Date   : Mon, 31 Jan 2005 09:51:50 +0000
From   : Richard_Talbot-Watkins@...
Subject: Re: Video ULA shenanigans

Thanks for all that, jgh.

And cheers for finally explaining this:

jgh wrote on 30/01/2005 17:26:01:

> MA12 causes hardware screen wrap-around. In MODE 0 if you set the screen
> start address to &4000, then 3/4 of the way down the screen the CRTC gets
> to &7FFF, and then gets to &8000 with MA12 set. When MA12 is set, an
adder
> circuit between the CRTC and the memory adds an offset to the base of
> screen memory, so when the CRTC tries to access &8000 it accesses &3000.

I noticed that the address wrap-around appeared to only work once in a
screen refresh, e.g. if a 20k screen was displayed with the latch set for
an 8k screen, and I always presumed this to be a rather odd bug.  Now it
all makes sense - thanks!

Rich



**********************************************************************
This email and any files transmitted with it are confidential and
intended solely for the use of the individual or entity to whom they
are addressed. If you have received this email in error please notify
postmaster@...

This footnote also confirms that this email message has been checked
for all known viruses.

**********************************************************************
Sony Computer Entertainment Europe