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From: jkg@gitpyr.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: changing character sets
Message-ID: <3139@gitpyr.gatech.EDU>
Date: Tue, 24-Feb-87 20:09:44 EST
Article-I.D.: gitpyr.3139
Posted: Tue Feb 24 20:09:44 1987
Date-Received: Fri, 27-Feb-87 05:08:58 EST
References: <1303@erix.UUCP>
Reply-To: jkg@gitpyr.UUCP (Jim Greenlee)
Distribution: world
Organization: Georgia Institute of Technology
Lines: 51

OK, I'm going to stick my neck WAAAAYYYY out and answer this one. This
applies to the MDA or CGA in character modes ONLY. I have no experience
whatever with Hercules cards or the EGA.

In article <1303@erix.UUCP> joe@erix.UUCP (Joe Armstrong) writes:
>        Dynamically  (i.e.  not at boot time)  Change the  system character
>sets to my own character sets - at any time I'd  like to be able  to poke a
>new character definition into the machine.

>        1) Are the characters sets in RAM

No, there is a mask-programmed ROM on both the monochrome and color graphics
adaptors which cannot be changed (except by removal and replacement with a
different character ROM). It is usually a MOSTEK series 36000 8K x 8 ROM which 
handles ASCII character to pixel translation.

>        2) If so at what address

The ROM is not even in the CPU address space! The address lines of the ROM
are driven by the data outputs of the video RAM plus the row address lines 
of the 6845 CRT controller. You couldn't access the ROM directly if you
wanted to.

>        3) How are the characters packed

Interesting question. IBM used some goofball addressing scheme that makes 
the raster rows non-contiguous. I think the reason is so they could support 
all the character modes on the CGA without duplicating pixel data. I had
quite a bit of fun figuring this out (luckily, I had a Technical Reference
Manual handy).

>        4) If I change the RAM character set (I assume it is in RAM)
>           can I use bios calls to output the characters or what?

Don't know about this one. It should be possible to do define your own 
characters in graphics mode (this is how programs like PC-Paint and PC
StoryBoard get away with it), but it would require a fair amount of CPU 
overhead to manage the whole thing - in other words it would be really
ssllloooooowwwwww. My guess is that you would have to do direct screen 
writes to make it work.

Any MDA or CGA compatible card should use this same method for character 
formation. All bets are off for the EGA - I don't know anything about how 
it handles character formation. 

						Jim Greenlee
-- 
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