Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!zephyr.ens.tek.com!tektronix!percy!parsely!bucket!leonard
From: leonard@bucket.UUCP (Leonard Erickson)
Newsgroups: comp.sys.ibm.pc
Subject: Re: MSDOS's recognition (?) of screen modes
Message-ID: <1869@bucket.UUCP>
Date: 31 Dec 89 01:35:33 GMT
References: <4703@itivax.iti.org>
Organization: Rick's Home-Grown UNIX; Portland, OR.
Lines: 41

dhw@itivax.iti.org (David H. West) writes:

>The part I don't understand is as follows: I invoke the utility to
>set the card into, say, EGA graphics mode - I think the mode number
>is 16, but the manual is at home.  I then get a DOS prompt in a
>different text font.  What is happening here - is command.com
>temporarily returning the screen to (some other) text mode?  Or is
>the card clever enough to know it needs to simulate text even though
>it's "really" in a graphics mode?  Or does the card vendor's utility 
>not actually set a specific graphics mode, but rather enable a
>particular kind of emulation (even though I'm passing it a "mode 
>number", specifying number of pixels and colors, as a parameter)?
>And how does this square with the fact that if I set a "wrong"
>graphics mode, I don't even get a DOS prompt afterwards?

What's happening is that when you set those modes, the BIOS data area
in low RAM gets some data values changed. Video mode (of course!),
number of columns on screen (normally 40 or 80, but both 90 and 132
are settable and usable with the proper hardware). screen size (in K)
number of rows of characters. Height of Characters (in scan lines)
etc etc

The height in scan lines is what triggered the font change. The system
saw that you now were in a mode that should be using the 8x8 font
instead of the 9x14 font. If you think waaaay back... to BASIC (anybody
remember BASIC) it had to be able to print text in graphics modes. As
I recall, the CGA modes are "hardwired to the 8x8 fonts and mode 7 is
hardwired to 9x14. Everything else is free to set things up as they
please. (It was ega where the rows and character height varaibles
were added)

I'm all *too* familiar with this after helping put together a TSR to 
allow us to use the 132 column capability of some of our video boards
with a terminal emulator package (Thank God the people who wrote the
emulator had a sample chunk of code available!) We were going nuts until
we noticed the columns variable... duh! 
-- 
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