Path: utzoo!utgpu!jarvis.csri.toronto.edu!helios.physics.utoronto.ca!ists!yunexus!maccs!cs4g6ag
From: cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Why no graphics in text mode?
Message-ID: <25B0D02D.14525@maccs.dcss.mcmaster.ca>
Date: 14 Jan 90 19:17:00 GMT
References: <25820@cup.portal.com>
Reply-To: cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn)
Organization: McMaster University, Hamilton, Ontario
Lines: 36

In article <25820@cup.portal.com> cliffhanger@cup.portal.com (Cliff C Heyer) writes:
$I'm trying to figure out why there is this division 
$between text and graphics modes with PCs. 
$I'm assuming that each screen pixel is represented 
$by a bit in memory (or several bits) even in text 
$mode - but in text mode the bits are written with 
$bit maps from the BIOS.

   Well, I think we need a little lesson on how memory usage differs between
text and graphics modes.

   In text mode, each byte in memory represents one character, and the byte
holds the ASCII code of the character to be displayed.  A second byte is used
to hold the attribute (colours, underlining, blinking, etc.).  When the
display card displays this on the screen, the ASCII code is used as an
offset into a table stored in ROM (the character generator) which provides
the actual bitmap of the character.  Do not confuse this ROM with the BIOS,
because it isn't part of it.

   In graphics mode, each byte in memory holds bitmap information (the
number of pixels represented by one byte depends on the graphics mode - in
Hercules graphics mode, for example, where a bit can be either on or off,
eight pixels can be stored in one byte).  If you display text on such
a screen, what actually happens is that the computer looks up the bitmap for
the character in the character generator and places that bitmap into the
display memory.

   So you can see that in text mode, there is no way to store graphics,
and in graphics mode, the computer performs the character generator lookup
itself when you display the text, rather than having the display card do
this lookup every time it refreshes the screen.
-- 
Stephen M. Dunn                               cs4g6ag@maccs.dcss.mcmaster.ca
          <std_disclaimer.h> = "\nI'm only an undergraduate!!!\n";
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    If it's true that love is only a game//Well, then I can play pretend