William Stallings Computer Organization and Architecture 6th Edition

1
William Stallings Computer Organization and
Architecture6th Edition

• Chapter 17
• Microprogrammed Control

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Structure - The Control Unit
Control Unit
CPU
Sequencing Logic
ALU
Control Unit
Internal Bus
Control Unit Registers and Decoders
Registers
Control Memory
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Table 16.1
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Microinstructions

• Each micro-operation is described in symbolic
  notation
• A notation that describes a set of
  micro-operations occurring at one time is called
  a microinstruction
• A sequence of microinstructions is a
  microprogram, or firmware
• A microprogram is midway between hardware and
  software

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Using Microprogramming in Control Unit

• Each control line from the control unit can be
  represented by a binary bit
• All the control lines construct a control word
• Each micro-operation can be represented by a
  different control word
• Put a sequence of control words into a memory,
  give each word an unique address, and a way to
  determine the address of next control word to be
  executed

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Microinstruction Formats
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Horizontal vs. Vertical
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Execute a Microinstruction

• To execute a microinstruction, turn on all the
  control lines indicated by a 1 bit
• Cause one or more micro-operations to be
  performed
• If the condition bit is false, execute the next
  microinstruction in sequence
• If the condition bit is true, the next
  microinstruction to be executed is indicated in
  the address field

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Organization of Control Memory
Jump to indirect or execute
Jump to execute
Jump to fetch
Fetch cycle routine
Indirect cycle routine
Interrupt cycle routine
Execute cycle routine
Jump to opcode routine
Jump to fetch or interrupt
ADD routine


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Control Unit Microarchitecture
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Key Elements of Control Unit

• Control memory
• Store microinstructions
• Registers
• Control address register
• Control buffer register
• Sequencing logic
• Loads control address register
• Issues READ command to control memory

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Fig 17.4 Functioning of Microprogrammed Control
Unit
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Wilkess Microprogrammed Control Unit
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Adv. and Disadv. of Mircoprogramming

• Advantage
• Simplifies the design of the control unit
• Cheaper
• More reliable
• Disadvantage
• Slower
• Micropragrammed control unit is used in CISC
  processors
• Hardwired implementation is used in RISC
  processors