If you have more than 8MB of RAM installed in a Macintosh, you can access only
8MB of that RAM unless you have 32-bit addressing turned on in System 7 or
you're using A/UX. With 32-bit addressing turned on, you can access up to 128MB
of physical RAM. under System 7.
To turn 32-bit addressing on, turn 32-Bit Addressing on in the Memory control
panel and restart.
If you're running System 7 in 24-bit addressing mode, or any version of system
software before 7.0, any RAM beyond 8MB is treated as if it is already being
used by the System heap, and is unavailable for your use.
In your case, a Macintosh Quadra 700 has 20MB of RAM, 12MB of which is
automatically allocated to the System heap and is unused. If the System is
reported as using 13MB, then the System is actually occupying 1MB of the
available 8MB of RAM.