12008 컴퓨터 구조 : P & H

3 장 : 컴퓨터 연산

22008 컴퓨터 구조 : P & H

• Bits are just bits (no inherent meaning)— conventions define relationship between bits and

numbers

• Binary numbers (base 2)0000 0001 0010 0011 0100 0101 0110 0111 1000 1001...decimal: 0...2n-1

• Of course it gets more complicated:numbers are finite (overflow)fractions and real numbersnegative numberse.g., no MIPS subi instruction; addi can add a negative

number

• How do we represent negative numbers?i.e., which bit patterns will represent which numbers?

Numbers

32008 컴퓨터 구조 : P & H

• Sign Magnitude: One's Complement Two's Complement

000 = +0 000 = +0 000 = +0001 = +1 001 = +1 001 = +1010 = +2 010 = +2 010 = +2011 = +3 011 = +3 011 = +3100 = -0 100 = -3 100 = -4101 = -1 101 = -2 101 = -3110 = -2 110 = -1 110 = -2111 = -3 111 = -0 111 = -1

• Issues: balance, number of zeros, ease of operations

• Which one is best? Why?

Possible Representations

42008 컴퓨터 구조 : P & H

• 32 bit signed numbers:

0000 0000 0000 0000 0000 0000 0000 0000two = 0ten

0000 0000 0000 0000 0000 0000 0000 0001two = + 1ten

0000 0000 0000 0000 0000 0000 0000 0010two = + 2ten

...0111 1111 1111 1111 1111 1111 1111 1110two = + 2,147,483,646ten

0111 1111 1111 1111 1111 1111 1111 1111two = + 2,147,483,647ten

1000 0000 0000 0000 0000 0000 0000 0000two = – 2,147,483,648ten

1000 0000 0000 0000 0000 0000 0000 0001two = – 2,147,483,647ten

1000 0000 0000 0000 0000 0000 0000 0010two = – 2,147,483,646ten

...1111 1111 1111 1111 1111 1111 1111 1101two = – 3ten

1111 1111 1111 1111 1111 1111 1111 1110two = – 2ten

1111 1111 1111 1111 1111 1111 1111 1111two = – 1ten

maxint

minint

MIPS

52008 컴퓨터 구조 : P & H

• Negating a two's complement number: invert all bits and add 1

– remember: “negate” and “invert” are quite different!

• Converting n bit numbers into numbers with more than n bits:

– MIPS 16 bit immediate gets converted to 32 bits for arithmetic

– copy the most significant bit (the sign bit) into the other bits

0010 -> 0000 0010

1010 -> 1111 1010

– "sign extension" (lbu vs. lb)

Two's Complement Operations

62008 컴퓨터 구조 : P & H

• Just like in grade school (carry/borrow 1s) 0111 0111 0110+ 0110 - 0110 - 0101

• Two's complement operations easy

– subtraction using addition of negative numbers 0111+ 1010

• Overflow (result too large for finite computer word):

– e.g., adding two n-bit numbers does not yield an n-bit number 0111+ 0001 note that overflow term is somewhat misleading, 1000 it does not mean a carry “overflowed”

Addition & Subtraction

72008 컴퓨터 구조 : P & H

• No overflow when adding a positive and a negative number

• No overflow when signs are the same for subtraction

• Overflow occurs when the value affects the sign:

– overflow when adding two positives yields a negative

– or, adding two negatives gives a positive

– or, subtract a negative from a positive and get a negative

– or, subtract a positive from a negative and get a positive

• Consider the operations A + B, and A – B

– Can overflow occur if B is 0 ?

– Can overflow occur if A is 0 ?

Detecting Overflow

82008 컴퓨터 구조 : P & H

• An exception (interrupt) occurs

– Control jumps to predefined address for exception

– Interrupted address is saved for possible resumption

• Details based on software system / language

– example: flight control vs. homework assignment

• Don't always want to detect overflow— new MIPS instructions: addu, addiu, subu

note: addiu still sign-extends!note: sltu, sltiu for unsigned comparisons

Effects of Overflow

92008 컴퓨터 구조 : P & H

• More complicated than addition

– accomplished via shifting and addition

• More time and more area

• Let's look at 3 versions based on a gradeschool algorithm

0010 (multiplicand)

__x_1011 (multiplier)

• Negative numbers: convert and multiply

– there are better techniques, we won’t look at them

Multiplication

102008 컴퓨터 구조 : P & H

Multiplication: Implementation

DatapathControl

MultiplicandShift left

64 bits

64-bit ALU

ProductWrite

64 bits

Control test

MultiplierShift right

32 bits

32nd repetition?

1a. Add multiplicand to product and

place the result in Product register

Multiplier0 = 01. Test

Multiplier0

Start

Multiplier0 = 1

2. Shift the Multiplicand register left 1 bit

3. Shift the Multiplier register right 1 bit

No: < 32 repetitions

Yes: 32 repetitions

Done

112008 컴퓨터 구조 : P & H

Final Version

Multiplicand

32 bits

32-bit ALU

ProductWrite

64 bits

Controltest

Shift right

32nd repetition?

Product0 = 01. Test

Product0

Start

Product0 = 1

3. Shift the Product register right 1 bit

No: < 32 repetitions

Yes: 32 repetitions

Done

What goes here?

•Multiplier starts in right half of product

122008 컴퓨터 구조 : P & H

Floating Point (a brief look)

• We need a way to represent

– numbers with fractions, e.g., 3.1416

– very small numbers, e.g., .000000001

– very large numbers, e.g., 3.15576 109

• Representation:

– sign, exponent, significand: (–1)sign significand 2exponent

– more bits for significand gives more accuracy

– more bits for exponent increases range

• IEEE 754 floating point standard:

– single precision: 8 bit exponent, 23 bit significand

– double precision: 11 bit exponent, 52 bit significand

132008 컴퓨터 구조 : P & H

IEEE 754 floating-point standard

• Leading “1” bit of significand is implicit

• Exponent is “biased” to make sorting easier

– all 0s is smallest exponent all 1s is largest

– bias of 127 for single precision and 1023 for double precision

– summary: (–1)sign significand) 2exponent – bias

• Example:

– decimal: -.75 = - ( ½ + ¼ )

– binary: -.11 = -1.1 x 2-1

– floating point: exponent = 126 = 01111110

– IEEE single precision: 10111111010000000000000000000000

142008 컴퓨터 구조 : P & H

Floating point addition

•

Still normalized?

4. Round the significand to the appropriate

number of bits

YesOverflow or

underflow?

Start

No

Yes

Done

1. Compare the exponents of the two numbers.

Shift the smaller number to the right until its

exponent would match the larger exponent

2. Add the significands

3. Normalize the sum, either shifting right and

incrementing the exponent or shifting left

and decrementing the exponent

No Exception

Small ALU

Exponentdifference

Control

ExponentSign Fraction

Big ALU

ExponentSign Fraction

0 1 0 1 0 1

Shift right

0 1 0 1

Increment ordecrement

Shift left or right

Rounding hardware

ExponentSign Fraction

152008 컴퓨터 구조 : P & H

Floating Point Complexities

• Operations are somewhat more complicated (see text)

• In addition to overflow we can have “underflow”

• Accuracy can be a big problem

– IEEE 754 keeps two extra bits, guard and round

– four rounding modes

– positive divided by zero yields “infinity”

– zero divide by zero yields “not a number”

– other complexities

• Implementing the standard can be tricky

• Not using the standard can be even worse

– see text for description of 80x86 and Pentium bug!

162008 컴퓨터 구조 : P & H

Chapter Three Summary

• Computer arithmetic is constrained by limited precision

• Bit patterns have no inherent meaning but standards do exist

– two’s complement

– IEEE 754 floating point

• Computer instructions determine “meaning” of the bit patterns

• Performance and accuracy are important so there are manycomplexities in real machines

• Algorithm choice is important and may lead to hardware optimizations for both space and time (e.g., multiplication)

• You may want to look back (Section 3.10 is great reading!)

172008 컴퓨터 구조 : P & H

4 장 : 성능

182008 컴퓨터 구조 : P & H

• Measure, Report, and Summarize

• Make intelligent choices

• See through the marketing hype

• Key to understanding underlying organizational motivation

Why is some hardware better than others for different programs?

What factors of system performance are hardware related?(e.g., Do we need a new machine, or a new operating system?)

How does the machine's instruction set affect performance?

Performance

192008 컴퓨터 구조 : P & H

Which of these airplanes has the best performance?

Airplane Passengers Range (mi) Speed (mph)

Boeing 737-100 101 630 598Boeing 747 470 4150 610BAC/Sud Concorde 132 4000 1350Douglas DC-8-50 146 8720 544

•How much faster is the Concorde compared to the 747?

•How much bigger is the 747 than the Douglas DC-8?

202008 컴퓨터 구조 : P & H

• Response Time (latency) [ 응답시간 ]

— How long does it take for my job to run?

— How long does it take to execute a job?

— How long must I wait for the database query?

• Throughput [ 처리량 ]

— How many jobs can the machine run at once?

— What is the average execution rate?

— How much work is getting done?

• If we upgrade a machine with a new processor what do we increase?

• If we add a new machine to the lab what do we increase?

Computer Performance: TIME, TIME, TIME

212008 컴퓨터 구조 : P & H

• Elapsed Time

– counts everything (disk and memory accesses, I/O , etc.)

– a useful number, but often not good for comparison purposes

• CPU time

– doesn't count I/O or time spent running other programs

– can be broken up into system time, and user time

• Our focus: user CPU time

– time spent executing the lines of code that are "in" our program

Execution Time

222008 컴퓨터 구조 : P & H

• For some program running on machine X,

PerformanceX = 1 / Execution timeX

• "X is n times faster than Y"

PerformanceX / PerformanceY = n

• Problem:

– machine A runs a program in 20 seconds

– machine B runs the same program in 25 seconds

Book's Definition of Performance

232008 컴퓨터 구조 : P & H

Clock Cycles

• Instead of reporting execution time in seconds, we often use cycles

• Clock “ticks” indicate when to start activities (one abstraction):

• cycle time = time between ticks = seconds per cycle

• clock rate (frequency) = cycles per second (1 Hz. = 1 cycle/sec)

A 4 Ghz. clock has a cycle time

time

seconds

program

cycles

program

seconds

cycle

(ps) spicosecond 2501210 9104

1

242008 컴퓨터 구조 : P & H

So, to improve performance (everything else being equal) you can either

(increase or decrease?)

________ the # of required cycles for a program, or

________ the clock cycle time or, said another way,

________ the clock rate.

How to Improve Performance

seconds

program

cycles

program

seconds

cycle

252008 컴퓨터 구조 : P & H

• Could assume that number of cycles equals number of instructions

This assumption is incorrect,

different instructions take different amounts of time on different machines.

Why? hint: remember that these are machine instructions, not lines of C code

time

1st

inst

ruct

ion

2nd

inst

ruct

ion

3rd

inst

ruct

ion

4th

5th

6th ...

How many cycles are required for a program?

262008 컴퓨터 구조 : P & H

• Multiplication takes more time than addition

• Floating point operations take longer than integer ones

• Accessing memory takes more time than accessing registers

• Important point: changing the cycle time often changes the number of cycles required for various instructions (more later)

time

Different numbers of cycles for different instructions

272008 컴퓨터 구조 : P & H

• Our favorite program runs in 10 seconds on computer A, which has a 4 GHz. clock. We are trying to help a computer designer build a new machine B, that will run this program in 6 seconds. The designer can use new (or perhaps more expensive) technology to substantially increase the clock rate, but has informed us that this increase will affect the rest of the CPU design, causing machine B to require 1.2 times as many clock cycles as machine A for the same program. What clock rate should we tell the designer to target?"

• Don't Panic, can easily work this out from basic principles 400Mhz 클록속도의 컴퓨터에서 10 초 걸리는 프로그램을 , 새로운 컴퓨터에서 6 초에

처리하려고 하면 클록속도가 어떻게 되어야 하나 ? 단 , 새 컴퓨터에서는 앞의 프로그램을 수행하는데 클록수가 1.2 배 증가한다 .답 : 800 MHz

- 원래 컴퓨터에서 프로그램을 수행하기 위해서 400107 사이클이 필요 . - 새로운 컴퓨터에서는 1.2 400107 사이클이 필요하는데 6 초에 처리하려니 ..

6 초 / 프로그램 = (1.2 400107 clocks/ 프로그램 ) (1/ 클록속도 )

클록속도 = 1.2 400107 / 6 = 800 MHz

Example

282008 컴퓨터 구조 : P & H

• A given program will require

– some number of instructions (machine instructions)

– some number of cycles

– some number of seconds

• We have a vocabulary that relates these quantities:

– cycle time (seconds per cycle)

– clock rate (cycles per second)

– CPI (cycles per instruction)

a floating point intensive application might have a higher CPI

– MIPS (millions of instructions per second)

this would be higher for a program using simple instructions

Now that we understand cycles

292008 컴퓨터 구조 : P & H

Performance

• Performance is determined by execution time

• Do any of the other variables equal performance?

– # of cycles to execute program?

– # of instructions in program?

– # of cycles per second?

– average # of cycles per instruction?

– average # of instructions per second?

• Common pitfall: thinking one of the variables is indicative of performance when it really isn’t.

302008 컴퓨터 구조 : P & H

• Suppose we have two implementations of the same instruction set architecture (ISA).

For some program,

Machine A has a clock cycle time of 250 ps and a CPI of 2.0 Machine B has a clock cycle time of 500 ps and a CPI of 1.2

What machine is faster for this program, and by how much?

• If two machines have the same ISA which of our quantities (e.g., clock rate, CPI, execution time, # of instructions, MIPS) will always be identical?

CPI Example

312008 컴퓨터 구조 : P & H

• A compiler designer is trying to decide between two code sequences for a particular machine. Based on the hardware implementation, there are three different classes of instructions: Class A, Class B, and Class C, and they require one, two, and three cycles (respectively).

The first code sequence has 5 instructions: 2 of A, 1 of B, and 2 of CThe second sequence has 6 instructions: 4 of A, 1 of B, and 1 of C.

Which sequence will be faster? How much?What is the CPI for each sequence?

• 2*1 + 1*2 + 2*3 = 10 사이클• 4*1 + 1*2 + 1*3 = 9 사이클 , 따라서 두번째가 10/9 배 빠르다

# of Instructions Example

322008 컴퓨터 구조 : P & H

• Two different compilers are being tested for a 4 GHz. machine with three different classes of instructions: Class A, Class B, and Class C, which require one, two, and three cycles (respectively). Both compilers are used to produce code for a large piece of software.

The first compiler's code uses 5 million Class A instructions, 1 million Class B instructions, and 1 million Class C instructions.

The second compiler's code uses 10 million Class A instructions, 1 million Class B instructions, and 1 million Class C instructions.

• Which sequence will be faster according to MIPS?

• Which sequence will be faster according to execution time?

MIPS example

332008 컴퓨터 구조 : P & H

• Performance best determined by running a real application

– Use programs typical of expected workload

– Or, typical of expected class of applicationse.g., compilers/editors, scientific applications, graphics,

etc.

• Small benchmarks

– nice for architects and designers

– easy to standardize

– can be abused

• SPEC (System Performance Evaluation Cooperative)

– companies have agreed on a set of real program and inputs

– valuable indicator of performance (and compiler technology)

– can still be abused

Benchmarks

342008 컴퓨터 구조 : P & H

Benchmark Games

• An embarrassed Intel Corp. acknowledged Friday that a bug in a software program known as a compiler had led the company to overstate the speed of its microprocessor chips on an industry benchmark by 10 percent. However, industry analysts said the coding error…was a sad commentary on a common industry practice of “cheating” on standardized performance tests…The error was pointed out to Intel two days ago by a competitor, Motorola …came in a test known as SPECint92…Intel acknowledged that it had “optimized” its compiler to improve its test scores. The company had also said that it did not like the practice but felt to compelled to make the optimizations because its competitors were doing the same thing…At the heart of Intel’s problem is the practice of “tuning” compiler programs to recognize certain computing problems in the test and then substituting special handwritten pieces of code…

Saturday, January 6, 1996 New York Times

352008 컴퓨터 구조 : P & H

SPEC ‘89

• Compiler “enhancements” and performance

0

100

200

300

400

500

600

700

800

tomcatvfppppmatrix300eqntottlinasa7doducspiceespressogcc

BenchmarkCompiler

Enhanced compiler

SP

EC

pe

rfo

rman

ce r

atio

362008 컴퓨터 구조 : P & H

SPEC CPU2000

372008 컴퓨터 구조 : P & H

SPEC 2000

Does doubling the clock rate double the performance?

Can a machine with a slower clock rate have better performance?

Clock rate in MHz

500 1000 1500 30002000 2500 35000

200

400

600

800

1000

1200

1400

Pentium III CINT2000

Pentium 4 CINT2000

Pentium III CFP2000

Pentium 4 CFP2000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

SPECINT2000 SPECFP2000 SPECINT2000 SPECFP2000 SPECINT2000 SPECFP2000

Always on/maximum clock Laptop mode/adaptiveclock

Minimum power/minimumclock

Benchmark and power mode

Pentium M @ 1.6/0.6 GHz

Pentium 4-M @ 2.4/1.2 GHz

Pentium III-M @ 1.2/0.8 GHz

382008 컴퓨터 구조 : P & H

Experiment

• Phone a major computer retailer and tell them you are having trouble deciding between two different computers, specifically you are confused about the processors strengths and weaknesses

(e.g., Pentium 4 at 2Ghz vs. Celeron M at 1.4 Ghz )

• What kind of response are you likely to get?

• What kind of response could you give a friend with the same question?

392008 컴퓨터 구조 : P & H

Execution Time After Improvement =

Execution Time Unaffected +( Execution Time Affected / Amount of Improvement )

• Example:

"Suppose a program runs in 100 seconds on a machine, with multiply responsible for 80 seconds of this time. How much do we have to improve the speed of multiplication if we want the program to run 4 times faster?"

How about making it 5 times faster?

• Principle: Make the common case fast

Amdahl's Law

402008 컴퓨터 구조 : P & H

• Suppose we enhance a machine making all floating-point instructions run five times faster. If the execution time of some benchmark before the floating-point enhancement is 10 seconds, what will the speedup be if half of the 10 seconds is spent executing floating-point instructions?

• We are looking for a benchmark to show off the new floating-point unit described above, and want the overall benchmark to show a speedup of 3. One benchmark we are considering runs for 100 seconds with the old floating-point hardware. How much of the execution time would floating-point instructions have to account for in this program in order to yield our desired speedup on this benchmark?

Example

412008 컴퓨터 구조 : P & H

• Performance is specific to a particular program/s ( 성능은 프로그램 성격에

좌우된다 ) – Total execution time is a consistent summary of performance

• For a given architecture performance increases come from:– increases in clock rate (without adverse CPI affects) : 가능함 , 돈 들음– improvements in processor organization that lower CPI : 어려움 , CPU

변경– compiler enhancements that lower CPI and/or instruction count : 가능함– Algorithm/Language choices that affect instruction count : 가능함

• Pitfall: expecting improvement in one aspect of a machine’s

performance to affect the total performance

• You should not always believe everything you read! Read carefully!

Remember