CONTROL OF THE PROCESSOR

CONTROL OF THE PROCESSOR

1. Functional Requirements

   • Definition: Those functions that the control unit must perform
     • Basic functional elements of the processor
       • Registers
         • Are used to store data internal to the processor.
         • Some records contain necessary status information to manage the instruction sequence.
       • Internal data paths
         • Are used to move data between registers and between Register and ALU.
       • ALU
         • Is the functional essence of the computer.
       • Control unit
         • Causes operations to happen inside the processor.
         • The control unit performs two basic tasks
           • Execution
           • Sequencing
       • External data paths
         • Are link registers to memory and I / O modules.
     • The execution of a program consists of operations involving these processor elements.
       • Micro operations
         • Transfer data from one register to another.
         • Transfer data from a register to an external interface (e.g., system bus).
         • Transfer data from an external interface to a register.
         • Perform an arithmetic or logic operation, using registers for input and output

2. Control Signals

   • General model of the control unit
     • Inputs
       • Clock
       • Instruction register
       • Flags
       • Control signals from control bus:
     • Outputs
       • Control signals within the processor
       • Control signals to control bus
   • Three types of control signals are used
     • Those that activate an ALU function.
     • Those that activate a data path.
     • Those that are signals on the external system bus or other external interface.
   • At a given point, it knows that the fetch cycle is to be performed next.
     • The first step is to transfer the contents of the PC to the MAR
       • The next step is to read a word from memory into the MBR and increment the PC.
         • The control unit does this by sending the following control signals simultaneously
           • A control signal that opens gates, allowing the contents of the MAR on to the address bus.
           • A memory read control signal on the control bus.
           • A control signal that opens the gates, allowing the contents of the data bus to be stored in the MBR.
           • • Control signals to logic that add 1 to the contents of the PC and store the result back to the PC.
   • Data Paths and Control Signals

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