Chapter 21. Advice on RAM

RAM can be a tricky thing to work out. In this chapter I will give a couple of tips to anyone having to choose between the various RAM products.

Bandwidth

Of course you want to choose the best and fastest RAM. It’s just not that easy to work out what type of RAM is the fastest in any given situation.

We can start by looking at the theoretical maximum bandwidth for the various systems. This is easy to calculate by multiplying the clock frequency by the bus width. This gives:

Module type	Max. transfer
SD RAM, PC100	800 MB/sec
SD RAM,  PC133	1064 MB/sec
Rambus, PC800	1600 MB/sec
Rambus, Dual PC800	3200 MB/sec
DDR 266 (PC2100)	2128 MB/sec
DDR 333 (PC2700)	2664 MB/sec
DDR 400  (PC3200)	3200 MB/sec
DUAL DDR PC3200	6400 MB/sec
DUAL DDR2-400	8600 MB/sec
DUAL DDR2-533	10600 MB/sec

Fig. 140. The highest possible bandwidth  (peak bandwidth) for the various types of RAM.

However, RAM also has to match the motherboard, chipset and the CPU system bus. You can try experimenting with overclocking, where you intentionally increase the system bus clock frequency. That will mean you need faster RAM than what is normally used in a given motherboard. However, normally, we simply have to stick to the type of RAM currently recommended for the chosen motherboard and CPU.

RAM quality

The type of RAM is one thing; the RAM quality is something else. There are enormous differences in RAM prices, and there are also differences in quality. And since it is important to have a lot of RAM, and it is generally expensive, you have to shop around.

One of the advantages of buying a clone PC (whether you build it yourself or buy it complete) is that you can use standard RAM. The brand name suppliers (like IBM and Compaq) use their own RAM, which can be several times more expensive than the standard product. The reason for this is that the RAM modules have to meet very specific specifications. That means that out a particular production run, only 20% may be “good enough”, and that makes them expensive.

Over the years I have experimented with many types of RAM in many combinations. In my experience, for desktop PC’s (not servers), you can use standard RAM without problems. But follow these precautions:

How much RAM?

RAM has a very big impact on a PC’s capacity. So if you have to choose between the fastest CPU, or more RAM, I would definitely recommend that you go for the RAM. Some will choose the fastest CPU, with the expectation of buying extra RAM later, “when the price falls again”. You can also go that way, but ideally, you should get enough RAM from the beginning. But how much is that?

If you still use Windows 98, then 256 MB is enough. The system can’t normally make use of any more, so more would be a waste. For the much better Windows 2000 operating system, you should ideally have at least 512 MB RAM; it runs fine with this, but of course 1024 MB or more is better. The same goes for Windows XP:

	128 MB	256 MB	512 MB	1024 MB
Windows 98	**	***	Waste	Waste
Windows 2000	*	**	***	****
Windows XP		*	***	****

Fig. 141. Recommended amount of PC RAM, which has to be matched to the operating system.

The advantage of having enough RAM is that you avoid swapping. When Windows doesn’t have any more free RAM, it begins to artificially increase the amount of RAM using a swap file. The swap file is stored on the hard disk, and leads to a much slower performance than if there was sufficient RAM in the PC.

RAM addressing

Over the years there have been many myths, such as ”Windows 98 can’t use more than 128 MB of RAM”, etc. The issue is RAM addressing.

Below are the three components which each have an upper limit to how much RAM they can address (access):

Windows 95/98 has always been able to access lots of RAM, at least in theory. The fact that the memory management is so poor that it is often meaningless to use more than 256 MB, is something else. Windows NT/2000 and XP can manage gigabytes of RAM, so there are no limits at the moment.

In Windows XP, you have to press Control+Alt+Delete in order to select the Job list. A dialog box will then be displayed with two tabs, Processes and Performance, which provide information on RAM usage:

Under the Processes tab, you can see how much RAM each program is using at the moment. In my case, the image browser, FotoAlbum is using 73 MB, Photoshop, 51 MB, etc., as shown in Fig. 143.

Modern motherboards for desktop use can normally address in the region of 1½-3 GB RAM, and that is more than adequate for most people. Server motherboards with special chipsets can address much more.

Standard motherboards normally have a limited number of RAM sockets. If, for example, there are only three, you cannot use any more than three RAM modules (e.g. 3 x 256 MB or 3 x 512 MB).

CPU’s have also always had an upper limit to how much RAM they can address:

Processor	Address bus width (bits)	Maximum System RAM
8088, 8086	20	1 MB
80286, 80386SX	24	16 MB
80386DX, 80486, Pentium, Pentium MMX, K5, K6 etc.	32	4 GB
Pentium Pro, Pentium II, III Pentium 4	36	64 GB

Fig. 144. The width of the CPU’s address bus determines the maximum amount of RAM that can be used.

Let me conclude this examination with a quote. It’s about RAM quantity: