INTRODUCTION - Structured Computer Organization - page 9

The Invention of Microprogramming
The first digital computers, back in the 1940s, had only two levels: the ISA
level, in which all the programming was done, and the digital logic level, which ex-
ecuted these programs. The digital logic level's circuits were complicated, difficult
to understand and build, and unreliable.
In 1951, Maurice Wilkes, a researcher at the University of Cambridge, sug-
gested designing a three-level computer in order to drastically simplify the hard-
ware and thus reduce the number of (unreliable) vacuum tubes needed (Wilkes,
1951). This machine was to have a built-in, unchangeable interpreter (the
microprogram), whose function was to execute ISA-level programs by interpreta-
tion. Because the hardware would now only have to execute microprograms,
which have a limited instruction set, instead of ISA-level programs, which have a
much larger instruction set, fewer electronic circuits would be needed. Because
electronic circuits were then made from vacuum tubes, such a simplification
promised to reduce tube count and hence enhance reliability (i.e., the number of
crashes per day).
A few of these three-level machines were constructed during the 1950s. More
were constructed during the 1960s. By 1970 the idea of having the ISA level be
interpreted by a microprogram, instead of directly by the electronics, was domi-
nant. All the major machines of the day used it.
The Invention of the Operating System
In these early years, most computers were ''open shop,'' which meant that the
programmer had to operate the machine personally. Next to each machine was a
sign-up sheet. A programmer wanting to run a program signed up for a block of
time, say Wednesday morning 3 to 5 A.M. (many programmers liked to work when
it was quiet in the machine room). When the time arrived, the programmer headed
for the machine room with a deck of 80-column punched cards (an early input
medium) in one hand and a sharpened pencil in the other. Upon arriving in the
computer room, he or she gently nudged the previous programmer toward the door
and took over the computer.
If the programmer wanted to run a FORTRAN program, the following steps
were necessary:
1. He † went over to the cabinet where the program library was kept,
took out the big green deck labeled FORTRAN compiler, put it in the
card reader, and pushed the START button.
2. He put his FORTRAN program in the card reader and pushed the
CONTINUE button. The program was read in.
† ''He'' should be read as ''he or she'' throughout this topic.