Chapter 7_The Control Unit and Its Design

Micro-Operations

• Smaller cycles that involves a series of steps, each of which involves the processor registers

• The execution of each subcycle involves one or more shorter operations, that is _____

micro

the prefix "_____" refers to the fact that each step is very simple and accomplishes very little.

instruction cycle

• The execution of a program consists of the sequential execution of instructions.

• Each instruction is executed during an _____ made up of shorter subcycles

Fetch,

Indirect,

Execute,

Interrupt

Sub cycles of the instruction cycle (4)

Fetch Cycle, Indirect Cycle, Execute Cycle, Interrupt Cycle

Instruction Cycle (4)

Memory Address Register (MAR),

Memory Buffer Register (MBR),

Program Counter (PC),

Instruction Register (IR)

Fetch Cycle (Four Registers)

Memory Address Register (MAR)

• Is connected to the address lines of the system bus.

• It specifies the address in memory for a read or write operation.

Memory Buffer Register (MBR)

• Is connected to the data lines of the system bus.

• It contains the value to be stored in memory or the last value read from memory

Program Counter (PC)

Holds the address of the next instruction to be fetched

Instruction Register (IR)

Holds the last instruction fetched.

Fetch Cycle Sequence

• The groupings of micro-operations must follow two simple rules:

  • The proper sequence of events must be followed.

  • Conflicts must be avoided. One should not attempt to read to and write from the same register in one time unit, because the results would be unpredictable.

• A final point worth noting is that one of the micro- operations involves an addition.

• To avoid duplication of circuitry, this addition could be performed by the ALU.

  • The use of the ALU may involve additional microoperations, depending on the functionality of the ALU and the organization of the processor.

Indirect Cycle

• The address field of the instruction is transferred to the MAR

• this is then used to fetch the address of the operand

• finally, the address field of the IR is updated from the MBR, so that it now contains a direct rather than an indirect address.

Execute Cycle

• Because of the variety of opcodes, there are a number of different sequences of micro-operations that can occur.

• Instruction Decoding

• The new feature introduced here is the conditional action.

Instruction Decoding

The control unit examines the opcode and generates a sequence of micro-operations based on the value of the opcode.

conditional action

• The PC is incremented if (MBR) = 0. This test and action can be implemented as one microoperation.

  • Note also that this micro-operation can be performed during the same time unit during which the updated value in MBR is stored back to memory.

Interrupt Cycle

• At the completion of the execute cycle, a test is made to determine whether any enabled interrupts have occurred.

• if so, the _____ occurs.

• The nature of this cycle varies greatly from one machine to another.

Instruction Cycle Code (ICC)

A new 2-bit register that designates the state of the processor in terms of which portion of the cycle it is in.

00,

01,

10,

11

Instruction Cycle Codes (4)

00

Instruction Cycle Code

• Fetch

01

Instruction Cycle Code

• Indirect

10

Instruction Cycle Code

• Execute

11

Instruction Cycle Code

• Interrupt

basic elements,

micro-operations,

functions

steps in the process to a characterization of the Control Unit

• Define the _____ of the processor,

• Describe the _____ that the processor performs,

• Determine the _____ that the control unit must perform

ALU,

Registers,

Internal data paths,

External data paths,

Control unit

basic elements of the processor (5)

one register to another,

register to an external interface,

external interface to a register,

arithmetic or logic operation

micro-operations that the processor performs (4)

• Transfer data from _____

• Transfer data from a _____

• Transfer data from an _____

• Perform an _____ using registers for input and output

Sequencing

Execution

functions that the control unit must perform to cause the micro-operations to be performed (2)

Clock,

Instruction register,

Flags,

Control signals from control bus

Control Signals Inputs (4)

Clock

Control signal Input.

• This is how the control unit "keeps time";

• the control unit causes one micro-operation (or a set of simultaneous micro-operations) to be performed for each clock pulse.

• This is sometimes referred to as the processor cycle time, or the clock cycle time.

Control Unit Input

• Issues a repetitive sequence of pulses that determine the timing of micro-operations.

Instruction register

Control signal Input

• The opcode and addressing mode of the current instruction are used to determine which micro-operations to perform during the execute cycle.

Control Unit Input

• provides the opcode of the instruction, which the control unit uses to determine what actions to perform.

Flags

Control signal input.

• 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

Control signal input.

• The control bus portion of the system bus provides signals to the control unit.

Control signals within the processor,

Control signals to control bus

Control Signals Outputs (4)

Control signals within the processor

Control signal output.

• 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

Control signal output.

These are two types:

• control signals to memory,

• and control signals to the I/O modules.

ALU function,

data path,

signals on the external system bus

types of control signals used (3)

• Those that activate an _____,

• Those that activate a _____,

• Those that are _____ or other external interface

MAR onto the address bus,

memory read,

data bus to be stored in the MBR,

add 1 to the contents of the PC

The control unit sends the following control signals simultaneously

• A control signal that opens gates, allowing the contents of the _____;

• A _____ control signal on the control bus;

• A control signal that opens the gates, allowing the contents of the _____;

• Control signals to logic that _____ and store the result back to the PC.

Hardwired implementation,

Microprogrammed implementation

categories of Control Unit Implementation (2)

Hardwired implementation

The control unit is essentially a state machine circuit; its input logic signals are transformed into a set of output logic signals, which are the control signals.

Inputs to the Control Unit,

Flags and Control Bus Signals,

Decoder,

Clock,

Counter

Control Unit Inputs (5)

Inputs to the Control Unit

come from the

• Instruction Register (IR),

• the clock,

• the flags,

• and the control bus signals

Flags and Control Bus Signals

contain individual bits that each have specific meanings (e.g., overflow).

Decoder

• To simplify control logic, the opcode is sent to a _____

• Converts the encoded opcode into a unique output, with each opcode activating one specific output line.

• In general, a ______ with n binary inputs produces 2ⁿ binary outputs, each representing one possible input pattern.

• The _____ used in a control unit may need to handle variable-length opcodes, making it more complex.

clock pulses

The period of the _____ must be long enough for signals to propagate through data paths and processor circuitry.

Counter

• Used as input to the control unit to distinguish between different time units (T1, T2, etc.).

• At the end of the instruction cycle, the control unit sends a signal to the _____ to reinitialize it back to T1, starting the next cycle.

Control Unit

produces output control signals based on its input signal

P,

Q

Two control signals, _____ and _____, are defined to indicate the current cycle of operation:

PQ = 00,

PQ = 01,

PQ = 10,

PQ = 11

control signals that indicate the current cycle of operation (4).

PQ = 00

Control signal for cycle.

• Fetch Cycle.

PQ = 01

Control signal for cycle.

• Indirect Cycle.

PQ = 10

Control signal for cycle.

• Execute Cycle.

PQ = 11

Control signal for cycle.

• Interrupt Cycle.

Boolean equations

• Collectively, they describe the behavior of the control unit and therefore the processor itself.

• In modern processors, the number of required _____ is very large, making combinational circuit implementation complex.

Microprogramming

A more practical method used in modern processors instead of pure hardware logic to simplify design when the number of required Boolean equations is very large.