Microprogramming Andreas Klappenecker CPSC321 Computer Architecture

Implementation of the Finite State Machine Control Logic

Graphical Specification of FSM How many state bits are needed?

Two-Level Logic Any logic function can be written in a two-level representation, e.g. the sums-of-products form E = AB ~C + AC ~B + BC ~A It uses the logical OR of products (AND ops) Contains variable or complemented variable

PLA Implementation Two level logic

Implementation: Finite State Machine for Control

Microprogramming

Motivation The full MIPS instruction set contains over 100 instructions Instructions can take from 1 clock cycle to more than 20 clock cycles Pipelining will lead to explosion of states It follows that the control will be quite complex => FSM can be cumbersome Use microprogramming!

Microprogramming Implementation of MIPS instructions as a sequence of simpler instructions Design of the microinstruction format Write a program Eases implementations of pipelined processor

Micro-Instructions What is needed? ALU control signals Program counter control signals Complete data path What do we do? Analyze what is happening at each step Express in a column oriented way Translate it into binary Store it in an appropriate form

A Simple Implementation

Microprogram Physical implementation: ROM or PLA Each micro-instruction has an address Sequentially ordered Each sequence step is one cycle Selection of next instruction Address increment (sequencing field = seq) Fetch: begins fetching the next micro-instruction (sequencing field = fetch) Dispatch: jump to the next micro-instruction, the number i indicates the location in the dispatch table

Microinstruction format

Microcode: Trade-offs Specification Advantages: Easy to design and write Design architecture and microcode in parallel Implementation (off-chip ROM) Advantages Easy to change since values are in memory Can emulate other architectures Can make use of internal registers Implementation Disadvantages: SLOWER Control is implemented on same chip as processor ROM is no longer faster than RAM No need to go back and make changes