The Making of the MCM/70 - Inventing the PC - page 50

located 32K of memory for that purpose (the so-called APL
workspace) to each APL user. This was much more than the
8008 could handle directly; the chip could address only 16K of
memory, 6K of which was already allocated for the core portion
of ROM . That left the MCM engineers with no more than 10K
of APL workspace, which was insufficient for all but the most
trivial applications.
To solve this “ RAM shortage” problem, MCM built a virtual
memory system into the MCM /70. Briefly, a virtual memory
system uses external storage to extend its RAM capacity. When
the execution of a program requires more RAM than is cur-
rently available, some part of the RAM that contains informa-
tion not vital to the current state of the program's execution
is copied into the external storage, freeing enough RAM space
to complete the computation. The relocated information is
brought back from the external storage to RAM when needed.
Virtual memory on MCM 's computer was a matter of necessity.
Without it, there would be no APL on the MCM /70; most of the
memory of the little computer would be consumed to store a
program and input data, and very little (if any) would be left for
the program's execution.
“Virtual memory was mine,” said Arpin. “There were a num-
ber of things that were obvious. Like … we had to be able to
save the workspace. And it became obvious when we worked
with the [prototype of the] machine that it was too small.” In
the 1960s Arpin was developing software for Canada Life In-
surance Company “that took advantage of loading programs in
and out and actually loaded data in and out as well. So it was
obvious to me,” continued Arpin, “that you could do that [for
the MCM /70].” What André Arpin and José Laraya came up
with was the implementation of virtual memory using a spe-
cially designed digital cassette drive as external storage. Look-
ing back at the adopted solution for virtual memory, Arpin
commented that “doing that on a digital tape, on a cassette