Processor Technology and Architecture 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 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

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 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? 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 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 Draw diagram from handout

CPU Fetch Execute Cycle

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 Based on this instruction format calculate how many instructions are possible

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? 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

Instructions 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 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. 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 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 Note text uses command “MOVE” not included in set for simple machine

Data Transformation Show truth tables for each operation

Logical shift Used to isolate the value of specific bit in a register

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) 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 http://www.heyrick.co.uk/assembler/psr.html This is an example of how the PSW is used for a processor manufactured by ARM, a processor manufacturer in Australia http://www.arm.com/

Arithmetic shift Numbers (formulas) can be rewritten to take advantage of shift

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 Demo in simple machine use # 5

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 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 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 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) Example 133 mhtz is 133,000,000 cycles per second

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 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 General purpose Collection of registers that can be used to store intermediate input and output of ALU operations Example 34 + 31 + 44 first 34 is added to 31 and placed in a register, then 44 is added to the register

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 Explain other uses of flags

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 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 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

Enhancing Processor Performance Memory caching (See Chapter 5.) Pipelining Method of organizing CPU circuitry to enable multiple instructions to execute simultaneously in different stages Branch prediction and speculative execution Ensure pipeline is kept full while executing conditional branch instructions Multiprocessing Duplicate CPUs or processor stages execute in parallel

Range of Possible Approaches for Multiprocessing Duplicate circuitry for some or all processing stages within a single CPU Duplicate CPUs implemented as separate microprocessors sharing main memory and a single system bus Duplicate CPUs on a single microprocessor that also contains main memory caches and a special bus to interconnect the CPUs

The physical CPU

Gate design for addition Show how an adder works

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. 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 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