Hardware Reference
In-Depth Information
*
JOB, 5494, BARBARA
*
XEQ
*
FORTRAN
FORTRAN
program
*
DATA
Data
cards
*
END
Figure 1-3.
A sample job for the FMS operating system.
program'' instruction. A level with only two instructions was not much of a level
but it was a start in that direction.
In subsequent years, operating systems became more and more sophisticated.
New instructions, facilities, and features were added to the ISA level until it began
to take on the appearance of a new level. Some of this new level's instructions
were identical to the ISA-level instructions, but others, particularly input/output in-
structions, were completely different. The new instructions were often known as
operating system macros
or
supervisor calls
. The usual term now is
system call
.
Operating systems developed in other ways as well. The early ones read card
decks and printed output on the line printer. This organization was known as a
batch system
. Usually, there was a wait of several hours between the time a pro-
gram was submitted and the time the results were ready. Developing software was
difficult under those circumstances.
In the early 1960s researchers at Dartmouth College, M.I.T., and elsewhere de-
veloped operating systems that allowed (multiple) programmers to communicate
directly with the computer. In these systems, remote terminals were connected to
the central computer via telephone lines. The computer was shared among many
users. A programmer could type in a program and get the results typed back al-
most immediately, in the office, in a garage at home, or wherever the terminal was
located. These systems were called
timesharing systems
.
Our interest in operating systems is in those parts that interpret the instructions
and features present in level 3 and not present in the ISA level rather than in the
timesharing aspects. Although we will not emphasize it, you should keep in mind
that operating systems do more than just interpret features added to the ISA level.
The Migration of Functionality to Microcode
Once microprogramming had become common (by 1970), designers realized
that they could add new instructions by just extending the microprogram. In other
words, they could add ''hardware'' (new machine instructions) by programming.
