CALectureWeek3.ppt

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CARC103 – Computer Architecture
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Prescribed Text
Bird, S. D. (2017), Systems Architecture, 7th ed, Cengage Learning
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Systems Architecture,

Seventh Edition
Chapter 4
Processor Technology and Architecture
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Systems Architecture, Seventh Edition

Chapter Objectives
In this chapter, you will learn to:
Describe CPU instruction and execution cycles
Explain how primitive CPS instructions are combined to form complex processing operations
Describe key CPU design features including instruction format, word size, and clock rate
Describe the function of general-purpose and special-purpose registers
Describe the principles and limitations of semiconductor-based microprocessors

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Systems Architecture, Seventh Edition

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FIGURE 4.1 Topics covered in this chapter
Courtesy of Course Technology/Cengage Learning

Systems Architecture, Seventh Edition

CPU Components & Functions
The central processing unit (CPU) is the computer system “brain”:
Executes program instructions including computation, comparison, and branching
Directs all computer system actions including processing, storage, input/output, and data movement
CPU components include:
Control unit – directs flow of data to/from memory, registers, and the arithmetic logic unit
Arithmetic logic unit (ALU) – executes computation and comparison instructions
Registers – storage locations within the CPU that hold ALU inputs, ALU outputs, and other data for fast access

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Systems Architecture, Seventh Edition

CPU and Other Computer System Components
Figure 4.2 CPU components
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Instruction and Execution Cycles
The CPU constantly alternates between two stages (or cycles):
Instruction cycle:
Also called the fetch cycle
The control unit reads an instruction from primary storage
The control unit increments the instruction pointer (address of the next instruction to be read)
The control unit stores the instruction is stored in the instruction register
If there are data inputs embedded in the instruction they’re loaded into registers as inputs for the ALU
If the instruction includes memory addresses of data inputs they’re copied from memory and loaded into registers as inputs for the ALU
Execution cycle:
Data movement instructions are executed by the control unit itself
Computation and comparison instructions are executed by the ALU in response to a signal from the control unit. Data inputs flow from registers through processing circuitry and the output(s) flows to one or more registers

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Systems Architecture, Seventh Edition

Instruction and Execution Cycles – Continued
Figure 4.3 Control and data flow during the fetch and execution cycles
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Instruction Format
An instruction is a command to the CPU to perform a single processing function on specific data inputs
As stored in memory or a register, an instruction is a sequence of bits that must be decoded to extract the processing function and data inputs (or the location of the data inputs
Instruction components:
Op code – a unique binary number representing the processing function and a template for extracting the operands
Operands – one or more groups of bits after the op code that contain data to be processed or identify the location of that data (a register or memory address)

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Systems Architecture, Seventh Edition

Instruction Format – Continued
Different kinds of operands have different lengths depending on the type of data or address stored therein
The same processing function may correspond to many different op-codes with different operand formats (e.g., an ADD instruction for integers stored as operands, another for integers stored in registers, and another for integers stored in memory)

FIGURE 4.4 An instruction containing one op code and two operands
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Instruction Summary
MOVE – Copy data from:
A memory address to a register (a load operation)
A register to memory address (a store operation)
A register to another register
Boolean logic – convert individual bits within a bit string (bitwise operations) or treat entire bit strings as true or false and manipulate/combine them (logic operations)
NOT – flip every bit, or change true to false and vice versa
AND – two 1 bits yields a 1 but, all other combinations are 0, or two trues are true, all other combinations are false
OR – two 0 bits yields a 0, all other combinations are 1, or two falses are false, all other combinations are true
Exclusive OR (XOR) – 0 and 1 are 1, all other combinations are 0, or true and false is true, all other combinations are false

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Systems Architecture, Seventh Edition

Instruction Summary – Continued
ADD
Produce the arithmetic sum of two bit strings
Need multiple ADD instructions, one per data type/format
SHIFT
Move all bits left or right and fill in zeros
Can be used to extract single bit values (logical shift)
Can be used for binary multiplication and division (arithmetic shift)

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Systems Architecture, Seventh Edition

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FIGURE 4.5 Original data byte (a) shifted 2 bits to the right (b)
Courtesy of Course Technology/Cengage Learning

Systems Architecture, Seventh Edition

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FIGURE 4.6 Extracting a single bit with logical SHIFT instructions
Courtesy of Course Technology/Cengage Learning

Systems Architecture, Seventh Edition

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FIGURE 4.7 Multiplying and dividing unsigned binary values with SHIFT instructions
Courtesy of Course Technology/Cengage Learning

Systems Architecture, Seventh Edition

Instruction Summary – Continued
BRANCH
Also called JUMP
Alters next instruction fetched/executed
Unconditional branch – always changes sequence (e.g., a GOTO statement)
Conditional branch – changes only if the value true is stored in a register (value was stored as a result of a previous comparison instruction)
HALT – self-explanatory

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Systems Architecture, Seventh Edition

Simple and Complex Instructions
The instructions on the previous slides comprise the minimal set of instructions needed to implement a full-fledged CPU, for example:
More complex operations such as exponentiation and operations on non-integer data types can be implemented by complex combinations of the simple instructions
For example, subtraction can be implemented as:

710−310 = ADD(ADD(XOR(0011,1111),0001),0111)
= ADD(ADD(1100,0001),0111)
= ADD(1101,0111)
= 10100
Pros of providing only a minimal instruction set
Processor is simple to build and construct
Simple = cheaper CPUs with very fast clock rates
Cons of providing only a minimal instruction set
Programs that need complex processing/data are complex
Complex = expensive, slow, and error-prone program development

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Systems Architecture, Seventh Edition

Simple and Complex Instructions – Continued
Modern CPUs provide a far richer set of instructions than the minimal set:
Duplicate instructions for multiple data types (e.g., signed/unsigned, integer/real, and single/double-precision)
Higher-order computational functions (e.g., subtraction. Multiplication/division, exponentiation, trig functions)
Higher-order logical functions (e.g., greater than or equal to)
Instructions that combine data movement to/from memory with processing
Complex silicon CPU circuits are cheaper than programmers!

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Systems Architecture, Seventh Edition

RISC versus CISC
Reduced Instruction Set Computing (RISC)
Avoid “unnecessary” complex instructions – keep instruction count to several dozen to a few hundred
Minimize number/complexity of instruction formats
Minimize maximum instruction length
Avoid combining data movement with transformation (sometimes called load-store architecture)
“Less is more”
For example, IBM POWER CPUs
Complex Instruction Set Computing (CISC)
Opposite of RISC
For example, Intel Core and Xeon CPUs

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Systems Architecture, Seventh Edition

RISC versus CISC – Continued
CPU Complexity/Speed
RISC simplifies the job of the control unit by simplifying the instruction set
Simpler fetch = faster fetch = higher clock rate?
Program execution speed
Higher clock rate = faster program execution?
BUT:
No complex instructions
Thus, more instructions must be fetched/executed to do “complex” operations
Bottom line – it’s a trade-off among:
Clock rate
Number of complex operations in “typical” programs
Relative penalty/benefit of providing or not providing single CPU instructions for those complex operations
Also:
The contrast isn’t a stark as the manufacturer’s white papers might lead you to believe
Both “camps” borrow heavily from the others’ best ideas

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Systems Architecture, Seventh Edition

Clock Rate
The CPU has an internal clock that generates “ticks” at regular intervals:
The CPU clock rate is the frequency of those ticks
Typically stated in gigahertz (GHz) – billions of cycles (ticks) per second
Fetch/execute cycles are fractions of the clock rate
Clock rate is assumed to be the time needed to fetch (with no wait states) and execute the simplest instruction (e.g., NOT)
Modern CPUs are much too complex for that simplistic assumption to work (more on these topics later)!
Memory caches
Multiple core processors
Multiple ALUs per core
Pipelining

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Systems Architecture, Seventh Edition

MIPS and MFLOPS
Instruction-oriented performance measures include:
MIPS – millions of (fetched/executed) instructions per second – presumed to be integer instructions or a “typical” mix
MFLOPS – millions of (fetched/executed) floating point operations per second
Both terms are outdated as modern CPUs get faster (e.g., GFLOPS, TFLOPS, and PFLOPS)
Both terms can apply to performance of:
Processor in isolation
Entire computer system

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Systems Architecture, Seventh Edition

MIPS and MFLOPS – Continued
MIPS/MFLOPS may be lower than implied by clock rate – WHY?
Programs do more than execute NOT statements!
More complex operations require more execution time (multiple clock cycles)
Wait states for:
Access to memory
Access to system bus
Access to storage and I/O devices

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Systems Architecture, Seventh Edition

Benchmarks
A benchmark is a performance measure for a computer system or one of its components when performing a specific and realistic type of software task, for example:
Responding to an HTTP request
Processing a complex database transaction
Reading/writing a disk
Redrawing the screen in an animation
Combinations of the above
Benchmarks can roughly divided into 2 classes:
Artificial – a “made-up” workload that is supposed to be representative of a class of real workloads
Live-Load – a workload based on “real” tasks such as playing an online game, encoding a DVD, or responding to web server requests
Benchmarks have their limitations, but even the artificial ones are generally more realistic and reliable indicators of computer system performance than MIPS and MFLOPS.

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Systems Architecture, Seventh Edition

Sample Benchmarks
Standard Performance Evaluation Corporation (SPEC) provides a suite a benchmarks including:
SPEC CPU: computational performance with integers and floating point numbers
SPEC MPI: computational performance of problems distributed across a cluster
SPECviewperf: workstation graphics performance
SPECmail: email server performance

http://www.spec.org

TPC
Server-oriented performance for processing business or database transactions

http://www.tpc.org

PassMark
Test suite for microcomputers

http://www.passmark.com

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Systems Architecture, Seventh Edition

Registers
Registers can be roughly divided into two classes:
General-purpose
Used as high-performance scratchpad memory by the ALU(s)
More are better up to a point (law of diminishing returns)
Modern CPUs typically provide a few dozen per ALU
Special-purpose registers
Used primarily by the control unit in CPU management tasks
Examples include:
Instruction pointer – memory address for next instruction fetch, a.k.a. program counter
Instruction register – copy of most recently fetched instruction
Program status word (PSW) – Goes by many different names – Set of bit flags containing error and other codes related to processing results, for example:
Result of comparison operations
Divide by zero
Overflow and underflow

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Systems Architecture, Seventh Edition

Word Size
A word is:
A fixed number of bits/bytes
The basic “unit” of data transformation in a CPU
The size of a data item that the CPU manipulates when executing a “normal” instruction
The size of a memory address?
The term has fallen into disuse as ever more complex CPU designs employ multiple word sizes
For example, a 64-bit Intel Core CPU has word sizes ranging from 16 to 128 bits

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Systems Architecture, Seventh Edition

Word Size and Performance
ALU circuitry manipulates all bits of a word in parallel while executing a single instruction
Larger word size implies larger and more complex ALU and other circuitry thus increasing CPU expense and slowing clock rate (all other things being equal)
Mismatches between CPU word size and the size of data items manipulated by a program include:
CPU word size > program data size
Lots of zeros are carried through fetches, registers, and ALU circuitry
Performance is suboptimal – CPU is more complex than the program requires – more complex = slower
Cost is higher than needed since “extra” word size is unused
CPU word size = program data size
Performance and cost are both optimal – best case scenario
CPU word size < program data size Avoids cost of extra bits Incurs substantial performance penalty due to breaking data items into word-sized chunks and performing piece-wise operations on the words Performance penalty varies with the size mismatch and the complexity of the processing function(s) Cost of CPU is lower since small word size = simpler CPU = less expensive CPU Take the cost statements in the slide with half a shaker of salt – modern CPUs are so cheap that word size must be VERY large to significantly increase cost Bottom line – for best cost/performance ratio, match CPU word size to the size of data that will be processed (assuming that’s feasible) * Systems Architecture, Seventh Edition Word Size and Performance - Continued Typical “normal data sizes” “Business” applications – 32 or 64 bits “Scientific” applications – 64 or 128 bits Database and multimedia applications – highly variable, but more is generally better! Early CPUs had small word size (e.g., 8 or 16 bits) due to technology limitations and thus had suboptimal performance for all but the simplest applications The gap between needed and actual CPU word size continued until the early/mid 2000s Most modern CPUs have 64-bit word size Will 128-bit CPUs appear? When? Why? * Systems Architecture, Seventh Edition Performance Enhancement Techniques Thus far we’ve described a relatively simplistic view of CPU operation that matches CPUs of the 1960s-1980s As fabrication technology has improved, CPU designers have been able to employ ever more complex performance improvement techniques individually and in combination, including: Memory caching (Chapter 5) Pipelining Branch prediction and speculative execution Multiprocessing * Systems Architecture, Seventh Edition Pipelining Pipelining is a Henry Ford era technique (i.e., the sequential assembly line) applied to executing program instructions Execution stages: Fetch from memory Increment and store instruction pointer (IP) Decode instruction and store operands and instruction pointer Access ALU inputs Execute instruction within the ALU Store ALU output Pipelining attempts to overlap instruction execution by performing each stage on a different instruction at the same time * Systems Architecture, Seventh Edition * FIGURE 4.10 Overlapped instruction execution via pipelining Courtesy of Course Technology/Cengage Learning Systems Architecture, Seventh Edition Pipelining - Continued Sounds great in theory, but there are some complexities with which to deal: Is one instruction pointer enough? Is one instruction register enough? Is one set of general purpose registers enough? Is one ALU enough? What happens if a branch is encountered? Pipelining can be “finer-grained” than we’ve shown thus far For example, execution (usually the longest stage) could be (and often is) further subdivided into additional stages) * Systems Architecture, Seventh Edition Multiprocessing Pipelining goes hand-in-hand with at least some duplication of processor circuitry
Multiprocessing carries the duplication to higher levels, such as:
Multiple ALUs (with parallel execution of instructions) per CPU (common by late 1990s)
Multiple CPUs on a single motherboard (common by early 2000s)
Multiple CPUs on a single chip (common by late 2000s)
Operating systems are more complex because they now manage more processing resources and more complex application software
Application software that takes advantage of multiprocessing is more complex because it must be designed for parallel execution (a.k.a. multithreading as discussed in a later chapter)

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Systems Architecture, Seventh Edition

Branch Prediction and Speculative Execution
Branches cause problems with pipelining because they invalidate the partially executed instructions that follow them:
The wrong instructions (after the branch) were fetched and partially executed
Special- and general-purpose register contents are incorrect
The pipeline must be flushed and filling it with the proper set of instructions (the branch target) must being anew
Real programs have lots of branches
Thus, pipelining will often “fail” unless preventive measures are employed

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Systems Architecture, Seventh Edition

Branch Prediction and Speculative Execution – Continued
Preventive Measures:
Look-ahead – “watch” incoming instructions for branches and alter standard behavior accordingly
Branch prediction – if a conditional branch is fetched attempt to guess the condition result and load/execute the corresponding instructions (this is called speculative execution)
Speculatively execute both paths beyond a conditional branch
Requires multiple execution units
Half the results will be thrown away (half the effort is wasted)
Modern CPUs employ all three techniques to improve pipelining performance

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Systems Architecture, Seventh Edition

The Physical CPU
Complex system of interconnected electrical switches
Contains millions of switches, which perform basic processing functions
Physical implementation of switches and circuits

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Systems Architecture, Seventh Edition

Switches and Gates
Switches and gates are building blocks of CPU and memory circuitry:
Switch – a device that can be open or closed to allow or block passage of electricity – implemented as a transistor
Gate – multiple switches wired together to perform a processing function on one bit:

NOT
AND
OR
XOR
NAND
FIGURE 4.12 Electrical component symbols for a signal inverter or NOT gate (a), an AND gate (b), an OR gate (c), an XOR gate (d), and a NAND gate (e)
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Circuits
Gates are wired into circuits to perform more complex processing (e.g., half and full adder below)

FIGURE 4.13 Circuit diagrams for half adder (a) and full adder (b)
Courtesy of Course Technology/Cengage Learning
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Systems Architecture, Seventh Edition

Electricity
Since circuits are electrical devices they benefit and suffer from electricity advantages/limitations:
Speed – electrons move through circuitry at approximately 70% of light speed – Speed of processing is thus directly proportional to circuit length
Conductivity – circuits must be constructed of highly conductive material – e.g., copper or gold
Resistance – even good conductors turn some electrical energy into heat
Circuit length is limited because energy loss accumulates
Heat must be dissipated to prevent higher resistance or physical damage to conductors

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Systems Architecture, Seventh Edition

Electrical Properties
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Conductivity Capability of an element to enable electron flow

Resistance Loss of electrical power that occurs within a conductor

Heat Negative effects of heat:
Physical damage to conductor
Changes to inherent resistance of conductor
Dissipate heat with a heat sink

Speed and circuit length Time required to perform a processing operation is a function of length of circuit and speed of light

Systems Architecture, Seventh Edition

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FIGURE 4.14 A heat sink attached to a surface-mounted microprocessor
Courtesy of Course Technology/Cengage Learning

Systems Architecture, Seventh Edition

Processor Fabrication
Modern CPUs are fabricated as microprocessors – silicon chips containing billions of transistors and their wiring implementing multiple CPUs, memory caches, and memory/bus interface circuitry
Speed has been improved over time by shrinking the physical size of the wires and transistors – currently 22 nanometers

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Systems Architecture, Seventh Edition

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FIGURE 4.15 The Intel 4004 microprocessor containing 2300 transistors
Courtesy of Intel Corporation

Systems Architecture, Seventh Edition

Processor Fabrication – Continued
FIGURE 4.17 A wafer of processors with 410 million transistors each
Courtesy of Intel Corporation
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Systems Architecture, Seventh Edition

Processor Fabrication – Continued
Copyright © 2009 IBM Corporation
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Systems Architecture, Seventh Edition

Processor Fabrication – Looming Problems
Moore’s Law – transistor count on a chip doubles every 18-24 months at no cost increase
Implies greater power and/or speed IF the additional transistors are used as effectively as the previous ones
Rock’s Law – cost of a processor fabrication facility doubles every four years
Currently >10 billion dollars
Process shrinkage has limits that we’ll soon hit:
Etching process requires higher and higher wavelength beams (currently using X-Rays)
Fabrication errors accumulate (e.g., material impurities)
Molecular width of conductors is a theoretical lower bound (single-digit nanometers)

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Systems Architecture, Seventh Edition

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FIGURE 4.18 Increases in transistor count for Intel microprocessors
Courtesy of Course Technology/Cengage Learning

Systems Architecture, Seventh Edition

Processor Fabrication – Where to From Here?
± 10 years of improvements left to current silicon-based fabrication processes
Optical interconnects
Reduces or eliminates wiring
Logical extension of current technology
Unknown price/performance characteristics
Many manufacturing issues yet to be worked out
Optical CPUs – none yet demonstrated in lab
Quantum processors – we don’t fully understand the physics let alone the physical implementation!

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Systems Architecture, Seventh Edition

Summary
CPU operation
Instruction set and format
Clock rate
Registers
Word size
Physical implementation
Future trends

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