This article is from the Macintosh system software FAQ, by Elliotte Harold firstname.lastname@example.org with numerous contributions by others.
Under system versions earlier than 7.0 or under System 7.x
without 32-bit addressing turned on the Mac cannot access more than
eight megabytes of real memory. If you have more physical RAM
installed, the Mac knows it's present but can't do anything with it.
When About this Macintosh (About the Finder in System 6) is selected
from the Apple menu, the system reports all the memory it can't use
as part of the system memory allocation.
To use the memory you need to install System 7 and turn on
32-bit addressing in the Memory control panel. If you have a Mac
with dirty ROMs (a II, IIx, SE/30, or IIcx) you also need MODE32.
MODE32 is free from the mythical friendly neighborhood dealer or
The original Mac II also needs the FDHD ROM upgrade to use 4 megabyte
or larger SIMMs in Bank A. Without it SIMMs larger than one megabyte
can only be put in the second bank of memory on a Mac II. If you're
staying with System 6, Maxima from Connectix ($45 street) allows you
to use up to fourteen megabytes of real memory and can allocate
anything beyond that to a RAM disk.
If you have an LC or an LC II with four megabytes of RAM
soldered to the motherboard, you still need to add two four-megabyte
SIMM's to reach the ten megabyte maximum imposed by the LC ROM.
This means you'll always have two unused megabytes which About this
Macintosh and About the Finder report as part of the system memory
allocation. Unfortunately there is no current means of accessing
this extra memory.
If you've turned on 32-bit addressing or if you have eight
megabytes or less of RAM, check your disk cache (RAM cache in
System 6) in the Memory Control Panel (General Control Panel in
System 6) to make sure it isn't set exceptionally high. All
memory allotted to the cache comes out of the System's
Finally if you recently upgraded to System 7.1 by updating your
system software rather than by doing a clean reinstall, (See question
4.6 in the general FAQ) you should move all fonts out of your system
file as these can take up an extraordinary amount of memory.