Re: Mode 13h question

[Bob Pendleton <bobp@pendleton.com>]

Cobra176@aol.com wrote:
> 
> In a message dated 3/8/00 3:05:57 PM Central Standard Time,
> anime6@earthlink.net writes:
> 
> << The best way I found to plot pixels is to use an unsigned char far pointer
> to the memory address 0xA0000000L, which is the begginning memory address for
> the video memory in mode 13h.  So to plot a pixel all you need to do is after
> you put the video card in mode 13h: >>
> Question: When plotting a pixel within that addressed memory, why doesn't the
> pixel go to the system memory? Does the video card do funky stuff to
> intercept the memory call or something?

Yep. In the original lay out of the original PC back in the early '80s a
64K chunk of the original 1 megabyte of address space was reserved for
the video display. That is, the video display just looks like a chunk of
memory. You write to the memory and what you write is displayed
depending on the display mode. If the mode is a character mode then what
you write is displayed as displayed as pixels. 

It took a long time for the memory used by the display to be bigger than
64k. The VGA display is designed around the idea of having 256K of
display memory mapped into a 64k window. The Vesa display modes were
added to support displays a lot bigger than 256k mapped into a 64K
window. Now days most video displays are mapped into high memory (above
1 meg barrier) where there is enough address space to map a
multimegabyte display.

Of course, over time more display functions were moved into the video
card in the form of graphic accelerators which reduces the value of
having a memory mapped display. But, it is still handy for getting
pixels BACK from the video display.

Because operating systems assume that there is a VGA display parked in
the original 64k reserved for the display in the original PC, most
displays still provide an interface that looks just like an old VGA
display.

			Bob P.

> =================================================================
> The GameProgrammer.Com mailing list is for the open discussion
> of any topic related to the art, science, and business of
> programming games. This list is especially tolerant of beginners.
> We were all beginners once
> 
> To SUBSCRIBE or UNSUBSCRIBE please visit:
> http://gameprogrammer.com/mailinglist.html
=================================================================
The GameProgrammer.Com mailing list is for the open discussion
of any topic related to the art, science, and business of
programming games. This list is especially tolerant of beginners.
We were all beginners once

To SUBSCRIBE or UNSUBSCRIBE please visit:
http://gameprogrammer.com/mailinglist.html