In the next subsection, we will discuss specific encoding techniques. The remainder
of this subsection deals with the effects of encoding and the various terms
used to describe it.
Based on the preceding, we can see that the control signal portion of the microinstruction
format falls on a spectrum. At one extreme, there is one bit for each
control signal; at the other extreme, a highly encoded format is used. Table 16.4
shows that other characteristics of a microprogrammed control unit also fall along a
spectrum and that these spectra are, by and large, determined by the degree-ofencoding
spectrum.
The second pair of items in the table is rather obvious. The pure Wilkes
scheme will require the most bits. It should also be apparent that this extreme presents
the most detailed view of the hardware. Every control signal is individually
controllable by the microprogrammer. Encoding is done in such a way as to aggregate
functions or resources, so that the microprogrammer is viewing the processor at
a higher, less detailed level. Furthermore, the encoding is designed to ease the microprogramming
burden. Again, it should be clear that the task of understanding
and orchestrating the use of all the control signals is a difficult one. As was mentioned,
one of the consequences of encoding, typically, is to prevent the use of certain
otherwise allowable combinations.