🚀 go-pugleaf

In article <6567@bwdls58.bnr.ca>, mlord@bwdls58.bnr.ca (Mark Lord) writes:

> <But trapping individual machine language instructions that access video
> <memory would be incredibly slow no matter how smart you did it.  Windows 3.0
> 
> The "smart" method, used by DesqView-386 and perhaps others, is to simply use
> the 386 virtual memory to remap "normal" memory in place of the task's 
> apparent "video memory", and let it write away at will.  The supervisory code
> periodically refreshes the window on the "real" screen from the virtual windows
> of each such nasty program.

For graphics this doesn't really work since EGA and VGA use 4 (or more for
SVGA) bits per pixel and a simple graphics processor to load the data. Thus
what appears to be a simple memory write to a memory manager is really
more like an IO operation to a device register. It's necessary to know
how the graphics processor is set up to know what is actually being done
with a write to graphics memory. Each memory address is really affecting
several color planes and/or a mask of bits, so just mapping the video
memory into another RAM block won't capture what is happening on the screen.

Ralph