🚀 go-pugleaf

etljmme@etlxd10h.etlxd (jim moore xd/nda) writes:

>Hi Paal.

>  Yes you can access a buffer which you declare in unused video ram but if you do,
>why not use page swapping rather than copy it across?  All page swapping involves is
>telling the VGA to start the next scan from another place in VGA memory anyway!

    [code deleted]

>I know this works in 320x400x256 mode so it should work in 320x200 mode.  Be sure to
>clear your buffer or page before you use it otherwise your screen will fill will
>garbage.

     I don't mean to say you're wrong, but unless you're using ModeX or
     a derivative of it, this won't work.  Standard VGA mode 13h takes
     up 64K (320x200x1byte) which nicely fits into the video memory
     page frame located at segment A000.  There is no way to write to 
     the rest of the video memory.  Which is why there is a modeX.  In
     this mode, there are 4 64K pages (0-3), where each page 'n' stores all
     pixels that OFFSET MOD 4=n, so the offset that you use into the 64K
     page frame is really divided by 4, so in the 64K page frame you can
     access all 4 pages of video memory, which is 256K.  So writing one
     byte will light up 4 adjacent pixels.  Then, to only light up 
     individual pixels, you turn on/off the cooresponding bit planes by
     using standard VGA port writes.  

     Then, and only then, can you access anything past the first quarter 
     of VGA memory and use the code that was posted for setting the
     starting address.

     Hope this clears things up.

-- 
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  John A. Slagel, email: jas37876@uxa.cso.uiuc.edu