Date   : Thu, 24 Feb 2005 23:50:11 +0000
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: Mode 2 text

"A.Weston" <a.weston2@...> wrote:
> There was a weird bug with VDU22 though on my old BBC Micro in that when
> I used it to change mode I got a few residual pixels at the top left of
> the screen.
 
I think you mean that there was a weird bug in your programs that
used VDU 22. If you don't ensure HIMEM is moved below where the
screen is going to occupy, then BASIC's data will appear in the
screen. It sounds like you've got the variable heap crawling into
screen space, like with:
 
MODE 7        :REM Select mode 7, reset BASIC's stack to &7C00
LOMEM=&300    :REM Put BASIC's heap at &3000
VDU 22,1      :REM Change screen mode to occupy &3000-&7FFF
fred$="Hello" :REM Add data to BASIC's heap, visible in the screen
 
Same thing would happen at the bottom right with data added to
BASIC's stack:
 
PROCfred(x)   :REM parameter pushed onto stack, visible on screen
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
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