The Making of the MCM/70 - Inventing the PC - page 49

The full description of MCM / APL appeared in the MCM /70
User's Guide . In the guide, as well as in the MCM promotional
literature, MCM / APL was declared a powerful and natural
small-system language, easy to learn and easy to use. “You
will find that, after only a brief exposure to the [ APL ] language
and to the MCM /70, you will be performing computer appli-
cations which normally take you weeks to do with any other
language.” 9 The 270-page guide, authored by Smyth, is a well-
written early document on the general architecture, functional-
ity, and programming of one of the first microcomputers.
When Ramer agreed to lead the software development for the
MCM /70 computer, he understood quite well that his success
would hinge upon the hardware engineers finding some creative
way to substantially extend the 16K of memory that the 8008
chip was designed to operate with directly. The first solution
to the memory problem Laraya and his group adopted was to
divide the computer's ROM into the core portion of 6K and a
number of 2K blocks, called banks. The core portion of ROM
contained the minimum software necessary to operate the com-
puter at any given time. The banks, on the other hand, con-
tained separate pieces of software that could be accessed indi-
vidually and only when needed. This technique, known as bank
switching, allowed MCM to store its software first in 14K of
ROM memory and later, with an improved version of APL , in
22K of ROM .
Bank switching took care of one memory problem only: cre-
ating enough ROM space to store MCM / APL . The execution of
an APL program on an MCM /70 was another issue altogether.
A user's application program and its input data together re-
quired additional memory for their storage. This is what com-
puter random access memory ( RAM ) is for. Clearly, the size of
RAM delimits the size of programs and the amount of input
data that can be processed. Both APL \360 and York APL al-