Computer Memory

Computer memory is very important to your system's functions.

A computer system's memory is its workspace. Everything the computer does is processed somewhere in memory.

The two main types of memory are Random Access Memory (RAM) and Read Only Memory (ROM).

Random Access Memory (RAM):
This is the computer's working memory and it has two distinct characteristics. (1) RAM is volatile which means all information stored in the chip will be lost when the system is turned off and (2) RAM can be written to and read from.

Today's operating systems (especially Windows XP) and application software require large amounts of RAM (Temporary Memory).

There are different types of RAM :

1. Dynamic Random Access Memory (DRAM)
   This is most commonly used for main memory. DRAM is also referred to as fast page-mode (FPM). Since this is one of the cheapest types of computer memory, it is used as main memory in most computer systems.

   DRAM must constantly be refreshed on a regular basis; hence, while it is being refreshed, it cannot be read or written to. This causes system slow-down, which can be compensated by two methods; namely, paging and interleaving.

   Paging:   Improves memory speed since the memory is accessed in chunks called pages; hence, the name Paging. Accessing computer memory in pages is almost like reading a book because as long as the book is open at a certain page, you can read from the top, the bottom or anywhere on the page.

   If you want to read the next page, you simply have to wait until the pages turn which takes longer than skipping around on the page to read different sections. This is how paged memory works. Because when a page of memory is accessed, data can be read from anywhere in that section of memory.

   Changing to a different page requires more time; therefore, the computer has to add a wait state. This wait state is time wasted for the Computer's CPU (Central Processing Unit) which has to wait until it can access memory (RAM).

   If you have fast enough RAM, the CPU does not have to wait to access data; hence the system has a "zero wait state". But since DRAM needs to be constantly refreshed, wait states are sometimes required.

   Interleaving:   Interleaved memory is accessed faster than paged memory. With interleaved memory, odd bytes are one bank and even bytes are in another bank. While one bank is being accessed, the other can be refreshed, saving time.

   By using interleaving, computer memory access can be almost doubled without using faster computer memory chips.

2. Extended Data Output (EDO)
   This is an enhanced version of DRAM.

3. Synchronous DRAM (SDRAM)
   This is a type of DRAM that runs in synchronization with the memory bus. SDRAM can operate at higher clock rates than standard DRAM.

   If SDRAM is used for main system memory, wait states are reduced or eliminated because the signals are already synchronized with the motherboard clock. SDRAM is the most popular type of memory used in new computer systems. At present, SDRAM can operate at speeds in excess of 800 MHz.

4. Static RAM
   This operates just like DRAM except for one important difference - It does not need to be refreshed; hence the word static which means fixed. Since this computer memory is fixed or static and doesn't need to be refreshed, it can be accessed much faster than DRAM.

   Access times for SRAM can be below 2 nanoseconds (ns) while access times for DRAM are between 60ns and 70ns. Since SRAM is faster than DRAM, why isn't SRAM used for all the system memory? Unfortunately, SRAM is physically larger, cost more to produce and consumes more power than DRAM.

This information doesn't change once its written - it's stored in the chip permanently. ROM is used on motherboards, modems, video cards, etc. to store data, programs and instructions that must be retained when the system is powered down.

One good example is the BIOS (Basic Input/Output System) which is used to boot your system.

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