Re: IIgs peculiarities

On Feb 22, 10:28 am, demp...@xxxxxxxxxxxxx (David Empson) wrote:
vladitx <vlad...@xxxxxxxxxxxxx> wrote:

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.

Probably because of a reason like some third party software was
expecting to be able to use text page 2 in bank $E0, and having it
permanently shadowing from bank 0 would have corrupted whatever was
there.

I suspected that. But then again, ROM 00/01 "Alternate Mode" will copy
data in banks $E0/E1 so a software relying on these 2x1KB chunks is
not very smart.
Also Memory Manager doesn't handle this region, which should further
stop developers from using it.

At least the ROM 3 Alternate Display Mode control panel simply toggles
the shadow bit, rather than having to copy memory in an interrupt
handler.

You're right! I just checked that - it does use the shadow bit. Neat!!

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?

You can't do this directly, because the KEYIN routine saves and restores
the register containing the speed. You have to call a subroutine which
sets the bit.

There's something fishy here. While I change system speed through
Control Panel (and make sure that it really changes), reading E0/C036
through Monitor still doesn't show change in bit7 - it is always set.

Maybe Sheppy will reveal the truth. BTW, this is on a 03.

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?

Anything saying 8.25 MB is simply wrong. The correct figure is 8.125 MB
(8 MB plus 128 KB).

The number 8.25 is found in several places in official Apple IIgs
documentation. Maybe a marketing misunderstanding.

4) Memory expansion - according to what I've seen the connector
provides for 2^10 rows/columns and 4 banks (CROW1/CROW0). This makes
4MB. How to go beyond this up-to 8MB? Is the signal "MSIZE"
responsible and how?

Memory cards which support more than four "rows" do so via a mechanism
which has compatibility issues. They watch for the bank register value
output by the CPU on the data bus in the first half of a cycle, and
latch at least one bit. Some of them latch the whole thing and ignore
the CROW1/0 bits, while others synthesize a third row select bit from
one bit of the bank select register.

This explanation fits well. Despite the marketing hype of "8.25MB",
hardware reference cites expansion of additional 4MB.

I still wonder what "MSIZE" signal does. It is shorted to ground on a
certain memory expansion board, so must be input to FPI.

The compatibility issue is that DMA cycles don't output a bank in the
first half of the cycle, so a memory card which uses this technique is
not compatible with cards in I/O slots which use DMA.

According to what I understand from a quick glance at ROM 01
schematics, I/O slots are bound only to MDBUS which is $E0. Which
should mean that DMA for the seven Apple ][ slots is only possible in
that bank.

Memory expansion is through FPI bus, where as you stated bank number
is output somewhere in the beginning of a cycle (sorry, still haven't
looked at 65C816 details).

There is one exception: the RamFAST SCSI card uses some trick to allow
DMA to work properly with these cards. I never heard the official story
of how they did this, but my speculation at the time was that the driver
does a normal CPU access to the target bank, which is sufficient to
latch the correct details into the memory card, then DMA cycles are
performed within that bank. It probably has to do with the CPU
repeatedly accessing the same bank to avoid bogus data appearing on the
data bus.

Are you sure DMA can have I/O in a bank different than $E0?

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 area in banks $F0 through $F7 is reserved for a ROM disk, which is
supported by the built-in SmartPort firmware (in slot 5). Some memory
cards provided ROM or static RAM in this area (I had an OctoRAM ESP).

So FPI doesn't block write access to this region? Nice for creating
runtime configurable ROM disks (devtools, for example).

The firmware just looks for signature bytes at the start of bank $F0 to
see if there is a valid ROMdisk present.

Banks $F8 and higher (up to $FB on a ROM 3 IIgs, or $FD on ROM 00/01)
are reserved for expanding the firmware ROM. I believe they do map to
the ROM expansion area on the memory expansion card, but nothing is
allowed to use them. Some cards might have bent the rules and used this
area to enlarge the ROMDISK, and been bitten when the built-in ROM got
bigger with ROM 3.

The split $F0..$F7 / $F8+ seems like just a software compatibility
guideline. FPI should output ROMSEL for the whole area.
Now it's quite easier - there won't be ROM 04. :-) So the whole area
$F0..$FB is there for ROMDISK, as no future firmware extensions would
be made.

To answer your specfic question: the FPI chips on the ROM 00/01 and ROM
3 are different. They even have different names: the ROM 3 on is "CYA"
(apparently short for "Control Your Apple").

Ok, sounds quite right.

Speaking of which, does anyone have schematics of ROM 03?

The ROM 3 version has different memory mapping rules to deal with the
larger built-in ROM, the greater number of fast RAM banks on the
motherboard, and the fact that they have one more multiplexed address
bit. The CROW0 and CROW1 signals are generated based on a higher start
address than for the ROM 00/01.

There is a signal on the 01 FPI named "1 Megabit". It could've been
only provision to support 1Mbit DRAMs, or something more ...

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?

It has to start with the word ROMDISK, but I don't recall the other
details. I think there is a special driver starting immediately after
the signature bytes, which is used by the SmartPort firwmare. Apart from
the driver, the rest can be anything the driver wants, but it would
typically be a ProDOS block device.

In principle, a ROMDISK device could use bank switching within the card
to implement even more memory than 512KB, as long as it stays within
banks $F0 through $F7.

I think $F7 is just a guideline. Nowadays we could exploit up-to $FB
without fear of breaking compatibility. ;-)

Thanks a lot for helping!
-- Vlad
.

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