From mdaniel@zeus.polsl.gliwice.pl Fri Jun 23 16:32:46 1995 From: mdaniel@zeus.polsl.gliwice.pl (Milosz Danielewski) Subject: x-mode base info Newsgroups: comp.lang.asm.x86 Date: 22 Jun 1995 11:22:44 GMT Organization: Politechnika Slaska w Gliwicach Path: fuw.edu.pl!news.nask.org.pl!news.nask.katowice.pl!hades.polsl.gliwice.pl!mdaniel Lines: 201 Message-ID: <3sbjq4$d3l@hades.polsl.gliwice.pl> NNTP-Posting-Host: zeus.polsl.gliwice.pl X-Newsreader: TIN [version 1.2 PL2] A few people asked me about VGA x-mode so I decided to write some info. X-mode is a video mode which allows you to use all 256K of VGA's memory. After switching in 13h mode ( 320x200 256 colors mode ) video memory is mapped >from address A0000h ( ie. A000:0000h ). Each byte is an index of pixel's color. It's usefull because memory is linear ( each pixel has got own address). After switching to x-mode video memory is still at A0000h but each address is conected witch 4 pixels.So how to choose one pixel ? This macro shows the way: macro bank nr mov ah,nr ; which pixel going to be set nr - mask mov al,02h ; Map Mask Register Index mov dx,03C4h ; dx = base address of sequencer out dx,ax endm What does it mean that 'nr' is a mask ? Only four less significant bits are important and each bit corresponds with pixel. Bit #0 with first pixel on the left , bit #1 with second , bit #2 with third and bit #3 with fourth. Example: ; processor is in real mode ; es = 0A000h di - offset on the screen ; We want to set all even pixels to color #15 mov ah,0101b ; mask for pixels mov al,02h mov dx,03C4h out dx,ax xor di,di mov cx,80*200 ; screen size ( 80 columns , 200 rows ) mov al,15 cld rep stosb ( All examples are NOT speed optimized but clear ) pixels at one address: OOO ### OOO ### ### - modificated pixel OOO ### OOO ### ### \ | \____ | OOO - non modificated pixel \ | OOO _\________| / \ / \ / \ / \ 1 0 1 0 = mask Bit complicated ? In practice you should write algorithms with minimal number of mask changes ( because 'out' is slow ). At the beginning you can have some troubles like drawing lines etc. but you need only some expirience. Point of x-mode is that you can plot,paint,draw etc. onto invisible page of video memory during the next is visible.How to change pages ? mov al,0Ch ; Start address high register mov dx,03D4h out dx,al inc dx mov al,xx ; xx = most significant part of page address out dx,al dec dx mov al,0Dh ; start address low register out dx,al inc dx mov al,yy ; yy = less significant part of page address out dx,al Actually , page address is an offset, base address is always A0000h. Changing start address you can scroll screen in each direction or change visible page etc. ( If you increase start address by 80 , screen will scroll up 1 line ). OK. How to switch into x-mode ? This part do it for us: mov ax,13h int 10h mov dx,3C4h mov al,4 out dx,al inc dx in al,dx and al,0F7h or al,4 out dx,al mov dx,3CEh mov al,5 out dx,al inc dx in al,dx and al,0EFh out dx,al dec dx mov al,6 out dx,al inc dx in al,dx and al,0FDh out dx,al mov dx,03C4h mov ax,0F02h out dx,ax mov ax,0A000h mov es,ax xor ax,ax xor di,di mov cx,32000 cld rep stosw mov dx,3D4h mov al,14h out dx,al inc dx in al,dx and al,0BFh out dx,al dec dx mov al,17h out dx,al inc dx in al,dx or al,40h out dx,al How does it work ? Soon I'll post full registers description. Aspect ratio in 320x200 is weird so people made new mode: 320x240. Rules are same as in 320x200 x-mode. This one is ripped by Dobrov from Pinbal Fantasies: mov ax,0013h int 10h mov dx,3D4h mov ax,0011h out dx,ax mov dx,3C4h mov al,4 out dx,al inc dx in al,dx and al,0F6h out dx,al mov dx,3D4h mov al,14h out dx,ax inc dx in al,dx and al,0BFh out dx,al mov dx,3d4h mov ax,0E317h out dx,ax mov dx,3CCh in al,dx or al,0C0h mov dx,3c2h out dx,al mov dx,3D4h mov al,11h out dx,al inc dx in al,dx and al,7Fh out dx,al mov dx,3D4h mov ax,0D06h out dx,ax mov ax,3E07h out dx,ax mov ax,0C009h out dx,ax mov ax,0EA10h out dx,ax mov ax,0C11h out dx,ax mov ax,0DF12h out dx,ax mov ax,0014h out dx,ax mov ax,0EA15h out dx,ax mov ax,0616h out dx,ax mov ax,0E317h out dx,ax Enough typing for me. See you ... P.S Many games and demos use x-mode ( DOOM etc.) but some groups on the demo scene don't use x-modes, maybe it's a new fashion ?