Help Stefan Campaign
Building your PC:
Memory
Memory can be considered as an area of very fast access temporary storage where a computer holds program code and data that the processor is currently working on. Data access to memory is much faster than to equivalent storage on the hard drive thus memory is used to speed-up the processing of data. As operating systems have become more sophisticated, allowing many applications to be executed at the same time CPUs spend far more time swapping code and data in and out of memory than they used to in the past. Thus having access to large amounts of fast memory can have a profound effect on the efficiency and running speed of your system. It is also one of the cheapest way of beefing-up your system.
Page Map |
|
|---|---|
| Memory History | Summary |
| Modern Memory | |
| DDR SDRAM | |
| DDR2 SDRAM |
Random Access Memory:
History:
Random Access Memory (RAM) is called that because the time required to access any item held in any area of memory is precisely the same as the time required to access an item held anywhere else. The main or 'core' memory in your computer is also termed 'volatile' in that all data is lost when the computer is powered down (this is in contrast with memory such as memory cards for cameras which are non-volatile). However, when most people thing of computer 'memory' what they think of is some form of Dynamic Random Access Memory 'DRAM'. Almost all main or 'core' memory is a descendant of the original DRAM package developed in 1966.
Electronically, DRAM memory is relatively simple. Each 'bit' (the 1s and 0s of memory) is stored in a transistor with an attached capacitor (this temporarily stores the charge maintaining the 'state' [the 1 (on) or 0 (off) nature of the transistor]). Over time, however, the capacitor loses its charge and for this reason the memory needs to be periodically 'refreshed' to maintain its integrity. Typically, most manufacturers specify that memory should be refreshed once every 64ms. As a result DRAM memory is very fast to read and write to (and easy to design) but it is only suited for short-term storage. This is not a problem in general as the CPU tends to continually write to and read from memory.
Up until the early 1990s memory came in various form factors each being essentially machine and manufacturer specific. It wasn't until the early 1990s that first the Single In-line Memory Module (SIMM) and its successor the Dual In-line Memory Module (DIMM) became standards allowing simple user upgrades of memory by inserting modules into sockets in a motherboard, as in the image, above left. The first of these packages was the SIMM (image, middle left) which had 72 pin connectors and was limited to eight memory modules on one side. The DIMM came next which had a larger number of connecting pins (168) and allowed eight memory modules per side, allowing the total amount of memory per module to be instantly doubled.
Modern Memory
The next evolution of Dynamic RAM was the Single Data Rate synchronous DRAM (SDR SDRAM) commonly referred to as SDRAM. Whilst the older forms of DRAM had an asynchronous interface, meaning that it reacts immediately to changes in its control inputs, SDRAM has a synchronous interface, meaning that it waits for a clock pulse before responding to its control inputs — it synchronizes with the computer's system bus. SDRAM is double sided, has 168 pins and two notches (as above). This form factor spawned the PC66, PC100 and PC133 line of memory modules which are still extant today (the PCxxx stands for the bus speed of the Motherboard and faster modules could generally be used in slower PCs. Thus PC133 modules were compatible with PC100 systems). These DIMMS came in two flavours parity and non-parity. Non-parity can be considered as 'standard' memory and contains exactly one bit of memory for every bit of data to be stored. Thus 8 bits are used to store each byte of memory. In contras, parity memory adds an extra single bit for every eight bits of data that's used solely for error detection and correction. Thus 9 bits of data are used to store each byte.
Parity memory can be used for parity checking, a basic form of error detection, on PCs that support it. It can also be used for an advanced form of error detection and correction that I'll talk about next. This error detection mechanism is called EEC (extended error correction) that's compatible with Pentium class or later systems that support ECC. This memory is specifically designed to allow the use of ECC on modern systems that have a chipset that supports it, much the way that parity memory can on some systems. However, as motherboards have become more complex there has become a de-emphasis on error detection in newer systems and the number of popular chipsets that don't support parity checking. Thus non-parity chips now predominate the market.
SDRAM is double-sided, has 168 pins and two notches. Many of these memory are still around but they are slowly becoming obsolete as faster motherboards with more demanding memory requirements become available. This has led to the latest generation of memory modules:
DDR SDRAM
Double data rate synchronous DRAM (DDR SDRAM) achieves greater bandwidth than ordinary SDRAM by transferring data on both the rising and falling edges of the clock signal (double pumped). This effectively nearly doubles the transfer rate without increasing the frequency of the front side bus (this being the bus speed of the motherboard). Thus a 100MHz DDR system has an effective clock rate of 200MHz when compared to equivalent SDR SDRAM. If you're designing your PC around a modern motherboard then you will need to buy DDR RAM modules. The precise type you buy will be dependent on the bus speed of your motherboard where RAM speed and motherboard bus speed need to match precisely. The current line of DDR chips includes the following:
- PC-1600: DDR-SDRAM memory module specified to run at 100 MHz using DDR-200 chips
- PC-2100: DDR-SDRAM memory module specified to run at 133 MHz using DDR-266 chips
- PC-2700: DDR-SDRAM memory module specified to run at 166 MHz using DDR-333 chips
- PC-3200: DDR-SDRAM memory module specified to run at 200 MHz using DDR-400 chips
DDR SDRAM DIMMs have 184 pins (as opposed to 168 pins on SDRAM and only a single notch (as above) and they operate at a voltage of 2.5V, compared with 3.3V for SDRAM which can significantly reduce power usage. It should also be noted that there is no actual architectural difference between DDR SDRAM designed for different clock frequencies. The number actually designates the maximum speed that the chip was designed to operate at. Thus, in the same manner as the original SDRAM modules memory rated at a higher bus speed will work in a system with a lower bus speed. However, this may only work with higher quality DDR memory and experience has taught me that this memory type can be a little fickle.
It should also be noted that some new chipsets use these memory types in dual or even quad channel configurations, which doubles or quadruples the effective bandwidth. However, in dual channel configurations it is recommended to use a pair of matched memory modules to optimize performance. In reality if you have a dual memory channel then you should really buy identical memory from the same manufacturer as I have seen memory with the same specification, but from different manufacturers fail.
DDR2 SDRAM
DDR SDRAM is slowly being replaced by the DDR-2 specification (Double Data Rate Two Synchronous Dynamic Random Access Memory). The main advantage of DDR2 is the ability of attaining much higher clock speeds. DDR2 modules are double-sided, have 240 pins and a single notch. The current line of DDR2 memory modules include:
- PC2-3200: DDR2-SDRAM memory stick specified to run at 200 MHz using DDR2-400 chips
- PC2-4200: DDR2-SDRAM memory stick specified to run at 266 MHz using DDR2-533 chips
- PC2-5300: DDR2-SDRAM memory stick specified to run at 333 MHz using DDR2-667 chips
- PC2-6400: DDR2-SDRAM memory stick specified to run at 400 MHz using DDR2-800 chips
Work is already underway on the next generation of memory technology DDR3 though the specification is not complete as yet.
Summary
As you can see, there are a number of memory specifications available and these formats are evolving quite rapidly. As soon as you have chosen a motherboard for your system sit down and read its specifications closely. Then select your memory. Match the memory specification DDR or DDR2 to that of the motherboard then match the clock speed of the memory to the bus speed of the motherboard (again the specifications of the motherboard will tell you what memory is supported). Buy as much memory as you can afford as this is one of the cheapest ways of improving your PC's performance. Finally check on whether the memory banks in your motherboard are dual channel. If they are and you're buying more than one memory module you will need to ensure that you match those modules in pairs and insert the matched pairs in the correct memory banks. Follow these simple rules and you will not have any problems with the memory you purchase and use.
