine, but it just doesn't run properly...can anyne PLEASE help me lol

thanks

P.S. Here's my problem code:

Code:

org 0x7c00
use16
                mov ax, 0x0000
                mov ds, ax

start:

macro print String{

        local .Printer
        local .Nextchar
        local .Return
        local a

        .Printer:
                mov si, a
                mov ah, 0Eh
                jmp .Nextchar
        
        .Nextchar:
                lodsb
                or al, al
                jz .Return
                int 10h
                jmp .Nextchar
                
        .Return:
                ret
        
        a db String,10,13,0
}

print "haha"
print "akimbo"

times 505-($-start) db 0  512 bytes
dw 0xaa55

11 Feb 2006, 13:08

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20343
Location: In your JS exploiting you and your system

revolution 11 Feb 2006, 15:06

The first instantiation of the macro will print "haha" and then RETurn to the caller (BIOS) so ANY code you place after your macro will not be executed. Try this:

Code:

org 0x7c00
use16 
                mov ax, 0x0000 
                mov ds, ax 

start: 

macro print String{ 

        local .Printer 
        local .Nextchar 
        local .Done
        local .a

        .Printer: 
                mov si, .a
                mov ah, 0Eh 
                jmp .Nextchar 
         
        .Nextchar: 
                lodsb 
                or al, al 
                jz .Done
                int 10h 
                jmp .Nextchar
                jmp .Done

        .a db String,10,13,0

        .Done:
} 

print "haha" 
print "akimbo"
      ret

times 505-($-start) db 0  ;512 bytes
dw 0xaa55

BTW: does the follwing line really compile properly?

Code:

times 505-($-start) db 0  512 bytes

I get an error,

11 Feb 2006, 15:06

oscar

Joined: 01 Feb 2006
Posts: 5

oscar 11 Feb 2006, 22:46

it does....but it may not actually be 512 bytes...i saw it in an example i was looking at and i never actaully figured out if stuff in the bootsector HAS to be 512 bytes or if it can be less than that.

point being it compiles for me...sould be

Code:

times 505-($-start) db 0 ;512 bytes

(the 512 bytes was a comment)

anyway,

Thanks heaps, i had the idea that "ret " just returned you to where you were in the main program (kind of like an exit in BASIC).

i can't beleive i spent 2 days trying to figure out something so simple

11 Feb 2006, 22:46

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8354
Location: Kraków, Poland

Tomasz Grysztar 12 Feb 2006, 06:24

oscar: I think you are using the macro where what you wanted was actually the procedure. The fact that you used "called" terminology and "ret" instruction seems to confirm it.

12 Feb 2006, 06:24

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20343
Location: In your JS exploiting you and your system

revolution 12 Feb 2006, 10:42

Quote:

i never actaully figured out if stuff in the bootsector HAS to be 512 bytes or if it can be less than that.

It must be 512 bytes to match the HDD sector size. The "dw 0xaa55" is also needed as the last two bytes to show the BIOS that the data is valid and not a blank sector.

12 Feb 2006, 10:42

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