CONTROL OF THE PROCESSOR

CONTROL OF THE PROCESSOR

1. Functional Requirements

   • A definition of these functional requirements is the basis for the design and implementation of the control unit.
     • The following three-step process leads to a characterization of the control unit:
       • 1. Define the basic elements of the processor. 2. Describe the micro-operations that the processor performs. 3. Determine the functions that the control unit must perform to cause the micro-operations to be performed.
         • The basic functional elements of the processor are the following:
           • • ALU • Registers • Internal data paths • External data paths • Control unit
             • the reader should see that all micro-operations fall into one of the following categories:
               • • 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.
                 • The control unit performs two basic tasks:
                   • • Sequencing: The control unit causes the processor to step through a series of micro-operations in the proper sequence, based on the program being executed.
                   • • Execution: The control unit causes each micro-operation to be performed.

2. Control Signals

   • • Clock: This is how the control unit “keeps time.”
     • • Instruction register: The opcode and addressing mode of the current instruction are used to determine which micro-operations to perform during the execute cycle.
       • • Flags: These are needed by the control unit to determine the status of the processor and the outcome of previous ALU operations.
         • • Control signals from control bus: The control bus portion of the system bus provides signals to the control unit.
           • The outputs are as follows:
             • • Control signals within the processor: These are two types: those that cause data to be moved from one register to another, and those that activate specific ALU functions.
               • • Control signals to control bus: These are also of two types: control signals to memory, and control signals to the I/O modules.
                 • The control unit does this by sending control signals simultaneously:
                   • • A control signal that opens gates, allowing the contents of the MAR onto 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
     • The inputs are

3. A Control Signals Example

   • • Data paths: The control unit controls the internal flow of data. For example, on instruction fetch, the contents of the memory buffer register are transferred to the instruction register.
   • • ALU: The control unit controls the operation of the ALU by a set of control signals.
   • • System bus: The control unit sends control signals out onto the control lines of the system bus (e.g., memory READ).

4. Internal Processor Organization

   • 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. The Intel 8085

   • • 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.