Memory Expansion #1
Questions and Answers
Revised by Mike Askins & Matt Deatherage (November 1988)
Written by Cameron Birse (April 1986)
This Technical Note documents many of the questions and answers
concerning the Memory Expansion Card which are not covered in its manual.
- Question: What screen holes does the Memory Expansion Card
- Answer: The Memory Expansion Card uses the following
- $478 + slot
- numbanks: number of 64K banks (256K = $04, 512 =
- $4F8 + slot
- powerup: powerup byte ($A5)
- $578 + slot
- These screen holes are not cast in concrete and may change with a
new revision of the firmware.
- Question: Why does RESET turn off the Memory Expansion Card
registers until an access to the $Cn00 space?
- Answer: The reason $Cn00 enables the registers was to
optimize speed and the number of pins and logic on the custom gate array.
The boot scan hits $Cn00 anyway and enables the registers.
- Question: Will any access (read, write, or status) to the
firmware cause the Memory Expansion Card to format itself?
- Answer: Yes, any access to the firmware will cause it to
format itself to the current operating system (DOS 3.3, Pascal, or
ProDOS), assuming it is not already formatted.
- Question: Why isn't the Memory Expansion Card marked as a
non-interruptible device? What if an interrupt occurred during access to
the card and the interrupt handler also accessed the card?
- Answer: The Memory Expansion Card is not marked as a
non-interruptible device because it would not be fatal to have an
interrupt occur during an access to the device. Obviously, the interrupt
handler would have to save and restore the registers as well as update the
"free block" bitmap, so when the handler returns control the program does
not overwrite the new data. The reason other devices are marked as
non-interruptible is due to timing dependent read and write
- Question: Why does the Memory Expansion Card fail to format
if the powerup screen hole contains the value $A0?
- Answer: The firmware checks the screen holes for $A0 values,
and if they are all $A0, it assumes that someone made a mistake and
cleared the screen improperly, filling the screen holes with spaces. In
this case, the firmware does not want to reformat and lose all the files
on the RAM disk.
- Question: The code at $Cn5A has the following sequence, and
does not seem to make sense:
Shouldn't the CMP #4 be a CPY #4?
- Answer: Yes, this is a known bug that will be fixed if the
ROMs are ever revised. The bug by itself was not considered significant
enough to justify a revision. Note that this is corrected in the Memory
Expandable Apple IIc.
- Question: If DOS formats the Memory Expansion Card, ProDOS
cannot reformat it without a power down or using a ProDOS application
which formats disks. In other words, it does not reformat itself when I
boot into a new operating system. Isn't that a bit severe?
- Answer: This is no different than any other disk device.
ProDOS does not have a format command, so you cannot just format from
ProDOS without having the formatter installed and some means for calling
it. Additionally this was done intentionally so that you could load DOS
files into the RAM card and be able to boot ProDOS and use the CONVERT
program to convert the DOS files to ProDOS.
This and all of the other Apple II Technical Notes have been converted
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