Re: Hopefully last dumb newbie question.

[dempson@actrix.gen.nz (David Empson)]

Slick <PAYSlick_RCBD@hotmail.com> wrote:

> Mitchell Spector wrote:
> > 
> >     It looks as though another side affect of my particular ROM 3
> > is Alternative Display Mode, by default, is always set to 'on'
> > and cannot be disabled. I'm not sure if this can affect my system
> > in any negative way (the CDA's being locked in are just annoying
> > mostly, since the couple of bytes of memory they use up aren't
> > anything to worry about). At least the hardware shadowing doesn't
> > slow down my system.
>
> One problem is it slightly slows down the system.

You are talking about a ROM 3, I assume?  Alternate Display Mode causes
quite a noticeable slowdown on a ROM 1 booted via ProDOS-8, less
noticeable if GS/OS was booted (which already slows things down a
little, due to VBL interrupts being enabled).

The only slowdown on a ROM 3 would be due to the synchronization delays
introduced by shadowing writes in locations $0800-$0BFF of bank 0 into
bank $E0, i.e. 1 MHz writes instead of 2.8 MHz.  Nothing outside that
address range is affected.

This would have a variable effect depending on what was using that area
of memory.  Applesoft BASIC programs normally run in that area, but they
aren't written to (except while editing or being loaded).  Under GS/OS,
it would tend to be application or toolbox direct page/stack space
(reasonably major impact).

> I noticed that it also slows down the text screen in GS/OS.

I wouldn't expect that (on a ROM 3).

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand