The address-selection signals determine which option is selected. This approach
reduces the number of address fields to one. Note, however, that the address field
often will not be used.Thus, there is some inefficiency in the microinstruction coding
scheme.
Another approach is to provide for two entirely different microinstruction formats
(Figure 16.8). One bit designates which format is being used. In one format, the
remaining bits are used to activate control signals. In the other format, some bits
drive the branch logic module, and the remaining bits provide the address.With the
first format, the next address is either the next sequential address or an address derived
from the instruction register.With the second format, either a conditional or
unconditional branch is being specified. One disadvantage of this approach is that
one entire cycle is consumed with each branch microinstruction.With the other approaches,
address generation occurs as part of the same cycle as control signal generation,
minimizing control memory accesses.