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Control Unit

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.

Control signals

Clock: The control unit causes

one micro-operation (or a set of simultaneous micro-operations) to be

performed for each clock pulse.

Instruction register: The opcode and addressing mode of the current

instruction are used to determine which micro-operations to perform

during the execute cycle.

The outputs are

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.

• 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.

Three types of control signals are used:

Activate an ALU

function.

Activate a data path.

signals on the

external system bus or other external interface

The fetch cycle

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.

Following this, the control unit sends a control signal that opens gates

between the MBR and the IR.

Destinations:

1-Data paths

2-ALU

3-System bus

CONTROL OF THE PROCESSOR

The Intel 8085

The inputs are

Incrementer/decrementer address latch: Logic that can add 1 to or subtract 1 from the contents of the stack pointer or program counter.

This saves time by avoiding the use of the ALU for this purpose.

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.