VGA video RAM

[nitroglycerin]

hi!
i've got some problems using vga video ram:
[tt]
   mov ax,0xb800
mov gs,ax

   mov al, 'X'
   mov byte [gs:0],al
   mov byte [gs:1],0x04[/tt]

I get it that this code will print a red "X" to upper left position (if text mode). Now, how to go 1 position right? if I do this:

[tt]   mov ax,0xb801
mov gs,ax
   mov al, 'X'
   mov byte [gs:0],al
   mov byte [gs:1],0x04[/tt]

it won't go 1 char to the right but 6 chars. howcome?

[Tim]

You might find that the second character is in column 7, not 6 as you imply.

This is because you've added 1 to the GS segment register (you went from 0xB800 to 0xB801). This had the effect of adding 16 to the final memory address, which is equal to 8 video memory cells.

You should leave GS as it is and use GS:2 and GS:3 instead. You should also revise real-mode addressing.

[df]

you seriously need to go and read a tutorial on how real-mode addressing works, to properly understand whats going on in your example.

see the link thread at the start of the os-dev board.

[nitroglycerin]

I read that one already before, and understand rmode addresing, just seems I overlooked offsets :-\
My bad.

Thanks!

[slacker]

nitro:
0xb8000 linear is the base of video memory but in real mode you need to use a segment:offset pair to reference this memory location. read Perica's tutorial:http://osdev.neopages.net/tutorials/rm_addressing.php

it is the best ive ever read and if you dont understand real mode addressing after reading this, you never will.

[nitroglycerin]

You don't get me. I know rmode addresing (and read perica's doc). The thing is that I totaly forgot to increase offsets instead of segs.