15 control-computer organization and archietecture-CO-COA
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The document discusses microprogrammed control unit implementation in processors. It describes how a control unit can be implemented using microcode stored in control memory. Microinstructions generate control signals and provide branching functionality. There are different approaches to microprogramming like horizontal vs vertical and direct vs indirect encoding. Key factors in the design of microprogrammed control units include speed, size of microinstructions, and address generation for branching.
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15 control-computer organization and archietecture-CO-COA
- 1. Chapter 16 Control Unit Implemntation
- 2. A Basic Computer Model
- 3. Example Simple Processor & Data Paths
- 4. MIPS Data Paths with Generation of Control Signals Note: This figure does not show J (jump) type instruction Data Paths
- 5. A Simple Computer & its Control Unit
- 6. Control Signal Sources • Clock — One micro-instruction (or set of parallel micro-instructions) per clock cycle • Instruction Register — Op-code for current instruction — Determines which micro-instructions are performed • Flags — State of CPU — Results of previous operations • From Control Bus — Interrupts / Bus Requests — Acknowledgements
- 7. Control Signals Outputs • Within CPU —Cause data movement —Activate specific functions • Via Main Bus —To memory —To I/O modules
- 9. The Internal Bus ?
- 10. Example Simple Processor & Data Paths
- 11. Example Simple Processor & Data Paths
- 12. State Machine for Example Simple Processor http://highered.mcgraw-hill.com/sites/dl/premium/0072467509/instructor/104653/figurec9.xls
- 13. Control Unit with Decoded Inputs
- 15. MIPS Data Paths with Generation of Control Signals Note: This figure does not show J (jump) type instruction Data Paths
- 16. MIPs Pipelined Machine with Controls IF | ID | EX | MEM | WB (64 bits) (142 bits) (107 bits) (71 bits)
- 17. Problems With Hard Wired Designs • Sequencing & micro-operation logic gets complex • Difficult to design, prototype, and test • Resultant design is inflexible, and difficult to build upon (Pipeline, multiple computation units, etc.) • Adding new instructions requires major design and adds complexity quickly
- 19. Example Simple Processor Micro-Programed Control
- 20. Control Unit Organization Tasks of Control Unit: • Microinstruction sequencing • Microinstruction execution The Control Memory contains sequences of microinstructions that provide the control signals to execute instruction cycles, e.g. Fetch, Indirect, Execute, and Interrupt. May be expected to complete instruction execution in “1” clock cycle. How is this possible?
- 22. Example of Control Memory Organization Microinstructions: • Generate Control Signals • Provide Branching
- 23. Horizontal vs Vertical Microprogramming Horizontal Microprogrammed or — Unpacked — Hard — Direct Vertical Microprogrammed or — Packed — Soft — Indirect
- 24. Microinstruction Encoding - Direct Encoding
- 25. Microinstruction Encoding - Indirect Encoding
- 26. Horizontal Micro-programming • Wide control memory word • High degree of parallel operations possible • Little encoding of control information • Faster
- 27. Vertical Micro-programming • Width can be much narrower • Control signals encoded into function codes – need to be decoded • More complex, more complicated to program, less flexibility • More difficult to modify • Slower
- 29. Example Microprogramming Formats • MicroProgram Counter • Subroutines • Stack • Control Register (MicroProgram Format)
- 30. Next Address Decision • Depending on ALU flags and control buffer register: — Get next instruction – Add 1 to control address register — Jump to new routine based on jump microinstruction – Load address field of control buffer register into control address register — Jump to machine instruction routine – Load control address register based on opcode in IR
- 32. Design Considerations • Necessity of speed • Size of Microinstructions • Address generation —Branches – Both conditional and unconditional – Based on current microinstruction, condition flags, contents of IR – Based on format of address information + Two address fields + Single address field + Variable format
- 33. Branch Control: Two Address Fields Branch based upon: • Instruction Opcode • Address 1 • Address 2 Does require a wide microinstruction, but no address calculation is needed
- 34. Branch Control: Single Address Field Branch based upon: • Next instruction • Address • Opcode Does require more circuitry, e.g. adder
- 35. Branch Control: Variable Format One bit determines microinstruction format: • Control signal format • Branch format Does require even more circuitry, and is slowest.
- 36. Advantages and Disadvantages of Microprogramming Advantage: • Simplifies design of control unit — Cheaper to design — Less error-prone — Much easier to modify — Supports having multiple versions / models Disadvantage: • Slower • More expensive to produce in quantities