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From: naughton%wind@Sun.COM (Patrick Naughton)
Newsgroups: comp.sys.ibm.pc,comp.graphics
Subject: Re: VGA palette & memory: HOW??
Keywords: vga, palette
Message-ID: <88069@sun.uucp>
Date: 2 Feb 89 17:12:49 GMT
References: <109@jarthur.Claremont.EDU> <15102@oberon.USC.EDU>
Sender: news@sun.uucp
Reply-To: naughton@sun.com (Patrick Naughton)
Organization: Sun Microsystems, Mountain View
Lines: 30

In article <15102@oberon.USC.EDU> annala@neuro.usc.edu (A J Annala) writes:
>I thought the CGA adaptor was capable of 256 simultaneous colors ... or
>is that another IBM PC adaptor?  In any case, can someone fill me in on
>the capabilities of the 256 color IBM PC adaptor?  Can it generate 256
>grey scales ... 256 independently controlled colors ... or is it simply
>confined to whatever existing palette it comes with.

CGA must have been a typo (or a joke).  The CGA can only display four colors
in a 320x200 pixel mode.  (There are four different sets of four ugly colors
to choose from)

The VGA displays 256 colors from a palette of 262144 colors.  You can control
which of the 256 colors are displayed by setting the palette with the 256
R, G, B triples you want.

The major restriction of this is that IBM wanted to save money on parts so
they used 6 bit DACs rather than 8 bit DACs.  So each channel can only have
64 levels, (1<<6), thus you cannot have 256 grey scales, or 256 blue scales,
only 64.

One good thing is that most VGAs on the market today will drive MultiSync
monitors to 800x600 and 1024x768 pixels, although the IBM standard only specs
640x480 as the highest resolution.

-Patrick

    ______________________________________________________________________
    Patrick J. Naughton				    ARPA: naughton@Sun.COM
    Window Systems Group			    UUCP: ...!sun!naughton
    Sun Microsystems, Inc.			    AT&T: (415) 336 - 1080