Explain the micro-instruction encoding methods with the help of suitable diagrams. If a machine has a very large number of instructions and registers, which of the two encoding methods is preferred? 10m Dec2005
The characteristics of highly encoded and unencoded micro-instructions:-
Unencoded micro-instructions
- One bit is needed for each control signal; therefore, the number of bits required in a micro-instruction is high.
- It presents a detailed hardware view, as control signal need, can be determined.
- Since each of the control signals can be controlled individually, therefore these micro-instructions are difficult to program. However, concurrency can be exploited easily.
- Almost no control logic is needed to decode the instruction as there is one to one mapping of control signals to a bit of micro-instruction. Thus, execution of micro-instruction and hence the micro-program is faster.
- The unencoded micro-instruction aims at optimising the performance of a machine.
Highly Encoded micro-instructions
- The encoded bits needed in micro-instructions are small.
- It provided an aggregated view that is a higher view of the CPU as only an encoded sequence can be used for micro-programming.
- The encoding helps in reduction in programming burden; however, the concurrency may not be exploited to the fullest.
- Complex control logic is needed, as decoding is a must. Thus, the execution of a micro-instruction can have propagation delay through gates. Therefore, the execution of micro-program takes a longer time than that of an unencoded micro-instruction.
- The highly encoded micro-instructions are aimed at optimizing programming effort.
In most of the cases, the design is kept between the two extremes. The LSI 11 (highly encoded) and IBM 3033 (unencoded) control units are close examples of these two approaches.
If a machine has a very large number of instructions and registers, then we can prefer highly encoding methods of micro-instructions.