Processing: Memory Addresses

Speed

Processing: Machine Cycle

The computer can only do one thing at a time. Each action must be broken down into the most basic steps. One round of steps from getting an instruction back to getting the next instruction is called the Machine Cycle.

The Machine Cycle
Fetch -	get an instruction from Main Memory
Decode -	translate it into computer commands
Execute -	actually process the command
Store -	write the result to Main Memory

For example, to add the numbers 5 and 6 and show the answer on the screen requires the following steps:

1.	Fetch instruction:	"Get number at address 123456"
2.	Decode instruction.
3.	Execute:	ALU finds the number. (which happens to be 5)
4.	Store:	The number 5 is stored in a temporary spot in Main Memory.
5 - 8 Repeat steps for another number (= 6)
9.	Fetch instruction:	"Add those two numbers"
10.	Decode instruction.
11.	Execute:	ALU adds the numbers.
12.	Store:	The answer is stored in a temporary spot.
13.	Fetch instruction:	"Display answer on screen."
14.	Decode instruction.
15.	Execute:	Display answer on screen.

The immense speed of the computer enables it to do millions of such steps in a second.
In fact, MIPS, standing for millions of instructions per second, is one way to measure computer speeds.

We need a method of naming the places where Main Memory stores data.
Each location needs a unique name, just like houses in a town need a unique street address.

Rather than a street name and house number, memory addresses are just numbers.

A memory address holds 1 byte of data where

1 bit =	0 or 1, on or off
1 byte =	8 bits
1 kilobyte (K or KB) =	1024 bytes
1 megabyte (MB) =	1024 kilobytes

You might wonder why 1024 instead of 1000 bytes per kilobyte. That is because computers don't count by tens like people. Computers count by twos and powers of 2. 1024 is 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2, that is 2 times itself ten times. It's a rather convenient size number (for computers!).

Update: Things are changing faster than I can type! The explanation above is no longer entirely true (July 2000). Different scientific and technical areas are using the words differently. For data storage devices and telecommunications a megabyte is 1 000 000 bytes. For data transmission in LANs a megabyte is 1 048 576 bytes as described above. But for data storage on a floppy disk a megabyte is 1 024 000 bytes!

A new set of words has been created to make it clear what size is really being used. See http://physics.nist.gov/cuu/Units/binary.html for a further explanation.

Дата добавления: 2014-12-23