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. 

A decoder is a device which is the reverse, undoing the encoding so that the original
information can be retrieved. The same method used to encode is usually just reversed in
order to decode.This unit receives the programming instructions and decodes them into
a form that is understandable by the processing units, i.e. The ALU or FPU Then, it
passes on the decoded instruction to the ALU or FPUs as desired.