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8.4 AUXILIARY MEMORY MANAGEMENT
Initializing The Card
When the Apple is first booted from a cold start, auxiliary bank 0 (zero) is
selected. This is the //c’s own built-in auxiliary 64K memory.
A program, however, should not assume bank 0 (zero) will he immediately
available to it. A program may have been run previously that left another bank
selected. Make it a practice to select bank 0 (zero) when first addressing the
card and when exiting your program. This will ensure error free operation for
your program and programs run after it.
Identifying MultiRam C Memory
After selecting auxiliary bank 0 (zero), your program should next determine how
such memory is available on the card for use by your program.
Memory banks can be identified by writing a particular bit pattern into a
specific location or locations in a memory bank. Next read the memory location
that the pattern was written into and compare what is found there against the
original pattern. A 64K auxiliary memory bank exists if there is a match.
Many different techniques can be used to perform the write/read/Compare
operation. A preferred method is to use a descending loop that selects the
highest memory bank available on the card, writes the expected auxiliary bank
number and a checksum value into two bytes of that bank, decrements the bank
counter and then repeats the process ending with bank 0 (zero). This would be
followed by a descending loop that reads the bytes from the two bytes of each
bank and compares them against the expected value.
A descending identification loop is preferable as it consistently identifies
useable 64K banks from the highest possible bank number to the lowest.
Using a descending loop starting with $18 rather than $8 would ensure the
software developed for MultiRam C would also work with MultiRam //e and MultiRam
ROB memory cards. These cards can be combined in the //e to give op to 1.5
megabytes of memory. The ending 64K bank of their 1.5 megabyte combination is
defined as siB. In the I/c. selecting non—existent banks from $1B to $9 and
writing the bank number associated with that bank into memory causes no
problems; bank 0 is selected for all values above $8 and the last value written
to bank $0 in a descending loop ending with $0 would be $0.