CS 300 – Lecture 1

Intro to Computer Architecture

/ Assembly Language

Welcome Back!

Calling All Seniors

Come see me about capstone projects!

The Text

We're using "Patterson / Hennessy". You should have this by now. You should also have the CD. If not, I'll help you out.

Note that some chapters are on the CD only! We'll use appendix B from the CD

Our Goals

* Understand how computing hardware works

* Investigate the differences between hardware and software and why they are important

* Understand low level programming (C and assembler)

* Find out how high level programs are translated into machine level programs

* Understand the performance of programs

* Look at parallelism

Course Outline

* Introduction (.5 wks)* History (.5 wks)* Logic circuits and digital design (2 wks)* Basic Machine language and the C programming language (2 wks)* Computer Arithmetic (2 wks)* Understanding performance (1 wk)* Datapath and control (1 wk)* Pipelining (2 wks)* Memory issues (2 wks)* Storage and IO (1 wk)* Multi-processing (1 wk)

Work Required

* Quizzes: 1 per chapter in the book, about 20 minutes each

* Homework assignments: 1 per week (approximately)

* Comprehensive final

* Classroom participation (come prepared!)

* No "big projects" but some homeworks will involve small amounts of software

Danger!

I've chosen a "classic" textbook for this class. It's normally used in a 2 or 3 semester sequence of classes.I plan to cherry-pick the good stuff and avoid getting into too much depth. Please let me know if the book gets over your heads.I'll also be doing some units using a simple embedded processor like the PIC that will be easier to understand.

What is a computer?

A computer is really just a collection of 4 basic things:•Computational element that performs reasoning•Storage elements that retain data•Communication elements that move data from one computer to another•Sensing and controls that allow the computer to interact with its environment

What are some examples of these elements?

History

Each element of a computer has evolved separately. What are the ancestors of these elements?

•Computing

•Storage

•Communication

•Sensing and control

A Historical Approach

We’re going to dive in to the innards of computing devices from a historical perspective. Why?•Older devices were a lot simpler yet addressed the same problems•It is interesting to address the context of each element of a computer from an evolutionary perspective•History is cool

The Computing Element

John Von Neumann, one of the pioneers of computing, used the word “Organ” to describe these elements. The biological metaphors started from day 1 …

The original computing element was the human brain. But eventually mechanical devices were created to speed up the calculation process.The apex of mechanical computing was Babbage’s “analytical engine”, a device too complex to ever work.This early computing was mathematical – building tables of numbers for navigation and engineering purposes.

Historical Computing Devices

Electronic Computing

The big innovation in computing was the replacement of mechanical computing devices by purely electronic ones.A gear or relay is too big / slow / unreliable to use in large quantities.An electronic switch has no moving parts – it operates by pushing electrons around.The original electronic computers used vacuum tubes – later transistors took over.

Electronic GatesA gate is a device in which one signal controls another. In a vacuum tube, the grid could block or allow flow from input to output. So this is just like a relay.

Transistors are very similar – just a lot smaller.

SiliconThe “computer revolution” came about when VLSI technology allowed a single chip to contain LOTS of transistors. A Pentium has about 50 million transistors. That would have been a lot of vacuum tubes. Manufacturing cost is something like $0.000001 per transistor.

Assessing Computation

How can we assess a computational technology?

This turns out to be REALLY HARD! Knowing how fast a device can do one task doesn’t tell us a lot about other tasks.

Approaches:•Clock rate (not very accurate)•MFLOP (only helps for numeric calculations)•Specific benchmarks

Units: tasks / second

Information Storage

Storing information is as important as processing it.

This all started with written language:

Important ideas:• Precise relationship between spoken and written languages• Ability to make a “perfect copy” of a document• A medium (clay, paper, …) is used to preserve information over time

Information Access

A large information repository is much more useful if it can be accessed quickly via mechanical means.

Punch cards predate computers (by a long shot!) and were used to store and process large volumes of information.

A key insight was that alphabetic information can be processed as if it is numeric

Herman Hollerith patented a system in which needles sensed the presence or absence of holes in a card. This converted information into electric impulses.His machine was used for the 1890 censusWhat company did he start?

Storage Media

Assessing Storage Technology

•Read/write or read-only•Latency (time it takes to find what you want) (time)•Transfer rate (how fast you get the information) (bits / second)•Capacity (bits)•Cost / bit ($)•Error rate (errors / bit)•Durability (time)

Interfacing

Getting (electronic) information from or to the real world is another BIG part of computing.

The first big breakthrough was a loom controlled by punched cards.

Interface Technology

The big idea here is converting between electronic representation and human sensing for audio and video objects.

Other interface technology includes pointing (mouse), typing (keyboard), and even GPS.

Interfaces usually come in two “layers” – one that is specific to the device and one “general” part like USB or Firewire.

Babbage’s Insight

Instead of programming a computer mechanically, use the storage to encode the program.

That is, instead of building a machine to accomplish just one task, build a general machine that could be programmed to do any task (a “stored program” computer).

The same data that a program manipulates can also be the program that controls the machine.

Building Electronic Computers

So what is it that makes a computer go?

As you peel back the layers (circuit boards, chips, memory, processing, …) you finally get to the “bottom” – the indivisible atoms that a computer is composed of: Logic Gates

Logic Gates

The basic building block of a digital computer is the “logic gate”, a hardware device that calculates a very simple function on 1’s and 0’s.

Logic gates have been constructed with mechanical relays, vacuum tubes, and (mainly) transistors on an integrated circuit. Logic gates use power as they switch – that’s why you have a fan in your computer.

We assess the “quality” of a logic gate in a number of ways: speed, size, energy use

Moore’s Law

Technology improves at an exponential rate.Integrated circuit capacity (number of gates) doubles every 18 months or so.Problems: some systems improve much faster than others (disk speed doesn’t increase as fast as processor speed). Circuit size can’t keep getting smaller forever.It’s very hard to make all of the transistors on a chip useful – doubling the number of transistors does not double the effective speed of a processor

How Logic Gates Work

A gate has “inputs” and “outputs”. The outputs are determined by the inputs.

For a relay: input is voltage on the coil, outputs are connections between terminals

Transistors are similar except a lot smallerKey characteristics:• Delay: how soon the output has the “right”

answer• Power dissipation: how much heat is generated• Size: how much silicon is needed on the chip

Building Logic Gates

Many different sorts of logic gates are used in computing

A Pentium has about 50,000,000 transistors

A Universal Logic Gate

Nand:

X Y Output ( 1 = True, 0 = False)0 0 10 1 11 0 11 1 0

Let’s create a “Circuit” using these gates …

Boolean Algebra

Logic designers use boolean algebra to understand their circuits. There is a lot of math in the boolean world; writing + for or and * for and, a*(b*c) = (a*b)*c a*(b+c) = a*b+a*c a+a’ = 1Truth tables are used to specify boolean functions

Adding Two Numbers

Truth table for addition:

x y Sum Carry

0 0 0 0

0 1 1 0

1 0 1 0

1 1 0 1

Inputs Outputs

Memory

Consider this circuit:

1

1 0

1

Wires, Clocks, and Words

A clock is the “heartbeat” a computer. The signal on a wire carries a new value at each clock. The clock determines the flow rate of information along a wire.If you want information to flow faster you use more wires (bits).A “word” is fixed number of bits/wires used in calculations and storage. A “byte” is an 8 bit word (often used to store characters)

More About Hardware

All the gates can work in parallel: hardware is not restricted to a “one step at a time” like programs generally are.

Wire length is just as big a problem as gate speed: signals move at just around the speed of light but that’s not very fast on a chip.

Scalability

Remember that sizes don’t mean much. A 64 bit machine doesn’t run 2x faster than a 32 bit one; a 50M transistor chip isn’t twice as fast as a 25M one, and a 4 GHz machine won’t execute programs twice as fast as a 2GHz one (why???).

Will a task get done twice as fast if you use twice as many workers?

Stored Programs

The key idea in a computer is to execute arbitrary programs for the user. Computers use "machine language" - language understood by the hardware that controls what the computer does.Different processors have different machine languages.Somehow, all of our programs need to get turned into this machine language.

Instructions

Machine language breaks the computational process into instructions. An instruction is just data (0s and 1s) that is executed.Instructions have two representations:* Binary (in the computer)* Textual (the assembler or textbook)The translation between binary and textual representations is done using an assembler (or disassembler).

Data

Data in an executing program can live in many different places:

* Memory

* Registers

* Disk

* Cache

An instruction mutates this data (the "processor state") in some predefined way

Programming

* Bad old days: Program by rewiring the computer logic

* Early computers: program written in binary

* Back in my day: program written in assembly language

* Low level languages (Fortran, C, Pascal)

* High level languages (Java, Haskell, C#, …)

What's the difference between low and high level languages?

About This Course

Why are we here?* Systems are composed of both "hard" and "soft" components. We need to understand what "hard" components can do* Hardware is fundamentally different from software: it is inherently parallel. We'll learn about exploiting parallelism in computation* All "soft" programs are executed on hardware – we can't understand how a program performs without knowing a lot about hardware* Many issues in hardware design are also present in software systems

A Plethora of Systems

* High performance computing* Servers* Desktop systems* Highly functional embedded applications (cell phones, PDAs, cameras)* Minimally functional embedded applications (calculators, toys, controllers)Understanding architecture helps a lot at both ends of this spectrum

A Short History of Computer Architecture

* 40's, 50's; Simple architectures, emphasis on building hardware capable of carrying out basic math and control* 60's: Development of "tricks" in the processor to make execution go faster* 70's: mini-computers (PDP8), super computers (Cray 1)* 80's: RISC vs CISC debate, Language driven architecture* 90's: Increasing clock speed, multi-processor architectures, memory concerns, networking* Recent: clock speed limitations: parallel processing, cheap embedded systems, pervasive use of computing. Networking becomes very important.