Control-Unit - Processor Architecture and Microprogramming - Lecture Slides, Slides of Microprocessor and Assembly Language Programming

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Control Unit Operation and

Microprogramming

Chap 16 & 17 of CO&A

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• Main components of the CPU
• Special Registers (Y and Z)
• The two cycles (fetch and execute phases)
• The basic functions of CU
  • Sequencing: stepping through microinstructions
  • Execution: executing microinstructions
• CU block diagram
  • Inputs ( IR, flags, clock, & control signals from system bus)
  • Outputs (control signals to system and internal buses)
• Internal processor organization (single bus architecture)

Introduction

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• Two Main techniques to implement CU:
  • Hardwired implementation
  • Microprogrammed implementation
• In hardwired implementations the CU is

considered as a combinational logic circuit the

produces its outputs (control signals) based on

its inputs (IR, flags, step counter, etc.)

• Ex: To derive a logical expression for Zin:
• Zin = T1. (fetch phase) + T6. Add + ………

Control Unit Implementation

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• All control steps for each instruction should be

specified then the logical expression for each

control signal is derived.

• This process becomes very complex with the

increase in processor complexity.

• An effective alternative is microprogrammed

implementation.

• The idea of microprogramming is to consider

each instruction as a sequence of much

simpler steps called microinstructions.

Control Unit Implementation (Cont.)

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• The previously described technique is called horizontal microinstruction.
• To execute such instructions:
  • Issue control signals for active bits
  • If condition is false, execute the following instruction
  • Otherwise, execute the instruction whose address is stored in the address field
• Another for of structuring the microinstruction is called vertical organization where the system bus and the internal CPU control signals are coded.
• Think of the pros and cons

Control Unit Implementation (Cont.)

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• Now the components of a microprogrammed CU are : - Decoder: to decode the IR - Sequencing logic: to determine the address Of the next microinstruction to be executed - Control address register: to hold the above address - Control memory: to store control words - Control buffer register: to store the currently read microinstruction - Decoder: to decode microinstructions if need

Control Unit Implementation (Cont.)

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• Realization of CUs.
• Emulation.
• Operating system support: Implement primitives that replace important portions of OS S/W.
• HLL support: various functions and data type can be implemented directly in firmware.
• Monitoring, detecting, isolating, and repairing system errors (microdiagnostics).
• User tailoring: Enable the use to write to the control memory by implementing it using RAM.

Microprogramming Applications