Memory is an integrated circuit (chip) that stores information electronically.
DRAM stands for Dynamic Random Access Memory andis the main memory used by the CPU tokeep copies of files, programs and other temporary informationused 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 byteis 1 million bytes.
Dynamic memory is a type of memory that must be continuouslyrefreshed in order to retain it's contents.
Think of each bit location as if it were a small bucket. The amount of water inthe bucket represents the state of the bit stored there. If the bucket is morethan half full of water, the bit is a 1. If it is less than half full the bitis 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 inexpensivecompared to static (non-leaky) memory. When you charge a cell, setting it to a1 bit, it immediately begins to lose charge. In a very short time the chargefalls below the half way mark and the 1 becomes a 0. Each cell must constantlybe refreshed with charge by the CPU or the self- refreshing circuits in newerDRAM devices.
The leaky DRAM cells are less expensive than SRAM cells which don't leak. IfSRAMs were used, PC prices would go up noticeably.
Why is the state of one bit so important? Imagine that this bit we've beentalking about is a status bit in a payroll program that the CPU uses todetermine if you are to be paid this week; 1 means you get paid and 0 means youdon't. You can see why a bit changing state when it's not supposed to can havedisastrous results!
Random Access means that any location in the memory can be read orwritten immediately. It's like a CD, which is also random access. When youselect 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'taccess any location you choose to instantly. You might have to rewind or fastforward 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. Bitscan only have one of two states, either on or off. These two states representthe 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 numbersystem, which is called a binary. All storage and calculation operations in aPC are done using binary arithmetic.
The reason there are only two states for a bit is because of the nature oftransistors, which are the building blocks for all digital electroniccircuits. 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 tenstates or values. The decimal digits 0,1,2,3,4,5,6,7,8 and 9 represent the tenvalues a number can have.
SIMMs and DIMMs
Up until recently DRAM memory boards were made in SIMM form, single in-linememory module. Today they are supplied in DIMM form, dual in-line memorymodule.