CPU Fundamentals

CPU Fundamentals

1. The Functions and Basic Structure of the CPU

The Basic Functions of the CPU

Control function:

• Controls the coordinated operation of all the components of the computer
• Executes the sequence of program instructions
• Manages the flow of data and the timing

Arithmetic function:

• Performs arithmetic operations (addition, subtraction, multiplication, division)
• Performs logical operations (AND, OR, NOT, XOR)
• Performs shift operations (left shift, right shift)

Coordination function:

• Coordinates the work of memory and I/O devices
• Manages interrupts and exceptions
• Controls bus transfers

The Basic Structure of the CPU

Arithmetic logic unit (ALU):

• Arithmetic logic unit: performs arithmetic and logical operations
• Accumulator: stores the result of an operation
• General-purpose registers: store operands and intermediate results
• Flag register: stores the status flags of the operation

Control unit:

• Instruction register: stores the instruction currently being executed
• Program counter: stores the address of the next instruction
• Instruction decoder: analyses the meaning of the instruction
• Timing controller: generates the control signals

Register set:

• General-purpose registers: R0-R15, store data
• Special-purpose registers: PC, SP, BP, and so on
• Status register: stores the status information of the CPU

Internal buses:

• Data bus: transfers data
• Address bus: transfers addresses
• Control bus: transfers control signals

2. The Instruction Execution Process

The Basic Execution Cycle

Fetch stage (IF):

• Fetches the instruction from memory
• Updates the program counter
• Moves the instruction into the instruction register

Decode stage (ID):

• Analyses the operation code of the instruction
• Determines the address of the operands
• Prepares the resources needed for execution

Execute stage (EX):

• Performs the arithmetic or logical operation
• Computes the effective address
• Performs the data transfer

Memory access stage (MEM):

• Accesses the data memory
• Reads or writes the data
• Handles memory access conflicts

Write-back stage (WB):

• Writes the result of the operation back into the register
• Updates the status flags
• Completes the execution of the instruction

An Example of Instruction Execution

The ADD R1, R2, R3 instruction:

• IF (fetch): fetches the instruction ADD R1, R2, R3 from memory
• ID (decode): determines that the operands are R2 and R3 and the destination is R1
• EX (execute): performs the addition R2 + R3
• MEM (memory access): no memory access is needed
• WB (write back): writes the result back into R1

The LOAD R1, [R2] instruction:

• IF (fetch): fetches the instruction LOAD R1, [R2] from memory
• ID (decode): determines the base register R2
• EX (execute): computes the effective address (the contents of R2)
• MEM (memory access): reads the data from memory
• WB (write back): writes the data back into R1

3. The Data Path

The Concept of the Data Path

Basic definition:

• The path along which data flows inside the CPU
• The data transfer channel that connects the functional components
• Realises the data transfers of instruction execution

Composition of the data path:

• ALU: performs the operations
• Register set: stores the data
• Multiplexers: select the source of the data
• Shifter: performs the shift operations

The Structure of the Data Path

The ALU part:

• Arithmetic unit: performs the arithmetic and logical operations
• Input multiplexer: selects the operands
• Output register: stores the result of the operation

The register part:

• Register set: stores general-purpose data
• Special-purpose registers: PC, SP, and so on
• Register file: multi-port access

The control part:

• Control signal generation: generates the control signals according to the instruction
• Timing control: coordinates the work of the components
• Status control: manages the state of the CPU

Data Path Operations

Reading data:

1. Read the operand from the register
2. Select the data through the multiplexer
3. Feed it into the ALU for the operation

Writing data:

1. The result of the ALU operation
2. Through the write-back path
3. Into the destination register

Transferring data:

1. Between registers
2. Between memory and registers
3. Between I/O devices and registers

4. The Control Unit

The Functions of the Control Unit

Instruction control:

• Controls the fetching, decoding, and execution of instructions
• Manages the order in which instructions execute
• Handles branch and jump instructions

Timing control:

• Generates the clock signal
• Coordinates the timing of the components
• Manages the instruction cycle

Data control:

• Controls the flow of data
• Manages the data transfers
• Handles data conflicts

Types of Control Unit

Hardwired control unit:

• Implementation: a combinational logic circuit
• Characteristics: fast, but inflexible
• Application: RISC processors

Microprogrammed control unit:

• Implementation: a sequence of microinstructions
• Characteristics: highly flexible, but slower
• Application: CISC processors

Control Signals

Timing signals:

• Clock signal: synchronises the work of the components
• Reset signal: initialises the system
• Enable signal: controls whether a component works

Data signals:

• Read signal: controls reading data
• Write signal: controls writing data
• Select signal: controls the selection of data

Status signals:

• Flag bits: the status of the result of an operation
• Interrupt signal: an external interrupt request
• Exception signal: internal exception handling

5. CPU Performance Metrics

Basic Performance Metrics

Clock frequency:

• The number of times the CPU oscillates per second
• Unit: Hz, MHz, GHz
• Impact: the speed of instruction execution

CPI (clock cycles per instruction):

• The average number of clock cycles needed per instruction
• Reflects the efficiency of instruction execution
• Goal: lower the CPI

MIPS (million instructions per second):

• The number of instructions executed per second
• Unit: MIPS
• Calculation: MIPS = clock frequency / (CPI × 10^6)

Advanced Performance Metrics

IPC (instructions per clock cycle):

• The number of instructions executed per clock cycle
• IPC = 1 / CPI
• Reflects the degree of parallelism

Throughput:

• The number of tasks processed per unit of time
• Reflects the overall performance of the system
• Influenced by several factors

Response time:

• The time needed to complete a task
• Includes the CPU time and the waiting time
• Impacts the user experience

Performance Optimisation

Hardware optimisation:

• Raise the clock frequency
• Add more cache
• Optimise the pipeline

Software optimisation:

• Optimise the algorithms
• Reduce the instruction count
• Raise the cache hit rate

System optimisation:

• Balance the performance of the components
• Reduce the bottlenecks
• Raise the overall efficiency

Exercises

Exercise 1

Briefly describe the basic structure of the CPU and its main functions.

Exercise 2

Which stages make up the instruction execution process?

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Glossary