CONTROL OF THE PROCESSOR

CONTROL OF THE PROCESSOR

1. 1.Functional Requirements

   • Have 3 Steps
     • Define the basic elements of the processor
     • Determine the functions that the control unit must perform to cause the micro-operations to be performed
     • Describe the micro-operations that the processor performs.
   • Basic functional elements of the processor
     • ALU
       • The ALU is the functional essence of the computer
     • Registers
       • Registers are used to store data internal to the processor
     • Internal data paths
       • Internal data paths are used to move data between registers and between register and ALU
     • Control unit
       • The control unit causes operations to happen within the processor
         • The control unit performs two basic tasks
           • Sequencing
           • Execution
     • External data paths
       • External data paths link registers to memory and I/O modules, often by means of a system bus

2. 2.Control Signals

   • The inputs are
     • Clock
       • This is how the control unit “keeps time.”
     • Flags
       • These are needed by the control unit to determine the status of the processor and the outcome of previous ALU operations
     • Instruction register
       • Used to determine which micro-operations to perform during the execute cycle
     • Control signals from control bus
       • The control bus portion of the system bus provides signals to the control unit
   • Outputs
     • Control signals within the processor
       • 2 types
         • those that cause data to be moved from one register to another
         • those that activate specific ALU functions
     • Control signals to control bus
       • 2 types
         • control signals to memory
         • control signals to the I/O modules

3. 3.A Control Signals Example

   • Control signals go to three separate destinations
     • ALU
       • The control unit controls the operation of the ALU by a set of control signals. These signals activate various logic circuits and gates within the ALU.
     • System bus
       • The control unit sends control signals out onto the control lines of the system bus
     • Data paths
       • The control unit controls the internal flow of data

4. 4.Internal Processor Organization

   • The complexity of this type of organization should be clear.
     • An operation to add a value from memory to the AC would have the following steps
       • t1: MAR ← (IR(address)), t2: MBR ← Memory, t3: Y ← (MBR), t4: Z ← (AC) + (Y), t5: AC ← (Z)

5. 5.The Intel 8085

   • Several key components that may not be self-explanatory are
     • Incrementer/decrementer address latch
       • Logic that can add 1 to or subtract 1 from the contents of the stack pointer or program counter
     • Interrupt control
       • This module handles multiple levels of interrupt signals
     • Serial I/O control:
       • This module interfaces to devices that communicate 1 bit at a time
   • These signals are the interface between the 8085 processor and the rest of the system
   • The control unit is identified as having two components labeled (1) instruction decoder and machine cycle encoding and (2) timing and control.
   • The timing of processor operations is synchronized by the clock and controlled by the control unit with control signals
   • The number of machine cycles is fixed for a given instruction but varies from one instruction to another
     • the number of machine cycles for an instruction depends on the number of times the processor must communicate with external devices