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Control Unit Operation and Microprogramming Chap 16 & 17 of CO&A Dr. Farag

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Control Unit Operation and Microprogramming Chap 16 & 17 of CO&A Dr. Farag Introduction Main components of the CPU Special Registers (Y and Z) The two cycles (fetch ... – PowerPoint PPT presentation

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Title: Control Unit Operation and Microprogramming Chap 16 & 17 of CO&A Dr. Farag


1
Control Unit Operation and MicroprogrammingChap
16 17 of COADr. Farag
2
Introduction
  • 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)

3
Basic Operations
  • Four basic operations can be used to construct
    more complex ones
  • Register transfer (LOAD MQ)
  • Performing an arithmetic or logic operation
  • Fetching a word from memory (Load M(100))
  • Storing a word into memory (Store M(100))
  • Complete example Write the control steps for
    executing IAS assembly instruction ADD M(3000)

4
Control Unit Implementation
  • 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

5
Control Unit Implementation (Cont.)
  • 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.

6
Control Unit Implementation (Cont.)
  • Each microinstruction basically activates a
    number of control signals.
  • Microinstructions can be stored into memory
    locations as control words.
  • Each control word contains a number of bits
    equals to the number of CPU bus control signals
    in addition to an address field and a jump
    condition field.
  • For those active control signals in a
    microinstruction corresponding bits will be set
    otherwise all other bits store zero.

7
Control Unit Implementation (Cont.)
  • 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

8
Control Unit Implementation (Cont.)
  • 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

9
Advantages and Disadvantages
  • Simplicity
  • Cheaper
  • Less error-prone
  • Flexibility The content of the control memory
    can be modified or even replaced to update the
    instruction set architecture of the computer.
  • This is impossible in can of Hardwires control.
  • The principle disadvantage of microprogramming is
    performance. Despite this, it is the dominant
    technique to implement CUs in CISC processors.
  • RISC processors usually use Hardwired control.

10
Microprogramming Applications
  • 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.
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