The Making of the MCM/70 - Inventing the PC - page 31

processor training environment and allow them to construct the
first prototype of the MCM /70. Intel's EPROM programming
aids would allow Ramer and his software group to create and
test the APL software for the KSI 's computer. Thus the story of
the design of the MCM /70 and its prototypes must begin with a
brief chronicle of systems activities that took place at Intel's Ap-
plications Research Group in 1971 and early 1972.
A new era in integrated electronics
The purpose of read-only memory ( ROM ) in a computing device
is to store programs that are needed by the device to control and
monitor its operations. For instance, a ROM chip can contain
the entire software of a traffic light controller or a program that
boots up a standard PC , preparing all its devices to function
when the PC is powered on. The content of an ordinary ROM
chip is set permanently during its fabrication at a semiconductor
factory. There is no direct way to correct errors or modify a
program already stored in a ROM other than to manufacture a
new chip with a new version of the program stored in it.
An EPROM serves the same purpose as a ROM chip. How-
ever, it can be reprogrammed many times over: its contents can
be erased by exposing the device to ultraviolet light, and then
reprogrammed anew using a relatively simple electronic device
called an EPROM programmer. EPROM s, therefore, presented
the systems engineers with cost effective alternatives to ROM s
for rapid system prototyping and development. The programs
destined for ROM chips could be developed and tested in-house
using EPROM s, and only when an engineer was satisfied with
the program (after, perhaps, several rounds of erasing and re-
programming an EPROM ) was the program's code sent to a
manufacturer and a non-erasable ROM chip produced.
These microprocessor and EPROM devices presented Intel
with the considerable marketing challenge of reaching a wider
systems engineering audience. The company's successful dis-