Re: ROM 3 Questions

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

Mitchell Spector <a_specto@alcor.concordia.ca> wrote:

> dempson@actrix.gen.nz (David Empson) wrote:
> 
> >william strutts <wrstrutts1@home.com> wrote:
> >
> >> I have two ROM 3 motherboards.  They are basically identical
> >> except for the actual ROM's.  The First machine has two ROM's
> >> marked 341-0748 and 341-0737.  The Control Panel Screen
> >> for this machine shows the options: Control Panel, Alternate
> >> Screen Mode.  The second machine has these ROM's marked
> >> 341-0728 and 341-0748 (this is a different size ROM than the
> >> other machine, it is four pins shorter than the other Machine).
> >
> >One of the machines probably has a pre-release version of the firmware
> >(most likely to be the 341-0728 ROM).  I don't have my ROM 3 handy, so I
> >can't check it against my own machine.  I recall looking into this
> >several years ago.
> 
>     That was more than seven years ago, and you indeed did. I recall
> your having done a complete disassembly and comparison between 
> the beta and standard (final; masked production) ROM 3 firmware in
> great detail.
> 
> >I don't remember the details off the top of my head, but there was only
> >a very minor change between the beta and final version of the ROM 3
> >firmware, consisting of one initialization routine somewhere in the
> >memory manager, I think.  My previous posting on the subject predates my
> >current News software, so I don't have a copy of my earlier posting on
> >hand.
> 
>     That's what Goggle/DejaNews is handy for. :) I just looked it up
> and found it without too much difficulty.

Thanks for digging that up.  I thought those archives had stopped
working, so I didn't even bother looking (besides, I was offline when I
wrote that article).

>     Speaking of the ROM 3, I noticed my replacement board either
> automatically activates hardware shadowing of text page 2 or it's
> always flagged to 'on' via the Alternate Display Mode CDA. I had 
> a look an emulated IIgs in KEGS32 (both a ROM 01 and ROM 3) 
> and it acts identically. I can't seem to disable it with the CDA so
> I'm not entirely clear on why the CDA is needed on the ROM 3.

If you have alternate display mode enabled (i.e. text page 2 shadowing
is on) for a ROM 3, there will be a small performance hit for all memory
writes in locations in the range $0800 to $0BFF in bank 0, with the
write being forced to run at 1 MHz so that it can be mirrored into bank
$E0.

Under ProDOS-8, while running BASIC.SYSTEM, this is the code area for an
Applesoft BASIC program, which would only be written when loading a
program into memory, during editing, or variables being written for a
very small program.

For other ProDOS-8 applications, it would be necessary to determine how
this area is being used on a case-by-case basis.

Under GS/OS, this area is almost always stack or direct page space for
the current application, and may get accessed frequently.

You probably wouldn't be able to notice the performance hit unless the
program was using this area very often.