🚀 go-pugleaf

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!

Here's how to flip the visual page from the default (at $A000:0000) to another.

Let's assume we have a page in video ram at $A000:8000. To select which page we want
to make visual, we can use this code (where p = page number, 0 or 1):

  PROCEDURE VisualPage(p:byte);
  VAR ofs : word;
  BEGIN
   ofs:=$8000*p;            { These lines calculate the offset for page 0 at 0000  }
   ofs:=ofs + $0C;          { or page 1 at 8000, the +$0C is arbitrary             }
   asm
    mov  dx,03d4h           { These lines redefine the VGA CRTC registers so that  }
    mov  ax,ofs             { the next scan starts at ofs, thereby making the page }
    out  dx,ax              { visual.  This is faster than copying from a buffer   }
   end;
  END;

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.

Good luck, Jim.