Control unit
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The document outlines the operation of a control unit in a computer, focusing on the fetch/execute cycle and its associated micro-operations. It details the processes involved in executing instructions, including fetching, indirect addressing, and interrupts, as well as the control signals that dictate the sequencing and execution of these operations. The discussion also highlights the challenges of hardwired control unit designs and the necessity for adaptable systems to accommodate new instructions.
Control unit
- 2. Micro-Operations • A computer executes a program • Fetch/execute cycle • Each cycle has a number of steps • see pipelining • Called micro-operations • Each step does very little • Atomic operation of CPU
- 3. Constituent Elements of Program Execution
- 4. Fetch - 4 Registers • Memory Address Register (MAR) • Connected to address bus • Specifies address for read or write op • Memory Buffer Register (MBR) • Connected to data bus • Holds data to write or last data read • Program Counter (PC) • Holds address of next instruction to be fetched • Instruction Register (IR) • Holds last instruction fetched
- 5. Fetch Sequence • Address of next instruction is in PC • Address (MAR) is placed on address bus • Control unit issues READ command • Result (data from memory) appears on data bus • Data from data bus copied into MBR • PC incremented by 1 (in parallel with data fetch from memory) • Data (instruction) moved from MBR to IR • MBR is now free for further data fetches
- 6. Fetch Sequence (symbolic) • t1: MAR <- (PC) • t2: MBR <- (memory) • PC <- (PC) +1 • t3: IR <- (MBR) • (tx = time unit/clock cycle) • or • t1: MAR <- (PC) • t2: MBR <- (memory) • t3: PC <- (PC) +1 • IR <- (MBR)
- 7. Rules for Clock Cycle Grouping • Proper sequence must be followed • MAR <- (PC) must precede MBR <- (memory) • Conflicts must be avoided • Must not read & write same register at same time • MBR <- (memory) & IR <- (MBR) must not be in same cycle • Also: PC <- (PC) +1 involves addition • Use ALU • May need additional micro-operations
- 8. Indirect Cycle • MAR <- (IRaddress) - address field of IR • MBR <- (memory) • IRaddress <- (MBRaddress) • MBR contains an address • IR is now in same state as if direct addressing had been used • (What does this say about IR size?)
- 9. Interrupt Cycle • t1: MBR <-(PC) • t2: MAR <- save-address • PC <- routine-address • t3: memory <- (MBR) • This is a minimum • May be additional micro-ops to get addresses • N.B. saving context is done by interrupt handler routine, not micro-ops
- 10. Execute Cycle (ADD) • Different for each instruction • e.g. ADD R1,X - add the contents of location X to Register 1 , result in R1 • t1: MAR <- (IRaddress) • t2: MBR <- (memory) • t3: R1 <- R1 + (MBR) • Note no overlap of micro-operations
- 11. Execute Cycle (ISZ) • ISZ X - increment and skip if zero • t1: MAR <- (IRaddress) • t2: MBR <- (memory) • t3: MBR <- (MBR) + 1 • t4: memory <- (MBR) • if (MBR) == 0 then PC <- (PC) + 1 • Notes: • if is a single micro-operation • Micro-operations done during t4
- 12. Execute Cycle (BSA) • BSA X - Branch and save address • Address of instruction following BSA is saved in X • Execution continues from X+1 • t1: MAR <- (IRaddress) • MBR <- (PC) • t2: PC <- (IRaddress) • memory <- (MBR) • t3: PC <- (PC) + 1
- 13. Instruction Cycle • Each phase decomposed into sequence of elementary micro- operations • E.g. fetch, indirect, and interrupt cycles • Execute cycle • One sequence of micro-operations for each opcode • Need to tie sequences together • Assume new 2-bit register • Instruction cycle code (ICC) designates which part of cycle processor is in • 00: Fetch • 01: Indirect • 10: Execute • 11: Interrupt
- 14. Flowchart for Instruction Cycle
- 15. Functional Requirements • Define basic elements of processor • Describe micro-operations processor performs • Determine functions control unit must perform
- 16. Basic Elements of Processor • ALU • Registers • Internal data pahs • External data paths • Control Unit
- 17. Types of Micro-operation • Transfer data between registers • Transfer data from register to external • Transfer data from external to register • Perform arithmetic or logical ops
- 18. Functions of Control Unit • Sequencing • Causing the CPU to step through a series of micro-operations • Execution • Causing the performance of each micro-op • This is done using Control Signals
- 19. 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
- 20. Model of Control Unit
- 21. Control Signals - output • Within CPU • Cause data movement • Activate specific functions • Via control bus • To memory • To I/O modules
- 22. 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
- 23. Data Paths and Control Signals
- 24. 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
- 26. Intel 8085 CPU Block Diagram
- 28. Intel 8085 OUT Instruction Timing Diagram
- 29. Hardwired Implementation (1) • Control unit inputs • Flags and control bus • Each bit means something • Instruction register • Op-code causes different control signals for each different instruction • Unique logic for each op-code • Decoder takes encoded input and produces single output • n binary inputs and 2n outputs
- 30. Hardwired Implementation (2) • Clock • Repetitive sequence of pulses • Useful for measuring duration of micro-ops • Must be long enough to allow signal propagation • Different control signals at different times within instruction cycle • Need a counter with different control signals for t1, t2 etc.
- 31. Control Unit with Decoded Inputs
- 32. Problems With Hard Wired Designs • Complex sequencing & micro-operation logic • Difficult to design and test • Inflexible design • Difficult to add new instructions