Chapter 16 Control Unit Operation

1. Micro-operations
   • An instruction cycle:
   • • Fetch sub cycle
     • • Access PC,PC contains address of next instruction
       • Address moved toMAR
       • Address placed on address bus
       • Control unit requests memory read
       • Result placed on data bus, copied toMBR
       • Then, toIR
       • MBR is now free for further data fetches
       • MeanwhilePC incremented by "1"(Determined by the length of instructions)
       • • in parallel with data fetch from memory
     • Indirect cycle
     • • t1:      MARß (IRaddress)  -  address field of IR,
       • t2:      Rß 1, MBR ß (memory)
       • t3:     IRaddressß MBR
     • Interrupt cycle
     • • t1:        MBRß(PC)
       • t2:        MARßsave-address,
         PCßroutine-address
       • t3:        Wß 1, memoryß (MBR)
     • Execute sub cycle: different for each instruction
   • The steps in sub cycles are called micro-operations
   • Micro-operations are the atomic operations of a processor
2. Rules for clock cycle grouping
   • Proper sequence must be followed
   • Conflicts must be avoided
   • PC ++: involving addition
   • • May need addtional micro-operations
3. Instruction cycle
   • A new 2-bit register called the instruction cycle code(ICC) is assumed
   • ICC = 
   • • 00: fetch
     • 01: indirect
     • 10: execute
     • 11: interrupt
     • 
4. Control of the processor
   • Basic element of processor
   • • ALU
     • Registers
     • Internal data paths
     • External data paths
     • Control unit
   • Types of micro-operation
   • • Data transfer
     • Perform arithmetic or logical ops
   • Functions of control unit
   • • Sequencing: causes the processor to step through a series of micro-operationsin the proper sequence
     • Execution: causes each micro-operation to be performed
5. Control signals
   • Input
   • • Clock
     • Instruction register
     • Flags:
     • • Determine the state of CPU
     • Control signals from control bus
     • • Interrupts
   • Output
   • • Control signals to CPU
     • Control signals to control bus
6. Conclusions on the control unit
   • The control unit is the engine that runs the entire computer
   • It controls everything with a few controls
   • Sequencing micro-ops
   • Execute some micro-ops
7. Internal processor organization
   • In CPU, usually a single internal bus exists
   • Gates control the movement of data onto and off the bus
   • Control signals control data transfer to and from external system bus
   • Temporary registers may need for proper operation of ALU, except for AC:
   • • Input register: Y
     • Temporary output register: Z
8. Hardwired implementation
   • Hardwired implementation:
   • • Can be expressed by boolean expression
     • • Can be implemented in combinational circuit: rather complex
     • Pros:
     • • Higher speed operation
       • Suitable for smaller implementations
       • Favored approach in RISC style designs
     • Cons:
     • • Complex sequencing & micro-operation logic
       • Difficult to design and test
       • Inflexible design
       • Difficult to add new instructions(repetitive of 3rd?)
   • Micro-programmed implementation: simpler approach