Lecture #2: Computers and computer languages

CSE 5a

Bob Pitha

bpitha@cs.ucsd.edu

administrivia

• Class web page

http://www-cse.ucsd.edu/classes/fa99/cse5a

– not much there yet, but keep your eyes on it.

• Discussion section

– CSB 001, Weds. 6:50-7:40 PM

understanding computers

• Understanding programming requires that you understand computers

• How does a computer work?

– Idiot savant

– Very good with a small number of tasks

– Completely clueless otherwise

what can computers do well?

• Store and retrieve numbers

– very quickly

– very accurately

• Add, subtract, multiply, and divide

– also fast and accurate

• Compare numbers (with 0)

• Follow a list of instructions

– jump around in the list

that’s about it

• What about everything else?

– more complex math

• combination of atomic operations

– interaction with peripheral devices

• output: graphics cards and printers

• input: keyboards, mice, joysticks

• all sorts of specialized devices

– everything done using numbers

computer numbers

• To the computer, everything is numbers.

• Computers use binary (base 2)

– each digit is 0 or 1

– represents a power of 2

human numbers

• You are familiar with decimal (base 10)

binary

• Example:

teminology

• bit: binary digit

• byte: a group of 8 bits

• nybble (obsolete): half a byte

• word: a group of bytes that a specific computer is optimized for working with (usually 4 or 8)

• kilobyte (K or KB): 1,000 bytes, or sometimes 1,024 bytes (2¹⁰)

• megabyte (M or MB): 1,000,000 bytes, or sometimes 1,028,576 bytes (2²⁰)

why all these powers of 2?

• Everything is little electronic switches

– off or on

– true or false

– yin or yang

– 0 or 1

• Decimal is foreign to the computer

– it can convert a number to decimal, but that’s really only for human convenience

• The computer works in base 2

other numbering systems

• octal – base 8

– used rarely, but you should at least recognize it

– digits are 0 through 7

other numbering systems

• hexadecimal – base 16

– used occasionally – you can’t really avoid it

– digits are 0-9, a-f

other numbering systems

• You do not have to be fluent in either of these (octal or hex) for this class

– recognize them

– don’t be scared of them

– know how to convert if you need to

• but you will rarely need to

byte

• A group of bits

– almost always 8

– some strange computers may have other byte size

– but they’re not an issue for this class

• The fundamental unit of data for the computer

• Can represent 256 values

– 0 to 255

– -128 to 127

word

• Also fundamental to the computer

• A group of bytes

– usually 4 or 8

– for the computers we’ll work with, it’s 4

• 4 bytes

– 32 bits

– 0 to 4,294,967,295

– or more usually, –2,147,483,648 to +2,147,483,647

inside the computer…

• Everything is bits

• Bits are always grouped

– into bytes

– into words

• Every byte is numbered

– sequentially

– called an address

– addresses are, of course, numbers themselves

computer instructions

• Computers execute instructions

• Instructions are numbers

– memory access (get/put)

– arithmetic (add, subtract, multiply, divide)

– compare values (equal, greater than, less than)

– change the order instructions are executed in

• often based on the result of a comparison

what kinds of numbers do computers work with?

• Instructions

• Addresses

• Data

– integers

– real numbers

– text

– all sorts of other things

what is a computer program?

A sequence of processor instructions designed to achieve a specific purpose

what is a computer program?

A sequence of processor instructions designed to achieve a specific purpose

• Computers execute programs sequentially

• Unlike, say, a shopping list

• More like driving directions

– each instruction depends on the last one being completed

what is a computer program?

A sequence of processor instructions designed to achieve a specific purpose

• Each instruction has a numerical code

• Examples:

– Get some data from memory

– Add two numbers

– If two numbers are equal, jump to a different part of the program

• To us, it all just looks like numbers

what is a computer program?

A sequence of processor instructions designed to achieve a specific purpose

• This is important!

• Instructions are numbers

• Random numbers can be executed

– but will not be a program

00401040 55 8B EC 83 EC 5C 53 56 57 6A 00 FF 15 B8 C2 42

00401050 00 89 45 FC 83 7D FC 00 7D 1B 68 0F 10 40 00 68

00401060 14 61 42 00 B9 E8 99 42 00 E8 32 05 00 00 8B C8

00401070 E8 90 FF FF FF 8D 45 EC 50 8B 0D 50 8D 42 00 51

00401080 FF 15 C0 C2 42 00 89 45 FC 83 7D FC 00 7D 20 68

00401090 0F 10 40 00 68 F8 60 42 00 B9 E8 99 42 00 E8 FD

004010A0 04 00 00 8B C8 E8 5B FF FF FF E9 9C 00 00 00 68

004010B0 0F 10 40 00 68 D8 60 42 00 B9 E8 99 42 00 E8 DD

004010C0 04 00 00 8B C8 E8 3B FF FF FF 8D 55 E4 52 68 28

004010D0 71 42 00 6A 01 6A 00 68 18 71 42 00 FF 15 C4 C2

004010E0 42 00 89 45 FC 83 7D FC 00 7C 1D 68 0F 10 40 00

004010F0 68 B4 60 42 00 B9 E8 99 42 00 E8 A1 04 00 00 8B

00401100 C8 E8 FF FE FF FF EB 43 8B 45 FC 50 68 88 60 42

00401110 00 E8 6A 25 00 00 83 C4 08 68 0F 10 40 00 68 84

00401120 60 42 00 8B 4D FC 51 68 60 60 42 00 B9 E8 99 42

00401130 00 E8 6A 04 00 00 8B C8 E8 33 01 00 00 8B C8 E8

00401140 5C 04 00 00 8B C8 E8 BA FE FF FF FF 15 BC C2 42

00401040 push ebp

00401041 mov ebp,esp

00401043 sub esp,5Ch

00401046 push ebx

00401047 push esi

00401048 push edi

00401049 push 0

0040104B call dword ptr [impCoInitialize@4 (0042c2b8)]

00401051 mov dword ptr [ebp-4],eax

00401054 cmp dword ptr [ebp-4],0

00401058 jge main+35h (00401075)

0040105A push offset @ILT+10(endl) (0040100f)

0040105F push offset string "CoInitialize failed." (00426114)

00401064 mov ecx,offset cout (004299e8)

00401069 call ostream::operator<< (004015a0)

0040106E mov ecx,eax

00401070 call @ILT+0(ostream::operator<<) (00401005)

00401075 lea eax,[ebp-14h]

00401078 push eax

00401079 mov ecx,dword ptr [pid (00428d50)]

0040107F push ecx

00401080 call dword ptr [impCLSIDFromProgID@8 (0042c2c0)]

00401086 mov dword ptr [ebp-4],eax

00401089 cmp dword ptr [ebp-4],0

0040108D jge main+6Fh (004010af)

0040108F push offset @ILT+10(endl) (0040100f)

00401094 push offset string "CLSIDFromProgID failed." (004260f8)

00401099 mov ecx,offset cout (004299e8)

0040109E call ostream::operator<< (004015a0)

004010A3 mov ecx,eax

004010A5 call @ILT+0(ostream::operator<<) (00401005)

004010AA jmp main+10Bh (0040114b)

004010AF push offset @ILT+10(endl) (0040100f)

004010B4 push offset string "CLSIDFromProgID succeeded." (004260d8)

machine language

• The one “true” computer language

– …except there isn’t just one

– Every computer has its own machine language

• All other computer languages created for human convenience

– The computer doesn’t understand C

– Must be converted into machine language

computer languages

• Assembler

– essentially just machine language with some text codes

• Compiled languages

– converted into machine language before use

– examples: C, Pascal, Fortran

• Interpreted languages

– converted into machine language during use

– examples: Basic, Java

why more than one?

• Meet different needs

• Tradeoffs

• Different approaches

• Special features

meet different needs

• Application domain

– programmer prefers to think about the application

• Computer domain

– programmer is willing to think about the computer

tradeoffs

• Control

• Ease-of-programming

• Performance

• Flexibility

different approaches

• Procedural

• Object-oriented

special features

• There are lots of special-purpose languages

– video games

– robots

– artifical intelligence

– embedded systems

– etc.