Date   : Fri, 16 Aug 2002 15:15:44 +0100 (BST)
From   : jim <jim@...>
Subject: Re: Firetrack emulation again

On Fri, 16 Aug 2002 Richard_Talbot-Watkins@... wrote:
> 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.

This is in fact the case (well, RA3 anyway, RA4 isn't used). As far as the
CRTC is concerned it's just got a 10-line character to display, but
because of the way its address pins are connected up lines 0-7 read
successive bytes of memory and 8-9 are blanked. 

The beeb's display hardware is wired up in a really wacky way. The full
circuit diagram is quite instructive if you can get your head around it ;)

jim
-- 
http://madeira.physiol.ucl.ac.uk/people/jim/
                                              - Bill Stickers is innocent!