Memory Guide - The Answers

Written by KingJackal aka Sam Evans

Date: 11/6/02

This article is a guide which will hopefully outline the most important aspects of computer memory. It is by no means the be-all and end-all of memory. I certainly haven't had the time to mention everything I know and have researched about memory, let alone everything there is to know. However, if you understand everything in this article then there is a good chance you'll be head and shoulders above most of the population. And you'll definitely be able to make some sound decisions when it comes time to upgrade your machine's memory.

If any of this whets your appetite to learn more, I have included some links in a section down at the end of the article.


There are three main types of memory. Namely SD-RAM, RAMBUS and flash-RAM. Different memory types are not interchangeable.


SD-RAM is what most PC's use. 'DDR' and older 'SDR' are both types of SD-RAM.

SD-RAM comes in many flavours. The most common being sold now is 184pin DDR SD-RAM.

A 184pin DDR SD-RAM module.

A 184pin DDR SD-RAM module.

A 168pin SDR SD-RAM module.

A 168pin SDR SD-RAM module.

Memory, much like everything else in your PC, runs on a clock. With each 'tick' of the clock, the memory can send or receive some data or the CPU can calculate something etc. DDR SD-RAM differs from ordinary SDR SD-RAM by transmitting or receiving data TWICE every clock cycle (on the rising and falling edges of the clock, which is similar to a sine wave), rather than once. While this effectively doubles it's speed, the actual performance of DDR RAM isn't double that of SDR RAM, due to similar latencies in the two types of RAM.


RDRAM is a more expensive, higher bandwidth variety of memory that you'll only find on higher-end Intel systems. It is commonly called Rambus, because the technology is very closely guarded by a company of the same name.

While RDRAM is just memory that can be read from or written to, it is fundamentally different to SD-RAM. It is designed to provide higher bandwidth, and hence performance than normal SD-RAM. DDR SD-RAM seems to have taken a large bite out of it's advantage though.

RDRAM not only provides a differing memory signalling process, but it also runs separate memory modules in serial, rather than in parallel like SD-RAM. This has the side effect of increasing latency significantly, partially offsetting the bandwidth advantage it gives over SD-RAM.


Flash-RAM is what you'll find in digital camera's and the like.

Flash RAM has a key property that none of the other types of RAM mentioned have. Specifically, if you turn off the power to flash-RAM, the bits stored inside it don't die. This means that it's fast like other RAM, but can be used to store data much like a hard-drive. The one drawback being cost. If it wasn't so expensive, we'd probably all be using flash-RAM hard-drives. This does however suit it to the world of digital camera's, where you don't need huge volumes of data storage. Many digital camera memory sticks are just a few flash-RAM chips in a plastic holder.

The ability to store memory without power is called non-volatility. Hence SD-RAM is volatile memory, while flash is non-volatile memory. There are other forms of non-volatile memory, such as EPROM and EEPROM. An EPROM chip can only be written to once (like a CD-R), while EEPROMs can be re-written (like a CD-RW). These types of memory are typically used for computer BIOS chips.


« Next Page »