Re: IIgs peculiarities

[Sheppy <the.sheppy@gmail.com>]

On Feb 21, 7:34 pm, vladitx <vlad...@nucleusys.com> wrote:

I'll answer the ones I know off the top of my head.

> 1) IIgs ROM 03 has additional bit in the shadowing register (and the
> default 'enable') to include Text Page 2 ($800..$BFF), but the
> firmware keeps it disabled for some reason. And the "Alternate Display
> Mode" option is still available in the Control Panel.
> Where is the logic to this? Just leaving the register to shadow Text
> Page 2 is the way to go, and no "Alternate Display Mode" is needed at
> all.

I believe this is for compatibility with ROM 01.

> 2) I tried manually setting "Slow" speed by changing the high-bit of
> the appropriate register (through Monitor), but nothing happens. And
> the content of the register doesn't change. Is there IRQ subroutine
> keeping overwriting it? Or is there a special way to access it?

It's been a while since I did this but it's not quite as simple as it
seems.  I'd have to go look at some old code to remember exactly what
the deal is though.

> 3) IIgs maximum RAM - banks $00 .. $7F are specified as RAM range.
> This makes exactly 8MB. But the manuals keep saying 8.25MB. What is
> the truth and where those 256KB came from?

There's 128K of memory in banks $E0 and $E1, which is the shadow RAM
for banks $00 and $01.  I've not seen the 8.25 MB figure; it's usually
quoted as just 8 MB.

> 5) The memory expansion board may contain ROM, banks $F0+. Does the
> FPI know the installed ROM on the motherboard and redirect the other
> memory range to the expansion? Is pin #57 of FPI configuring this or
> FPI chips on ROM 03 are newer?

The built-in ROM is located in banks $FE-$FF on the ROM 00 an ROM 01,
and $FC-$FF on the ROM 3.  The expansion slot ROM is always in the $F0-
$F8 range.

> 6) If a ROM disk is to be present in banks $F0 .. $FB, what the format
> should be? Just a ProDOS image which then shows with it's volume name?

IIRC, the format is that the first bytes in the ROM memory need to be
"ROMDISK" followed by a disk image.

Sheppy