Apple LaserWriter 12/640 PS - Manual (Page 180)

Installing SIMMs
The printer’s circuit board has two SIMM slots, marked BANK C/D (on the
left) and BANK A/B (on the right), for installing additional memory. RAM
is provided on SIMMs (Single Inline Memory Modules) that you insert into
the slots.
Depending on the amount of memory you’re installing, you may need to
install one or two SIMMs, and perhaps remove a SIMM from a slot in order
to insert one that has more memory. For example, when installing 20 MB of
RAM, you must remove the 4 MB SIMM from bank A/B and reinstall it in
bank C/D.
When removing SIMMs, always begin by removing the SIMM in bank A/B
first. Removing a SIMM in the C/D slot first can be difficult because of the
proximity of the slots.
When installing SIMMs, always begin by installing the SIMM in bank C/D
first. Installing a SIMM in the A/B slot first can make installing a second
SIMM in bank C/D difficult because of the proximity of the slots.
Bank C/D
Bank A/B
155 Installing Options