NetNews Usenet Archive 1992 #27/NN_1992_27.iso/spool/comp/sys/ibm/pc/hardware/30734

Internet Message Format | 1992-11-24 | 1.9 KB

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From: [e-mail] (robert.k.nichols)
Subject: Re: Graphics Accelerator Cards
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: AT&T
References: <[e-mail]> <[e-mail]> <[e-mail]>
Date: Tue, 24 Nov 1992 01:56:12 GMT
Lines: 26

In article <[e-mail]> [e-mail] writes:
...(Quoted article deleted)...
>  On the contrary, reducing colors from 256 to 16 reduces the number of
>possible colors by 16, but will reduce the number of bits/pixel by a factor of
>only 2 (8-bits vs 4-bits per pixel).
...

Actually, it's a bit more complex (and potentially better) than that.
A VGA card in 256-color mode uses a simple 1-byte-per-pixel mapping in
the (paged) video memory space.  When you switch to 16-color mode, the
memory is arranged as 4 separate bitplanes, so that a 1-byte access
gets you 1 bit from each of 8 pixels.  Now this can be a real pain if
you're trying to update all 4 bits for each pixel.  However, for things
like text scrolling, all you're trying to do is switch pixel colors
between the foreground color and the background color.  A good text
scrolling program will arrange the pallette such that the foreground
and background colors differ in only one bitplane.  Thus, the program
can work within a single bitplane and manipulate 8 (or even 16) pixels
with each video memory access.  This yields a 8-to-1 speedup over the
256-color mode.

Of course, this all breaks down if you're displaying multi-colored text.

Bob Nichols
AT&T Bell Laboratories
[e-mail]