What is in the box?
The system unit is what contains the processing part of the computer.
The motherboard contains the primary electronics of the system unit; the CPU, memory, and connections to power, expansion slots, and ports.
Bits and Bytes
How high can you count on ten figures?
By using the binary system, you can count to 1024.
All data within the computer is represented in terms of pattern of off and on, or equivallently in terms of binary numbers.
Thought of as a binary number, the pattern 01100101 equals
.
Each 0/1 is called a bit. A collection of eight bits can represent different possibilities, which starts to get useful. A byte is a collection of eight bits.
Character codes
Since one byte gives 256 possibilities, and there are less than 256 different characters on a keyboard, we can assign each character a distinct 8-bit pattern and use it to represent the character.
A character code is an assignment of all the characters to distinct bit patterns. In order for programs to work with text consistantly, all programs must use the exact same character.
Thus standard character codes have been defined. ASCII is the most common 8-bit character code. Less common is EBCDIC, used only on mainframes.
Unicode is a new 16-bit international
code designed to represent every single
symbol in every language on earth.
Since 
, there is just enough room!
Parity
The reliability of computer storage and computations are an essential part of why they have become ubiquitous. A commonly used method to check on the accuracy of memory is to store an extra parity bit.
The extra parity bit can be used to detect (but not correct) single bit errors in a byte.
There are two conventions. In even parity, it is always the case that the total number of 1 bits is even - if necessary by setting the parity bit to be 1. In odd parity, it is always the case that the total number of 1 bits is odd.
Which is better? Neither - they are equally good but unfortunately unstandardized conventions.
Sometimes we need more than detecting errors. By adding extra parity bits, we can correct single-bit and even larger errors. For example, CD-ROMS use very sophisticated error-correcting mechanisms to compensate for scratches.
Parity also becomes an issue with transmitting data from one computer to another via a modem. It is important that both computers agree on whether to use even, odd, or no parity so the bits can be received properly.
The CPU
The CPU (central processing unit) is the ``brain'' of the computer. It is a piece of hardware that knows how to do a given set of very simple things, such as
.This collection of simple things a processor can do is called its instruction set. Different types of computers have different instruction sets.
Microprocessors
Microprocessors are CPUs which sit on a single VLSI `chip' or integrated circuit.
Important types of microprocessors include
The system clock speed measures how fast the electronics are getting run. The speed is given in megahertz or ``millions of cycles per second''.
Each processor has a word-size, describing how many bytes it can work with at one time. Typical sizes are 16-bits, 32-bits, and now 64-bits.
Memory
The memory in a computer is split between RAM and ROM.