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Microprogramming

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

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Microprogramming

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  1. Microprogramming Andreas Klappenecker CPSC321 Computer Architecture

  2. Implementation of the Finite State Machine Control Logic

  3. Graphical Specification of FSM How many state bits are needed?

  4. 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

  5. PLA Implementation Two level logic

  6. Finite State Machine for Control Implementation:

  7. Microprogramming

  8. 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!

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

  10. 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

  11. A Simple Implementation

  12. 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

  13. Microinstruction format

  14. 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

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