17 micro programmed control
- 1. William Stallings Computer Organization and Architecture 7th Edition Chapter 17 Micro-programmed Control
- 3. Micro-programmed Control • Use sequences of instructions (see earlier notes) to control complex operations • Called micro-programming or firmware
- 4. Implementation (1) • All the control unit does is generate a set of control signals • Each control signal is on or off • Represent each control signal by a bit • Have a control word for each micro- operation • Have a sequence of control words for each machine code instruction • Add an address to specify the next micro- instruction, depending on conditions
- 5. Implementation (2) • Today’s large microprocessor —Many instructions and associated register-level hardware —Many control points to be manipulated • This results in control memory that —Contains a large number of words – co-responding to the number of instructions to be executed —Has a wide word width – Due to the large number of control points to be manipulated
- 6. Micro-program Word Length • Based on 3 factors —Maximum number of simultaneous micro- operations supported —The way control information is represented or encoded —The way in which the next micro-instruction address is specified
- 7. Micro-instruction Types • Each micro-instruction specifies single (or few) micro-operations to be performed — (vertical micro-programming) • Each micro-instruction specifies many different micro-operations to be performed in parallel —(horizontal micro-programming)
- 8. Vertical Micro-programming • Width is narrow • n control signals encoded into log2 n bits • Limited ability to express parallelism • Considerable encoding of control information requires external memory word decoder to identify the exact control line being manipulated
- 9. Horizontal Micro-programming • Wide memory word • High degree of parallel operations possible • Little encoding of control information
- 11. Compromise • Divide control signals into disjoint groups • Implement each group as separate field in memory word • Supports reasonable levels of parallelism without too much complexity
- 13. Control Unit
- 14. Control Unit Function • Sequence login unit issues read command • Word specified in control address register is read into control buffer register • Control buffer register contents generates control signals and next address information • Sequence login loads new address into control buffer register based on next address information from control buffer register and ALU flags
- 15. 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
- 17. Wilkes Control • 1951 • Matrix partially filled with diodes • During cycle, one row activated —Generates signals where diode present —First part of row generates control —Second generates address for next cycle
- 18. Wilkes's Microprogrammed Control Unit
- 19. Advantages and Disadvantages of Microprogramming • Simplifies design of control unit —Cheaper —Less error-prone • Slower
- 20. Tasks Done By Microprogrammed Control Unit • Microinstruction sequencing • Microinstruction execution • Must consider both together
- 21. Design Considerations • Size of microinstructions • Address generation time —Determined by instruction register – Once per cycle, after instruction is fetched —Next sequential address – Common in most designed —Branches – Both conditional and unconditional
- 22. Sequencing Techniques • Based on current microinstruction, condition flags, contents of IR, control memory address must be generated • Based on format of address information —Two address fields —Single address field —Variable format
- 23. Branch Control Logic: Two Address Fields
- 24. Branch Control Logic: Single Address Field
- 26. Address Generation Explicit Implicit Two-field Mapping Unconditional Branch Addition Conditional branch Residual control
- 27. Execution • The cycle is the basic event • Each cycle is made up of two events —Fetch – Determined by generation of microinstruction address —Execute
- 28. Execute • Effect is to generate control signals • Some control points internal to processor • Rest go to external control bus or other interface
- 30. A Taxonomy of Microinstructions • Vertical/horizontal • Packed/unpacked • Hard/soft microprogramming • Direct/indirect encoding
- 31. Improvements over Wilkes • Wilkes had each bit directly produced a control signal or directly produced one bit of next address • More complex address sequencing schemes, • using fewer microinstruction bits, are possible • Require more complex sequencing logic module • Control word bits can be saved by encoding and subsequently decoding control information
- 32. How to Encode • K different internal and external control signals • Wilkes’s: — K bits dedicated — 2K control signals during any instruction cycle • Not all used — Two sources cannot be gated to same destination — Register cannot be source and destination — Only one pattern presented to ALU at a time — Only one pattern presented to external control bus at a time • Require Q < 2K which can be encoded with log2Q < K bits • Not done — As difficult to program as pure decoded (Wilkes) scheme — Requires complex slow control logic module • Compromises — More bits than necessary used — Some combinations that are physically allowable are not possible to encode
- 33. Specific Encoding Techniques • Microinstruction organized as set of fields • Each field contains code • Activates one or more control signals • Organize format into independent fields —Field depicts set of actions (pattern of control signals) —Actions from different fields can occur simultaneously • Alternative actions that can be specified by a field are mutually exclusive —Only one action specified for field could occur at a time
- 36. Required Reading • Stallings chapter 17