Apple II
Technical Notes
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                                                  Developer Technical Support


Memory Expansion Card
#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.
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Question:    What screen holes does the Memory Expansion Card firmware use?
Answer:      The Memory Expansion Card uses the following screen holes:

             $478 + slot  numbanks number of 64K banks (256K = $04, 512 = $08)
             $4F8 + slot  powerup  powerup byte ($A5)
             $578 + slot  power2

             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 requirements.

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:

                 LDA #$1
                 LDY $42
                 CMP #4
                 BCS Cn8E

             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.