Figure 16.4 shows two modules labeled decoder. The upper decoder translates
the opcode of the IR into a control memory address.The lower decoder is not used
for horizontal microinstructions but is used for vertical microinstructions (Figure
16.1b). As was mentioned, in a horizontal microinstruction every bit in the control
field attaches to a control line. In a vertical microinstruction, a code is used for
each action to be performed [e.g.,MAR ; (PC)4,and the decoder translates this
code into individual control signals. The advantage of vertical microinstructions is
that they are more compact (fewer bits) than horizontal microinstructions, at the expense
of a small additional amount of logic and time delay.