COMPUTER ORGANIZATION AND DESIGN 5th
Edition
The Hardware/Software Interface
Chapter 1
Computer Abstractions
and Technology
Classes of Computers
Personal computers
General purpose, variety of software
Subject to cost/performance tradeoff
Server computers
Network based
High capacity, performance, reliability
Range from small servers to building sized
Chapter 1 — Computer Abstractions and Technology — 2
Classes of Computers
Supercomputers
High-end scientific and engineering
calculations
Highest capability but represent a small
fraction of the overall computer market
Embedded computers
Hidden as components of systems
Stringent power/performance/cost constraints
Chapter 1 — Computer Abstractions and Technology — 3
What You Will Learn
How programs are translated into the
machine language
And how the hardware executes them
The hardware/software interface
What determines program performance
And how it can be improved
How hardware designers improve
performance
What is parallel processing
Chapter 1 — Computer Abstractions and Technology — 4
Understanding Performance
Algorithm
Determines number of operations executed
Programming language, compiler, architecture
Determine number of machine instructions executed
per operation
Processor and memory system
Determine how fast instructions are executed
I/O system (including OS)
Determines how fast I/O operations are executed
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§1.2 Eight Great Ideas in Computer Architecture
Eight Great Ideas
Design for Moore’s Law
Use abstraction to simplify design
Make the common case fast
Performance via parallelism
Performance via pipelining
Performance via prediction
Hierarchy of memories
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§1.3 Below Your Program
Below Your Program
Application software
Written in high-level language
System software
Compiler: translates HLL code to
machine code
Operating System: service code
Handling input/output
Managing memory and storage
Scheduling tasks & sharing resources
Hardware
Processor, memory, I/O controllers
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Levels of Program Code
High-level language
Level of abstraction closer
to problem domain
Provides for productivity
and portability
Assembly language
Textual representation of
instructions
Hardware representation
Binary digits (bits)
Encoded instructions and
data
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§1.4 Under the Covers
Components of a Computer
The BIG Picture Same components for
all kinds of computer
Desktop, server,
embedded
Input/output includes
User-interface devices
Display, keyboard, mouse
Storage devices
Hard disk, CD/DVD, flash
Network adapters
For communicating with
other computers
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Inside the Processor (CPU)
Datapath: performs operations on data
Control: sequences datapath, memory, ...
Cache memory
Small fast SRAM memory for immediate
access to data
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Inside the Processor
Apple A5 32-SoC
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A Safe Place for Data
Volatile main memory
Loses instructions and data when power off
Non-volatile secondary memory
Magnetic disk
Flash memory
Optical disk (CDROM, DVD)
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§1.5 Technologies for Building Processors and Memory
Technology Trends
Electronics
technology
continues to evolve
Increased capacity
and performance
Reduced cost
DRAM capacity
Year Technology Relative performance/cost
1951 Vacuum tube 1
1965 Transistor 35
1975 Integrated circuit (IC) 900
1995 Very large scale IC (VLSI) 2,400,000
2013 Ultra large scale IC 250,000,000,000
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Manufacturing ICs
Yield: proportion of working dies per wafer
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Intel Core i7 Wafer
300mm wafer, 280 chips, 32nm technology
Each chip is 20.7 x 10.5 mm
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Integrated Circuit Cost
Cost per wafer
Cost per die
Dies per wafer Yield
Dies per wafer Wafer area Die area
1
Yield
(1 (Defects per area Die area/2))2
Nonlinear relation to area and defect rate
Wafer cost and area are fixed
Defect rate determined by manufacturing process
Die area determined by architecture and circuit design
Chapter 1 — Computer Abstractions and Technology — 16
§1.6 Performance
Defining Performance
Which airplane has the best performance?
Boeing 777 Boeing 777
Boeing 747 Boeing 747
BAC/Sud BAC/Sud
Concorde Concorde
Douglas Douglas DC-
DC-8-50 8-50
0 100 200 300 400 500 0 2000 4000 6000 8000 10000
Passenger Capacity Cruising Range (miles)
Boeing 777 Boeing 777
Boeing 747 Boeing 747
BAC/Sud BAC/Sud
Concorde Concorde
Douglas Douglas DC-
DC-8-50 8-50
0 500 1000 1500 0 100000 200000 300000 400000
Cruising Speed (mph) Passengers x mph
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Response Time and Throughput
Response time
How long it takes to do a task
Throughput
Total work done per unit time
e.g., tasks/transactions/… per hour
How are response time and throughput affected
by
Replacing the processor with a faster version?
Adding more processors?
We’ll focus on response time for now…
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Relative Performance
Define Performance = 1/Execution Time
“X is n time faster than Y”
Performance X Performance Y
Execution timeY Execution timeX n
Example: time taken to run a program
10s on A, 15s on B
Execution TimeB / Execution TimeA
= 15s / 10s = 1.5
So A is 1.5 times faster than B
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Measuring Execution Time
Elapsed time
Total response time, including all aspects
Processing, I/O, OS overhead, idle time
Determines system performance
CPU time
Time spent processing a given job
Discounts I/O time, other jobs’ shares
Comprises user CPU time and system CPU
time
Different programs are affected differently by
CPU and system performance
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CPU Clocking
Operation of digital hardware governed by a
constant-rate clock
Clock period
Clock (cycles)
Data transfer
and computation
Update state
Clock period: duration of a clock cycle
e.g., 250ps = 0.25ns = 250×10–12s
Clock frequency (rate): cycles per second
e.g., 4.0GHz = 4000MHz = 4.0×109Hz
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CPU Time
CPU Time CPU Clock Cycles Clock Cycle Time
CPU Clock Cycles
Clock Rate
Performance improved by
Reducing number of clock cycles
Increasing clock rate
Hardware designer must often trade off clock
rate against cycle count
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CPU Time Example
Computer A: 2GHz clock, 10s CPU time
Designing Computer B
Aim for 6s CPU time
Can do faster clock, but causes 1.2 × clock cycles
How fast must Computer B clock be?
Clock CyclesB 1.2 Clock Cycles A
Clock RateB
CPU TimeB 6s
Clock Cycles A CPU TimeA Clock Rate A
10s 2GHz 20 109
1.2 20 109 24 109
Clock RateB 4GHz
6s 6s
Chapter 1 — Computer Abstractions and Technology — 23
Instruction Count and CPI
Clock Cycles Ins truction Count Cycles per Ins truction
CPU Tim e Ins truction Count CPI Clock Cycle Tim e
Ins truction Count CPI
ClockRate
Instruction Count for a program
Determined by program, ISA and compiler
Average cycles per instruction
Determined by CPU hardware
If different instructions have different CPI
Average CPI affected by instruction mix
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CPI Example
Computer A: Cycle Time = 250ps, CPI = 2.0
Computer B: Cycle Time = 500ps, CPI = 1.2
Same ISA
Which is faster, and by how much?
CPU Tim e Ins truction Count CPI Cycle Tim e
A A A
I 2.0 250ps I 500ps A is faster…
CPU Tim e Ins truction Count CPI Cycle Tim e
B B B
I 1.2 500ps I 600ps
B I 600ps 1.2
CPU Tim e
…by this much
CPU Tim e I 500ps
A
Chapter 1 — Computer Abstractions and Technology — 25
CPI in More Detail
If different instruction classes take different
numbers of cycles
n
Clock Cycles (CPIi Instruction Counti )
i1
Weighted average CPI
Clock Cycles n
Instruction Counti
CPI CPIi
Instruction Count i1 Instruction Count
Relative frequency
Chapter 1 — Computer Abstractions and Technology — 26
CPI Example
Alternative compiled code sequences using
instructions in classes A, B, C
Class A B C
CPI for class 1 2 3
IC in sequence 1 2 1 2
IC in sequence 2 4 1 1
Sequence 1: IC = 5 Sequence 2: IC = 6
Clock Cycles Clock Cycles
= 2×1 + 1×2 + 2×3 = 4×1 + 1×2 + 1×3
= 10 =9
Avg. CPI = 10/5 = 2.0 Avg. CPI = 9/6 = 1.5
Chapter 1 — Computer Abstractions and Technology — 27
Performance Summary
The BIG Picture
Instructions Clock cycles Seconds
CPU Time
Program Instruction Clock cycle
Performance depends on
Algorithm: affects IC, possibly CPI
Programming language: affects IC, CPI
Compiler: affects IC, CPI
Instruction set architecture: affects IC, CPI, Tc
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Multiprocessors
Multicore microprocessors
More than one processor per chip
Requires explicitly parallel programming
Compare with instruction level parallelism
Hardware executes multiple instructions at once
Hidden from the programmer
Hard to do
Programming for performance
Load balancing
Optimizing communication and synchronization
Chapter 1 — Computer Abstractions and Technology — 29
SPEC CPU Benchmark
Programs used to measure performance
Supposedly typical of actual workload
Standard Performance Evaluation Corp (SPEC)
Develops benchmarks for CPU, I/O, Web, …
SPEC CPU2006
Elapsed time to execute a selection of programs
Negligible I/O, so focuses on CPU performance
Normalize relative to reference machine
Summarize as geometric mean of performance ratios
CINT2006 (integer) and CFP2006 (floating-point)
n
n
Execution time ratio
i1
i
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§1.10 Fallacies and Pitfalls
Pitfall: Amdahl’s Law
Improving an aspect of a computer and
expecting a proportional improvement in
overall performance
Taffected
Timproved Tunaffected
improvement factor
Example: multiply accounts for 80s/100s
How much improvement in multiply performance to
get 5× overall?
80 Can’t be done!
20 20
n
Corollary: make the common case fast
Chapter 1 — Computer Abstractions and Technology — 31
Pitfall: MIPS as a Performance Metric
MIPS: Millions of Instructions Per Second
Doesn’t account for
Differences in ISAs between computers
Differences in complexity between instructions
Instruction count
MIPS
Execution time 106
Instruction count Clock rate
Instruction count CPI CPI 10 6
10 6
Clock rate
CPI varies between programs on a given CPU
Chapter 1 — Computer Abstractions and Technology — 32
§1.9 Concluding Remarks
Concluding Remarks
Cost/performance is improving
Due to underlying technology development
Hierarchical layers of abstraction
In both hardware and software
Instruction set architecture
The hardware/software interface
Execution time: the best performance
measure
Power is a limiting factor
Use parallelism to improve performance
Chapter 1 — Computer Abstractions and Technology — 33
