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)

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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

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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.

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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.

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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

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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

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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.

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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.