CSC 110 –Intro to Computing

Lecture 9:Computing Components

Announcements

First set of experience pages due Feb. 23

Quiz solutions will be posted on course web page tomorrow

Central Processing Unit

“Brains” of a computerPerforms all of the calculations in the ALUControl unit directs system to run the various

program(s)

Computer Memory

Computers contain two types of memoryROM (Read-Only Memory)

Once data written to ROM, it cannot be changed Contains small number of instructions that initialize

computer and load operating system

RAM (Random-Access Memory) What people mean when discussing computers

“memory” Contents disappear when computer turned off

Stored Program

Each byte in RAM has a unique addressEach address is just a number from 0 to ???

Locations in RAM contain a binary numberNumber could be part of a program, picture,

mp3, Word document…Each program directs processor how to

interpret this information

Finding a Program

Contents of RAM disappear when we turn off the powerBut I do not want to re-install Word each time I

boot computerSolve this problem by storing programs on

secondary storage devices E.g.,

Running a Program

Hard drive 10000x slower than RAMMost of us are not this patientOther storage devices are even slower!

Computer starts program by loading it into RAMThen uses executes the copy in memory

But RAM is still 200x slower than CPU

Processor Cache

Small amount of fast memory located on processor

CPU stores most recently used instructions in this memory Programs spend most time re-executing small number

of instructions What happens when program executes new

instructions?Do not want CPUs running at 1/200th speed

Fetch-Execute Cycle

Processor run like assembly line Fetch Get instruction from memory and

store in cache Decode Determine how processor will

execute instructionRetrieve data used by instruction and store it

in registers

Fetch-Execute Cycle

Execute Perform the actionsALU executes math/logic functionsControl unit handles “branch” instructionsProcessors use multiple ALUs so they can

execute multiple instructions at once Retire Record outcome of instruction

Write results back to memory Work like bucket brigade to do this quickly

Instruction Scheduling

add

sub

add

add

Instruction

r

7

f

1

f

d

2

f

d

e

3

f

d

e

r

4

d

e

r

5

e

r

6

Cycles needed w/o pipelining = 4 * 4 = 16 Cycles needed w/ pipelining = 7

Not All Instructions Are Equal

Add and subtract: Easy Multiply: Not as easy Divide: Hard Check if any other program modified a specific

memory address and, if it has not been modified, write a new value into that address: PricelessThis is also VERY hard

Instruction Scheduling

Processors take longer to execute harder instructions

Example (actual times depends on CPU):Add takes 1 cycleSubtract takes 1 cycleMultiply takes 3 cyclesDivide takes 5 cycles

Instruction Scheduling

add

sub

mult

add

Instruction

d

e

r

7

f

1

f

d

2

f

d

e

3

f

d

e

r

4

e

5

e

6

e

r

8

r

9

Cycles needed w/o pipelining = 18 Cycles needed w/ pipelining = 9

Why is My Computer So Slow?

If we only add & subtract numbers:Pipelining improves performance 2.125x

When we also multiply numbers:Pipelining improves performance 2x

Still an improvement, but not as good!

Mumble… Grumble… Stupid Hardware Problem is we have only one ALU

Can only do one add, sub, mult, div, AND, OR, etc. at a time

So, while our processor is busy multiplying Cannot execute other instructions… … which causes fetch & decode units to back up… … and causes our assembly line to stop

Obvious solution: add another ALU

Woo-Hoo, More Hardware!

add

sub

mult

add

Instruction

e

r

7

f

1

f

d

2

f

d

e

3

f

d

e

r

4

d

e

e

5

e

6

r

8

r

9

Cycles needed w/o pipelining = 18 Cycles needed w/ pipelining = 9

What Happened

Adding an ALU had no impact! Must finish first instruction before retiring

the next one What if we allow CPU to execute and

retire instructions in any order? Imagine if we began with the multiply

instruction

One More Time

add

sub

mult

add

Instruction

r

7

f

1

f

d

2

f

d

e

3

f

d

e

r

4

d

e

e

5

e

r

e

6

r

8 9

Cycles needed w/o pipelining = 18 Cycles needed w/ pipelining = 8

Success!

Running instructions out-of-order reduces execution time by 6%

Modern processors actually execute MANY instructions at once

Even fancier things happen in a compiler (program which creates other programs)Compiler research also makes people smarter

and more attractive

Your Turn

What impact does 1 or 2 ALUs have on these instructions?How about executing in-order or out-of-order?

addaddmultsubadd

addadddivsubadd

addmultmultsubadd

multaddsubdivadd

divadddiv

multsub

Boolean Properties

Law of Double Negation: a a

Property AND OR

Commutative a·b = b·a a+b = b+a

Associative a·(b·c) = (a·b)·c a+(b+c)=(a+b)+c

Distributive a·(b+c) = (a·b)+(a·c) a+(b·c) = (a+b)·(a+c)

Identity a·1 = a a+0 = a

Complement a·ā = 0 a+ā = 1

DeMorgan a·b = ā+b a+b = ā·b

Idempotency a·a = a a+a = a

For next lecture

Relax and enjoy the (relative) breakNo quiz and no homework!

Start reading Section 12 Be ready to discuss:

Information systems and databases