Hardware Reference
In-Depth Information
This revelation led to a virtual explosion in machine instruction sets, as designers
competed with one another to produce bigger and better instruction sets. Many of
these instructions were not essential in the sense that their effect could be easily
achieved by existing instructions, but often they were slightly faster than a se-
quence of existing instructions. For example, many machines had an instruction
INC
(INCrement) that added 1 to a number. Since these machines also had a gener-
al
ADD
instruction, having a special instruction to add 1 (or to add 720, for that
matter) was not necessary. However, the
INC
was usually a little faster than the
ADD
, so it got thrown in.
For the same reason, many other instructions were added to the microprogram.
These often included
1. Instructions for integer multiplication and division.
2. Floating-point arithmetic instructions.
3. Instructions for calling and returning from procedures.
4. Instructions for speeding up looping.
5. Instructions for handling character strings.
Furthermore, once machine designers saw how easy it was to add new instructions,
they began looking around for other features to add to their microprograms. A few
examples of these additions include
1. Features to speed up computations involving arrays (indexing and
indirect addressing).
2. Features to permit programs to be moved in memory after they have
started running (relocation facilities).
3. Interrupt systems that signal the computer as soon as an input or out-
put operation is completed.
4. The ability to suspend one program and start another in a small num-
ber of instructions (process switching).
5. Special
instructions for processing audio,
image, and multimedia
files.
Numerous other features and facilities have been added over the years as well,
usually for speeding up some particular activity.
The Elimination of Microprogramming
Microprograms grew fat during the golden years of microprogramming (1960s
and 1970s). They also tended to get slower and slower as they acquired more bulk.
Finally, some researchers realized that by eliminating the microprogram, vastly
