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Lecture 15

Lecture 15. Microarchitecture Level: Level 1. Microarchitecture Level. The level above digital logic level. Job: to implement the ISA level above it. The design depends on: ISA being implemented Cost Performance goal. Microarchitecture Level.

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Lecture 15

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  1. Lecture 15 Microarchitecture Level: Level 1

  2. Microarchitecture Level • The level above digital logic level. • Job: to implement the ISA level above it. • The design depends on: • ISA being implemented • Cost • Performance goal

  3. Microarchitecture Level • Many modern ISAs, e.g. RISC, have simple instructions that can usually be executed in a single clock cycle. • Other complex ISAs, e.g. Pentium IV, may require many clock cycles to execute single instruction.

  4. Basic Elements of Processor • ALU • Registers • Internal data paths • External data paths • Control Unit

  5. Types of Micro-operation • Transfer data between registers • Transfer data from register to external • Transfer data from external to register • Perform arithmetic or logical operations

  6. Functions of Control Unit • Sequencing • Causing the CPU to step through a series of micro-operations • Execution • The CU causes each micro-op to be performed. • This is done using Control Signals

  7. Control Signals • 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 • Acknowledgements

  8. Model of Control Unit

  9. Control Signals - output • Within CPU • Cause data movement • Activate specific functions • Via control bus • To memory • To I/O modules

  10. Example Control Signal Sequence - Fetch • MAR <- (PC) • Control unit activates signal to open gates between PC and MAR • MBR <- (memory) • Open gates between MAR and address bus • Memory read control signal • Open gates between data bus and MBR

  11. Example: Data Paths and Control Signals

  12. Internal Organization • Usually a single internal bus • Gates control movement of data onto and off the bus • Control signals control data transfer to and from external systems bus • Temporary registers needed for proper operation of ALU

  13. Microprogrammed Control • A microprogram has a sequence of instructions in a microprogramming language. • These are very simple instructions that specify micro-operations.

  14. Microprogrammed Control • A microprogrammed control unit is a simple logic circuit that is capable of: • Sequencing through microinstructions • Generating control signals to execute each microinstruction. • As in a hardwired control unit, the control signal generated by a microinstruction are used to cause register transfers and ALU operations.

  15. Control Unit Organization

  16. Micro-programmed Control • Use sequences of instructions to control complex operations. • Called micro-programming or firmware

  17. Micro-programmed Control • 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.

  18. Micro-programmed Control • 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

  19. 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

  20. 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)

  21. 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

  22. Horizontal Micro-programming • Wide memory word • High degree of parallel operations possible • Little encoding of control information

  23. Typical Microinstruction Formats

  24. 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

  25. Organization ofControl Memory

  26. Control Unit Store a set of microinstructions For vertical microinstructions only

  27. Control Unit Function • Sequence logic unit issues read command • Word specified in control address register is read (from control memory) into control buffer register • Control buffer register contents generates control signals and next address information • Sequence logic loads new address into control address register based on next address information from control buffer register and ALU flags

  28. 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

  29. Advantages and Disadvantages of Microprogramming • Advantage: • Simplifies design of control unit • Cheaper • Less error-prone • Disadvantage: • Slower Microprogramming is the dominant technique for implementing control unit in pure CISC.

  30. Hard Wired Control Unit • Disadvantage: • Complex sequencing & micro-operation logic • Difficult to design and test • Inflexible design • Difficult to add new instructions • Advantage: • Faster Hardwired control unit is typically use for implementing control unit in pure RISC.

  31. Tasks Done By Microprogrammed Control Unit • Microinstruction sequencing • Microinstruction execution • Must consider both together

  32. 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

  33. 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

  34. Branch Control Logic: Two Address Fields

  35. Branch ControlLogic: Single Address Field

  36. Branch Control Logic: Variable Format

  37. Execution • The cycle is the basic event • Each cycle is made up of two events • Fetch • Determined by generation of microinstruction address • Execute • Effect is to generate control signals • Some control points internal to processor • Rest go to external control bus or other interface

  38. Control Unit Organization

  39. A Taxonomy of Microinstructions • Classification of microinstruction: • Vertical/horizontal • Packed/unpacked • Hard/soft microprogramming • Direct/indirect encoding

  40. How to Encode • K different internal and external control signals • Wilkes’s (first proposed the use of microprogrammed control unit in 1951): • K bits dedicated • 2K possible combination of control signals. • 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.

  41. How to Encode • Require Q < 2K which can be encoded with log2Q < K bits • Not done • As difficult to program as pure decoded (Wilkes) scheme. • It is complex and therefore slow control logic module. • Compromises • More bits than necessary are used. • Some combinations that are physically allowable are not possible to encode.

  42. Microinstruction Encoding • In practice, microprogrammed control units are not designed using: • a pure unencoded or • horizontal microinstruction format. • At least some degree of encoding is used to reduce control memory width and to simplify the task of microprogramming.

  43. 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

  44. Microinstruction EncodingDirect Encoding

  45. Microinstruction EncodingIndirect Encoding

  46. The Data Path: Example The data path is the part of CPU containing ALU, its input and its output. Mic-1

  47. The Data Path: Example( cont…) Useful combinations of ALU signals and the function performed. ALU functions is determined by the 6 control lines.

  48. Microinstruction Control: The Mic-1 • To control the datapath of Mic-1, we need 29 signals: • 9 signal to control writing data from the C bus into registers. • 9 signal to control enabling registers onto the B bus for ALU i/p. • 8 signal to control the ALU and shifter functions. • 2 signal (not shown) to indicate memory read/write via MAR/MDR. • 1 signal (not shown) to indicate memory fetch via PC/MBR.

  49. Microinstruction Control: The Mic-1 • The values of 29 control signals specify the operations for one cycle of the data path. • A data path consists of: • Gating values out of registers and onto the B bus. • Propagating the signals through the ALU and shifter. • Driving them onto the C-bus. • Writing the results in the appropriate register(s). • In addition, if a memory read data signal is asserted, the memory operation is started at the end of the data cycle, after MAR has been loaded.

  50. We use 9+4+8+2+1=24 signals to control data path for one cycle. Microinstruction Control: The Mic-1 • Issues: • It may be desirable to write the o/p on the C bus into many registers. • It is never desirable to enable more than one register write onto the B bus at a time – physical damage. By adding a 4-to-16 decoder, we can reduce the number of bits needed to select among the possible sources for driving the B bus. How to determine the next cycle? Need 2 additional fields: • NEXT_ADDRESS field • JAM field.

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