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William Stallings Computer Organization and Architecture 6 th Edition

William Stallings Computer Organization and Architecture 6 th Edition. Chapter 17 Microprogrammed Control. 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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William Stallings Computer Organization and Architecture 6 th Edition

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  1. William Stallings Computer Organization and Architecture6th Edition Chapter 17 Microprogrammed Control

  2. Structure - The Control Unit Control Unit CPU Sequencing Logic ALU Control Unit Internal Bus Control Unit Registers and Decoders Registers Control Memory

  3. Microinstruction Table 16.1

  4. 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

  5. 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

  6. Microinstruction Formats

  7. Horizontal vs. Vertical

  8. 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

  9. Organization of Control Memory Fetch cycle routine Indirect cycle routine Interrupt cycle routine Execute cycle routine ADD routine • • • • • •

  10. Control Unit Microarchitecture

  11. 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

  12. Fig 17.4 Functioning of Microprogrammed Control Unit

  13. Wilkes’s Microprogrammed Control Unit

  14. 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

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