Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!umich!samsung!think!mintaka!mit-eddie!uw-beaver!zephyr.ens.tek.com!tektronix!psueea!parsely!bucket!leonard
From: leonard@bucket.UUCP (Leonard Erickson)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Why no graphics in text mode?
Message-ID: <1920@bucket.UUCP>
Date: 15 Jan 90 09:00:25 GMT
References: <25820@cup.portal.com>
Organization: Rick's Home-Grown UNIX; Portland, OR.
Lines: 26

You are only half right. In graphics mode a given byte of screen RAM
is displayed as a bit pattern on the screen. It *is* that bit pattern.

In text mode, that byte is either:
1. a character code
2. an attribute code

If it is a character code, it represents an index value into a character
table. But there are only 256 entires in the table, and they are all used
by characters. They are also not arbitrarily placable on the screen.
So they can't be used for graphics except in very odd cases of limited
applicability.

If it is an attribute code, it defines a "mask" to to be applied to the data
before displaying it. This is handled in *hardware*. The attributes on
are foreground/background colors on a color display, and things like
inverse, invisible, and underline on a mono screen.

Look at it another way. In text mode you only have 4k of memory for the
screen data. There's no way that can contain the required info for
graphics.
-- 
Leonard Erickson		...!tektronix!reed!percival!bucket!leonard
CIS: [70465,203]
"I'm all in favor of keeping dangerous weapons out of the hands of fools.
Let's start with typewriters." -- Solomon Short