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Microprogramming

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Implementation (off-chip ROM) Advantages. Easy to change since values are in memory ... ROM is no longer faster than RAM. No need to go back and make changes ... – PowerPoint PPT presentation

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Title: Microprogramming


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
    gt 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
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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

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Microinstruction format
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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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