[newbie] Normaly using 65xx user needs some adivce ;)

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BastetFurry

Joined: 28 Jul 2008
Posts: 4
Location: Essen, NRW, Germany

BastetFurry 28 Jul 2008, 22:15

Hi!

Coming from the land of the C64 and the 6510 i want to learn x86 assembler for some oldschool DOS hacking, maybe even build a cool demo in 2 or 3 years and win the Wild at Breakpoint Wink

(No DOS in PC-Demo Compo Sad

)

I want to build a loop, for learning purposes, that increases every pixel on a $13 screen, but nothing shows up.
Could anyone please look at my code and tell me my mind-mistake?

Code:

format MZ ; DOS executable format

stack $1000
heap $200

start:
    ;Make code and data segment the same
    push cs
    pop ds
    
    ;Get into graphic mode $13 (320x200x8bit)
    mov ax, 0x13
    int 0x10

mainloop:    
    push es
    mov ax,0xa000
    mov es,ax
    
    mov bx,0xFA00
    mov di,bx

    mov ax,0
myloop:
    inc byte [di]
    dec di
    jz loopend
    jmp myloop
    
loopend:
    
    pop es
    jmp mainloop

    
    ;DOS end of programm routine
    mov ax,0x4c00
    int 0x21
    ret

28 Jul 2008, 22:15

windwakr

Joined: 30 Jun 2004
Posts: 827

windwakr 28 Jul 2008, 22:38

simple thing that does what you want:
This makes a '.com' not '.exe' like your code does.
com is easier to work with for smaller programs.

Code:

org 100h         ;Make it .com

push 0a000h      ;Where the screen area is
pop es           ;^^

mov ax,13h       ;Video mode
int 10h          ;^^

mov cx,64000     ;screen has 64,000 pixels
a_loop: 
inc byte [es:di] ;YOU HAVE TO USE [es:di], NOT [di]
inc di           ;Next pixel
loop a_loop      ;Loop 64k times

key:             ;Simple checking for escape
in al,60h        ;Don't use for anything you'll actually release..
dec al           ;People will complain...
jnz key          ;
ret              ;

28 Jul 2008, 22:38

BastetFurry

Joined: 28 Jul 2008
Posts: 4
Location: Essen, NRW, Germany

BastetFurry 28 Jul 2008, 23:00

First, thanks Smile

Some question about your code, i dont see you decrementing register cx, does loop do that for you?

28 Jul 2008, 23:00

windwakr

Joined: 30 Jun 2004
Posts: 827

windwakr 28 Jul 2008, 23:03

loop basically does two things in one. It decrements cx and does jnz

BTW: Your code has a few flaws. It never stops looping because at loopend you have it jump back to the first loop. The next dec di would make di FFFF and cause the whole thing to start back up. Also, you zero out ax but don't actually use it.

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28 Jul 2008, 23:03

BastetFurry

Joined: 28 Jul 2008
Posts: 4
Location: Essen, NRW, Germany

BastetFurry 28 Jul 2008, 23:17

Um, i wanted it to go trough all colors, i assume that when some memory location contains $ff and i "inc" that, that it turns to $00.

28 Jul 2008, 23:17

windwakr

Joined: 30 Jun 2004
Posts: 827

windwakr 28 Jul 2008, 23:18

Ohhhh, I thought you wanted it to go through once, sorry.

Edit: Is this like what you wanted?

Code:

org 100h         ;Make it .com

push 0a000h      ;Where the screen area is 
pop es           ;^^ 

mov ax,13h       ;Video mode 
int 10h          ;^^ 

start:

mov cx,64000     ;screen has 64,000 pixels
a_loop:  
inc byte [es:di] ;YOU HAVE TO USE [es:di], NOT [di] 
inc di           ;Next pixel 
loop a_loop      ;Loop 64k times 

key:             ;Simple checking for escape 
in al,60h        ;Don't use for anything you'll actually release.. 
dec al           ;People will complain... 
jnz start        ;
ret

It doesn't look all that great because it doesn't wait for vertical retrace.

BastetFurry wrote:

Um, i wanted it to go trough all colors, i assume that when some memory location contains $ff and i "inc" that, that it turns to $00.

Ya, it would go to 0

Take a look at This Thread for some good links on DOS coding. Especially check out Ralph Browns Interrupt list.

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28 Jul 2008, 23:18

BastetFurry

Joined: 28 Jul 2008
Posts: 4
Location: Essen, NRW, Germany

BastetFurry 28 Jul 2008, 23:40

windwakr wrote:

It doesn't look all that great because it doesn't wait for vertical retrace.

Trying to get that done ATM Smile

I know that under QB45 its: WAIT &h3da,8
So i have this, but he never leaves the loop.

Code:

    mov dx,0x03da
vblankloop:
    in ax,dx
    cmp ax,8
    jne vblankloop

windwakr wrote:

Take a look at This Thread for some good links on DOS coding. Especially check out Ralph Browns Interrupt list.

Thanks

EDIT:
Exchanged cmp with test and now it works. Question

28 Jul 2008, 23:40

edfed

Joined: 20 Feb 2006
Posts: 4354
Location: Now

edfed 29 Jul 2008, 00:20

try this

29 Jul 2008, 00:20

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