What is a Bus?

One of the set of conductors (wires, PCB tracks, or connections in an integrated circuit) connecting the various functional units in a computer. There are busses both within the CPU and connecting it to external memory and peripheral devices. The width of the data bus, i.e., the number of parallel connectors, and its clock rate determine its data rate (the number of bytes per second which it can carry). This is one of the factors limiting a computer’s performance.

Computers do their work with Ones and Zeroes

• This is called the binary number system

• A single binary digit (a one or a zero) is called a bit

• Binary works for computers because switches are on or off

• Eight bits make up a byte• We'll see more about this

soon. . .

Decimal Binary

0 0

1 1

2 10

3 11

4 100

5 101

etc etc

Serial

Parallel

Printed Circuit Boards

Cross section of multilayer printed circuit board

CPU RAMor

ROM

The bus’s address wires:10010111

The bus’s data wires:00110010

Hey RAM! Get me whatever is stored

at address 10010111

Ok boss. . . that would be the

number 00110010

bus's read/write wire0 (read from memory)

(Data on data wires would go in the other direction if read/write wire was 1 – CPU

would send, RAM would receive)

Okay, so that’s what thecircuit board is about. . .

What goes on behind the closeddoors of the chips?

The clock set the pace, and keeps the chips in sync. . .

Some metaphors for how a computeris organized. . .

A Little More About the CPU. . . All a CPU does is:• fetch instructions from RAM or ROM• execute those instructions

Executing an instruction generally involves either:• reading data from memory into a register (a very

small chunk of extremely fast volatile memory built into the CPU), or

• modifying the data in a register (adding two values, comparing them, testing if a value is zero, etc), or

• storing the contents of a register back into memory

Instructions are just ones and zeroesFor instance, in an 8086 microprocessor, the

instruction10110000 01100001

means "put the value 01100001 into register AL"

Booting a PC. . .

Adding Two One-Bit Binary Numbers

Input A Input B Decimal Output

Binary Output

In Other Words . . .

0 0 0 00 0 + 0 = 0

1 0 1 01 1 + 0 = 1

0 1 1 01 0 + 1 = 1

1 1 2 10 1 + 1 = 2

Think of the binary output as two separate bits:Rule for the right (ones) bit: (A and not B) or (B and not A)Rule for the left (twos) bit: ??

Getting Tricky with Ones and Zeroes

AND OR

2-way switchflashlight