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Chapter 4. Processor Technology and Architecture. Chapter goals. Describe CPU instruction and execution cycles Explain how primitive CPU instructions are combined to form complex processing operations

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Chapter 4

Chapter 4

Processor Technology and Architecture

Chapter goals

Chapter goals

  • Describe CPU instruction and execution cycles

  • Explain how primitive CPU instructions are combined to form complex processing operations

  • Describe the key CPU design features, including instruction format, word size, and clock rate

  • Describe the function of general-purpose and special-purpose registers

  • Compare and contrast CISC and RISC CPUs

  • Describe the principles and limitations of semiconductor-based microprocessors

Model of central processing unit

Model of Central Processing Unit

How the cpu works

How the CPU works

  • CPU is a complex electronic device that carries out instructions

  • Called the “brains” of a computer

  • Is a combination of parts that through a carefully coordinated process execute code

Cpu parts

CPU parts

Control Unit – moves data and instructions between main memory and registers

Arithmetic and Logic Unit – performs all computation and comparison operations

Registers – fixed size high speed storage locations that hold inputs and outputs for the ALU

How does cpu execute code

How does CPU execute code?

  • CPU can only execute machine code

  • Machine code is a predetermined set (defined by hardware manufacturer) of instructions CPU can execute

  • Machine code is in binary format (0s and 1s)

  • Process of executing code is called the “Fetch Execute Cycle”

The fetch execute cycle

The Fetch Execute Cycle

  • Program counter (pc) points to the next instruction to be execute

  • Instruction is loaded into instruction register and program counter is incremented

  • Instruction is de-coded or separated into OPCODE and addresses

  • Instruction is executed and results are stored if required

Cpu fetch execute cycle

CPU Fetch Execute Cycle

Cpu instructions

CPU Instructions

  • Instruction is a single command a CPU is capable of carrying out

  • Instruction is formatted as a bit string, i.e. a sequence of 0s and 1s

    • Opcode – unique binary number representing operation to be performed

    • Operand(s) – reference or pointer to data needed for operation

Instruction format

Instruction format

Opcodes and operands

Opcodes and operands

  • Opcodes – unique binary number representing an operation to be carried out

  • Operand(s) – reference(s) to location of data needed for operation

    • Register #

    • Memory address

    • Secondary storage or I/O device

How is instruction executed

How is instruction executed?

  • Instruction directs CPU to route data through a built-in set of circuitry (i.e. a series of logic gates) designed to carry out the desired function

  • Circuitry takes input signals and depending on sequence and number of logic gates produces the desired output signal

  • Output signal is stored in a register

  • Then may be stored in memory, secondary storage, or used by a subsequent instruction



  • Some instructions are just handled by the control unit

    • Moving or copying data

    • Halting or restarting the CPU

  • Other instructions require coordination with the ALU

    • Computation

    • Logic (comparisons)

Instruction set

Instruction set

  • The collection of all possible instructions CPU can execute is called the “instruction set”

  • Predetermined by hardware manufacturer

  • Vary greatly from machine to machine (even with the same manufacturer)

Instruction set cont

Instruction set cont.

  • Since instruction sets vary so much, we will describe what is generally in most machines

  • Specific “machine code” we will learn will be for the machine simulator presentation

General instruction categories

General instruction categories

  • Data movement (really a copy command, original bit pattern is unchanged)

    • Load – copies data from memory into a register

    • Store – copies data from a register into memory

Data transformation

Data Transformation

Logical shift

Logical shift

Using logical shift

Using logical shift

  • Computers often use Boolean (true false) values to control processes

  • These values (called flags) can be stored in a single bit

  • Therefore, a 32 bit register can contain 32 individual flags to identify 32 separate conditions

Program status word psw

Program status word (PSW)

  • See p. 133 in text

  • PSW used by CPU to store status information for currently executing instruction

    • Store the result of a comparison (equal or not equal, T or F)

    • Indicate overflow and underflow conditions

How a psw is used

How a PSW is used


  • This is an example of how the PSW is used for a processor manufactured by ARM, a processor manufacturer in Australia


Arithmetic shift

Arithmetic shift

Sequence control

Sequence control

  • Default sequence (order) of program instructions is one after another

  • Can override through BRANCH or JUMP

    • unconditional – new address of next instruction is loaded into PC (JUMP)

    • conditional – new address of instruction is loaded depending on result of some comparison (BRC & BRP in simple machine)

  • HALT – ends execution

Sequence control cont

Sequence control cont.

  • Allows loops (iteration)for (int 1=0; i <10; i++) cout << “\nHello”;

  • Allows decision statements if (speed >= 65) cout << “Speeding ticket”;else cout << “Legal speed”;

Variations in instruction format

Variations in instruction format

  • Formats can vary as to opcode size

  • meaning of opcode values

  • Number of operands

  • Data types used as operands

  • Length and coding format of each operand

Reduced instruction set computing

Reduced Instruction Set Computing

  • Analysis of actual software found that certain instructions made up the vast majority of machine code

  • Many instructions used very infrequently

  • CPU design that limited instruction set found to be much faster

Risc vs cisc


  • Pentium (RISC) vs. 486 (CISC)

  • CISC bloated instruction set slowed down execution time

  • CISC CPU larger and slower than necessary

Clock rate

Clock rate

  • System clock is a timing device that generates timing pulses or signals that are transmitted devices throughout the computer

  • Frequency or rate (clock rate) is measured in hertz (Hz) and megahertz (MHz)

Clock rate cont

Clock rate cont.

  • CPU uses timing of clock to trigger its actions (i.e. fetch, execute, store)

  • Clock is also used by other devices like secondary storage

  • CPU must often wait for slower devices (secondary storage, RAM)

    • Wait state – cycle where CPU is idle waiting for other devices

Measuring cpu speed

Measuring CPU speed

  • Clock rate – measured in mHtz

  • MIPS – millions of instructions per second (assumed to be instructions involving integer operations)

  • MFLOPS – millions of floating point operations per second

  • CPU instructions can vary greatly as to length of time for execution

Cpu registers

CPU registers

  • General purpose

    • Collection of registers that can be used to store intermediate input and output of ALU operations

    • Example34 + 31 + 44first 34 is added to 31 and placed in a register, then 44 is added to the register

Special purpose registers

Special purpose registers

  • Several registers in CPU are set aside for specific purposes:

    • Instruction register – holds the currently executing instruction

    • Program counter (PC) – points to the next instruction to be executed

    • Program status word (PSW) – set of flags (bits) indicating certain conditions

Word size

Word size

  • Unit of data that contains a fixed number of bits

  • Determines the amount of data CPU can process at one time

  • Corresponds to size of general purpose registers

Optimal word size

Optimal word size

  • Should be same size as system bus

    • If bus is smaller every load and store operation requires multiple transfers

  • Word size should correspond to size of data used in the machine

    • Int float data types are 4 bytes (32 bits)

      • Double is 8 bytes (64 bits)

Current word sizes

Current word sizes

  • Most desktop machines are 32 bit word size

  • Doubling word size to 64 increases CPU components by 2.5 to 3 times

  • Larger word increases CPU fabrication cost

  • Since the rest of the machine operates at 32 bit (system bus and secondary storage) this larger word size is not yet an advantage

The physical cpu

The physical CPU

Gate design for addition

Gate design for addition

Chapter summary

Chapter summary

  • The CPU continuously alternates between the instruction, or fetch cycle and execution cycle

  • Primitive CPU instructions can be classified into three types:

    • Data movement

    • Data transformation

    • Sequence control

Summary cont

Summary cont.

  • An instruction formation is a template describing the op code position and the length and the position, type and length of each operand

  • The CPU clock rate is the number of instruction and execution cycles potentially available in a fixed time interval

Summary cont1

Summary cont.

  • CPU registers are of two types:

    • General purpose

    • Special purpose

  • Word size is the number of bits that a CPU can process simultaneously

  • CPUs are electrical devices implemented as silicon-based microprocessors

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