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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

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 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

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

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


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

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