In 24-bit mode using virtual memory, total RAM can be set up to 13MB,
depending on the CPU and the number and placement of NuBus cards. This
memory is not a contiguous block, however. The largest contiguous block
will be roughly 7.5MB.
Viewing the About this Macintosh window can appear confusing because the
totals for RAM used by programs may seem to add up incorrectly. A total of
16MB of RAM is addressable under System 7. Some of this is memory
reserved for ROM (1MB), IO (1MB), video for the Macintosh IIci or
Macintosh IIsi (1MB), and NuBus cards (1MB each). This memory is mapped
starting above the 8MB mark. Any free memory in the upper 8MB of RAM (up
to the 5MB possible) forms an "upper memory block."
Under System 7, system heap first loads into the bottom 8MB of memory
then programs, starting with Finder, load into the upper memory block,
provided there is a large enough contiguous block there to hold the
program. If there is not a large enough contiguous block in the upper
memory, the program will load in the lower memory block. Thus it is
possible to have a lower unused block of about 7.5MB (the lower 8MB minus
system heap) and an upper unused block of about 4MB (or less). Viewing
About this Macintosh lists only the largest unused block of memory, not
the total unused memory.