Inside the CPUInside the CPU

COSC-100 (Elements of Computer Science)

Prof. Juola

What’s a computer?What’s a computer?

• Information-processing device

• Primary function : math, decisions

• Normally “digital” (vs. analog)

1:38 pm1:38 pm

Computers : A historyComputers : A history

• 3000 BC -- digital counting device (abacus)

• 16th c. -- Napier’s bones

• 19th c. -- Jacquard’s (programmable) loom

• 1830 -- Babbage’s Analytic Engine

• 1940 -- WWII, programmable coding and decoding machines (Colossus)

Computers: A history (cont.)Computers: A history (cont.)

• 1943-6 -- Programmable computers• ENIAC• 10 feet tall, 10,000 square feet footprint,

150kW of power, and 18,000 vacuum tubes

• 1945 -- First program “bug”• Moth found in computer wiring @ Harvard

• 1945 -- Von Neumann defines “computer”

Computers: A history (cont.)Computers: A history (cont.)

• 1947 -- First transistor (no more tubes!)• Begins “second generation” computers

• 1949 -- First programming language• 1958 -- First integrated circuit,

• Begins “third generation” computers

• 1971 -- VLSI microprocessor• Computer-on-a-chip• Begins “fourth generation” computers

Computers: A recent historyComputers: A recent history

• 1971 -- Intel builds 4004, eventually to become the Pentium (IV)

• 1975 -- Microsoft founded• 1981 -- IBM produces IBM-PC (Personal

Computer)• 1984 -- Apple Macintosh• Change in hardware since then mostly

incremental

Software : A historySoftware : A history

• 1950--1975 : Institutional computing• Banks, large corporations, scientific problems

• 1975--1995 : Personal computing• Document processing, data storage, calculations

• 1995--today : Interpersonal computing• Primarily networking ; Email, web, IM, gaming

Computing issuesComputing issues

• Computers are algorithmic; they only do what you tell them to do.

• Limited by validity of data : GIGO principle

• Primarily an enabling tool, but enabling of what?

Structure of CPUStructure of CPU

• CPU most salient part of computer

• E.g. “Intel Pentium 4 Processor at 3.4GHz with 800 MHz front-end bus”

• Describes type of computer (Pentium 4), and how fast it does stuff (3.4 billion clock ticks/second)

Electronically SpeakingElectronically Speaking

• Basic computer design hinges on electronic switch (valve?) controlling current

• Current is either flowing or not. • Fundamental representation : flowing/not

becomes a yes/no or a 1/0 “binary” representation.

• All quantities in computers represented by binary quantities

RepresentationsRepresentations

• Numbers : 10111 becomes 23• 2^4 + 2^2 + 2^1 + 2^0• 16 + 4 + 2 + 1 = 23

• Letters : 01000001 becomes ‘A’• American Standard Code for Information

Interchange (ASCII)

• Code : 00001101 becomes “add integers”• Programs (compilers) to convert from human

languages to “machine language”

Electrical manipulationsElectrical manipulations

• Binary patterns manipulated through electronic circuitry

StorageStorage

• More circuitry creates self-powering (stable) circuit for temporary data storage

CPU StructureCPU Structure

• Several sub-components• Arithmetic-Logical Unit -- does math/logic• Bus Unit -- talks to rest of computer• Prefetch/Decode Units -- load/figure out next

instruction• Control Unit -- executes instructions, controls

rest of CPU• Cache memory -- high-speed local memory for

scratch-pad style storage

TerminologyTerminology

• Bit -- one yes/no pattern• Byte -- 8 yes/no patterns, grouped • Word -- several bytes, grouped for easy

manipulation• 8-bit computer : microcomputer• 16-bit computer : minicomputer• 32-bit computer : mainframe• 64-bit computer : ??• 128-bit computer : supercomputer

More terminologyMore terminology

• Kilo : either 1000 or 1024 (2^10)

• Mega : one million

• Giga : one billion

• Tera : one trillion

Example : 1 GHz is one billion Hertz, and a 256 MB pen-drive stores 256 million bytes

Why different CPUs?Why different CPUs?

• Different capacities : bigger word size means more expensive, but also more powerful. Why buy more than you need?

• Incompatible designs. Different machines (e.g. PowerPC vs. Pentium) have different machine codes, different architectures

• Different philosophies : RISC vs. CISC

Virtual machinesVirtual machines

• “The CPU that isn’t.”

• Virtual CPU is software to interpret patterns for another machine. E.g. your Mac can run PC software.

• Java Virtual Machine exists nowhere but in software. As software, built into every web browser, so Java programs run everywhere the Web does.

Moore’s LawMoore’s Law

• Eniac had 18,000 tubes• Pentium 4 has 55 million transistors• Gordon Moore (1965) observed :

• The number of transistors that can be put on a chip doubles every eighteen months

• The cost of a given amount of computing power halves every 18 months

• The cost to develop a new chip doubles every 18 months.

Why so important?Why so important?

• Primary factor : transistor size

• Smaller transistor takes up less space

• … but also• draws less power• operates faster• costs less to fabricate (but more to design)

• Unknown how long Moore’s law will continue to be true.

Programs get biggerPrograms get bigger

• Demand for programs is getting larger

• PDP-11 text processor (troff) used less than 64K of memory. Compare to MS-Word

• Game design often pushes limits of both computer speed and memory capacity.

• How to make computers faster?

MultiprogrammingMultiprogramming

• Multiple CPUs per “computer,” all cooperating

• Extreme example : CM-2 Connection Machine had 65,536 CPUs.

• Alternatively, use networked computers (SETI@home)

The futureThe future

• Transistor density may hit fundamental limitations (can’t build a transistor smaller than an atom).

• Faster substances (GaAs?)

• DNA computing in experimental stage

• “Quantum computing” a possibility, but will require fundamental re-think of how computers work.

The future (cont.)The future (cont.)

• Smaller computers mean more uses (e.g. PDA’s, MP3 players)

• “Ubiquitous computing” aims to put Star Trek style computers everywhere for simple tasks.

• Computer assist for many tasks we can’t even imagine today.