Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!uxc.cso.uiuc.edu!uxc.cso.uiuc.edu!ux1.cso.uiuc.edu!uxe.cso.uiuc.edu!mcdonald
From: mcdonald@uxe.cso.uiuc.edu
Newsgroups: comp.sys.ibm.pc
Subject: Re: Why are bitmap save/restores so slo
Message-ID: <110200005@uxe.cso.uiuc.edu>
Date: 18 Sep 89 14:14:00 GMT
References: <3776@titan.camcon.co.uk>
Lines: 47
Nf-ID: #R:titan.camcon.co.uk:3776:uxe.cso.uiuc.edu:110200005:000:1742
Nf-From: uxe.cso.uiuc.edu!mcdonald    Sep 18 09:14:00 1989


>mcdonald@uxe.cso.uiuc.edu <110200003@uxe.cso.uiuc.edu> :
>-
>-There are extremely efficient ways of copying areas from region
>-to another of video memory. And, to copy to or from ordinary memory,
>-retaining the planar maps, is very fast indeed. So is drawing
>-lines or circles or text in a fixed color. The programmer just has
>-to understand how it works and choose an efficient way to do it.

>Okay.  Any tips on how to do this?

To copy from one part of EGA/VGA memory to another you first realize that
it is stored (in C) as

unsigned char screen[480][640];       

that is first the upper left corner, then proceeding across the screen,
then down.

To copy a region from one place in screen memory to another, just
write code to copy the bytes. In C, you might want to use memcpy or
memmove. In assembler, use the byte string move instructions.
This is done with the control registers of the EGA/VGA in their
normal state as set by a bios call to set mode 16 or 18.

For example, to copy the top two rows to the 3rd and forth rows:

char far *p1;
char far *p2;
int i,j;

p1 = (char far *)0xa000000;
p2 = p1 + 1280;
for(i = 0; i < 1280; i++)*p2++ = *p1++;

(Now for the weird part!!!!! You can replace *p2++ = *p1++ with
*p2++ = - *p1++ or *p2++ = *p1++ +12345; and it will still work.
Each byte operation in C copies all four bit planes.)

To read the color bitplanes to or from ordinary memory, you copy each
individual plane one at a time.This requires diddling the IO registers.
It is complicated enough that you will need to read a book to see how
to set them. Once set, however, the graphics memory behaves just
like ordinary memory (no four bytes copied at once). Just be sure to
return the registers to normal.

Doug MCDonald