Nasm (screen offset)

[niiize]

Started to look into assembly again after a bunch of years off. Didn't get very far with it so I'm a beginner still. I'm wondering how one can access the screen-offset in linux.


Thnk you.

[NathanB]

niiize -

Having a nice tutorial handy usually helps a great deal:

http://tronche.com/gui/x/xlib/

Feel free to post any code you are having trouble with.

[niiize]

I forgot to mention in the post that it is regular cosole mode I was referring to, and the letters offset on the screen and not pixels. also been googling alot but havn't found any thorough tutorials on linux assembly yet.

Ok. Here is some basic code for writing "hello world" to the screen using syscall 4
...
mov ecx, helloworld
mov edx, len
mov ebx, 1 ;basic output code?
mov eax, 4
int 0x80
...
How can I reach the screenoffset. Don't know if I'm remembering wrong but back in DOS this adress was stored in DS:DI ?

Thank you

[lisati]

Don't know if I'm remembering wrong but back in DOS this adress was stored in DS : DI ?

Thank you

I'd recommend brushing up on 32-bit programming (something I'll need to do if I want to learn ASM for Ubuntu as well)

[NathanB]

Modern operating systems will not allow a user-space program to have direct access to hardware (without special permission). Read up on 'protected mode' and 'virtual memory' for the details:

http://en.wikipedia.org/wiki/Protected_mode

http://en.wikipedia.org/wiki/Virtual_memory

If you are still determined to write your programs in the old style, then grab VirtualBox and load a FreeDOS image. This will allow your programs a simulated 'direct access to hardware' without any danger of borking your computer.

http://www.virtualbox.org/

http://www.freedos.org/

Here is an 'Etch-a-Sketch' example of using Video Mode 13h from a simple DOS program.

; To assemble with A86: A86 ETCH3.ASM
;
; Control Keys:
; Up Arrow: move up
; Down Arrow: move down
; Left Arrow: move left
; Right Arrow: move right
; SPACE BAR: toggles line drawing OFF and ON
; C: clears the drawing
; ESC: quit
;
code segment
;
row dw 0
col dw 0
write db 0
;
org 100h
mov ax,0013h
int 10h
mov write,11
mov di,0A000h
mov es,di
reset:
;
; Light up the VGA border.
;
mov ax,1001h ;function 10h, sub-function 1
mov bh,5 ;set color in BH (5 = magenta)
int 10h ;call ROM BIOS video services

mov row,32000
mov col,159

NextPixel:
mov ax,320
mov bx,row
mul bx
add bx,col
mov di,bx
mov byte [es:di],11
mov ah,0
int 16h
cmp write,0
jne YesWrite
mov bl,write
mov byte [es:di],bl
YesWrite:
cmp ah,2Eh
jne NotShake
mov ax,0013h
int 10h
jmp reset
NotShake:
cmp ah,48h
jne NotMoveUp
sub row,320
NotMoveUp:
cmp ah,50h
jne NotMoveDown
add row,320
NotMoveDown:
cmp ah, 4Bh
jne NotMoveLeft
sub col,1
NotMoveLeft:
cmp ah,4Dh
jne NotMoveRight
add col,1
NotMoveRight:
cmp ah,39h
je Toggle
cmp ah,01h
je exit
jmp NextPixel
Toggle:
cmp write,11
jne SetWrite
mov write,0
jmp NextPixel
SetWrite:
mov write,11
jmp NextPixel
exit:
mov ax,0003h
int 10h
int 20h
;
end

Get the 'a86' assembler here:
http://eji.com/a86/

[niiize]

Thank you guys