Instruction decoder
The instruction decoder of a processor is a combinatorial circuit sometimes
in the form of a read-only memory, sometimes
in the form of an ordinary combinatorial circuit. Its purpose is to
translate an instruction code into the address in the micro memory where the micro code for the
instruction starts.
In our example computer, the input to the instruction decoder is the
low 5 bits of the data bus, for a total of 32 different instruction
codes. The output is 6 bits to correspond to the width of the micro PC and the micro memory.