Date   : Fri, 16 Aug 2002 09:41:13 +0100
From   : Richard_Talbot-Watkins@...
Subject: Re: Firetrack emulation again

Michael Foot <mjfoot@...> wrote:

> Do you know how the CRTC is told there is only 8 lines of data?
> I'll like to be able to emulate this properly.

I believe it's simply to do with the hardware configuration, and as
such is set in stone.

Basically, the CRTC has two groups of address lookup lines, named
MA0-MA13 (memory address lookup) and RA0-RA4 (row address for
character lookup).  Without a particularly in-depth knowledge of
all things "hardwarey", I would guess that MA0-MA11 are connected
to address lines A3-A14, RA0-RA2 are connected to address lines
A0-A2.  It may be something like RA3/RA4 are connected to the
display enable pin, so that instead of the expected 'wraparound'
within character rows when the scanline count per character is
greater than 8, we simply get a blanked display.

Caveat: I don't know my hardware stuff at all, but the bottom line
is that I don't think it's worth emulating at that kind of level -
I'd say it was sufficient to know that characters after scanline 8
output blank lines :)

Rich




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