Memory

Memory is an integrated circuit (chip) that stores information electronically.

DRAM stands for Dynamic Random Access Memory and is the main memory used by the CPU to keep copies of files, programs and other temporary information used by the CPU when it is running.

When you see a computer ad that says '1.2Ghz, 128Mb, 10GB Hard Drive etc,' the 128Mb is the size in mega bytes of the DRAM memory in the PC. A mega byte is 1 million bytes.

Dynamic memory is a type of memory that must be continuously refreshed in order to retain it's contents.

Think of each bit location as if it were a small bucket. The amount of water in the bucket represents the state of the bit stored there. If the bucket is more than half full of water, the bit is a 1. If it is less than half full the bit is a 0. These buckets are leaky (dynamic) due to small holes in the bottom.

DRAM cell leak because the cell structure used is small and inexpensive compared to static (non-leaky) memory. When you charge a cell, setting it to a 1 bit, it immediately begins to lose charge. In a very short time the charge falls below the half way mark and the 1 becomes a 0. Each cell must constantly be refreshed with charge by the CPU or the self- refreshing circuits in newer DRAM devices.

The leaky DRAM cells are less expensive than SRAM cells which don't leak. If SRAMs were used, PC prices would go up noticeably.

Why is the state of one bit so important? Imagine that this bit we've been talking about is a status bit in a payroll program that the CPU uses to determine if you are to be paid this week; 1 means you get paid and 0 means you don't. You can see why a bit changing state when it's not supposed to can have disastrous results!

Random Access means that any location in the memory can be read or written immediately. It's like a CD, which is also random access. When you select a track on a CD it goes to that track and starts playing right away.

Serial Access on the other hand is like an audio or video cassette. You can't access any location you choose to instantly. You might have to rewind or fast forward past lots of stuff to get to the location you want.

What are bits and bytes?

A bit is a binary digit and is the smallest unit of information in a PC. Bits can only have one of two states, either on or off. These two states represent the two values a bit can have, either 1 or 0. A byte is group of 8 bits.

Everything in a computer is stored and calculated using this two value number system, which is called a binary. All storage and calculation operations in a PC are done using binary arithmetic.

The reason there are only two states for a bit is because of the nature of transistors, which are the building blocks for all digital electronic circuits. For technical reasons beyond the scope of this discussion, transistors used in digital circuits can be in only one of two stable states, either fully on or completely off.

In contrast, the number system we use every day is decimal and it has ten states or values. The decimal digits 0,1,2,3,4,5,6,7,8 and 9 represent the ten values a number can have.

SIMMs and DIMMs

Up until recently DRAM memory boards were made in SIMM form, single in-line memory module. Today they are supplied in DIMM form, dual in-line memory module.