Q. Decoding of slightly encoded micro-instructions?
Generally micro-programmed control unit designs are neither totally unencoded nor highly encoded. They are slightly coded. This decreases width of control memory and micro-programming efforts. The fundamental approach for encoding is demonstrated in Figure below. Micro-instruction is arranged as a set of fields. Every field comprises a code that upon decoding triggers one or more control signals. Execution of a micro-instruction means that each field is decoded and produces control signals. So with N fields N simultaneous actions can be specified. Every action results in activation of one or more control signals. Usually each control signal is triggered by no more than one field. Design of an encoded micro-instruction format can be stated in simple terms:
- Organize format in independent fields. Which is every field portrays a set of actions such that actions from different fields can take place concurrently.
- Define every field such that substitute actions which can be specified by field are mutually exclusive. That is only one of the actions specified for a given field cantake place at a time.