PC Architecture. Chapter 21. Advice on RAM. A book by Michael B. Karbo

Copyright Michael Karbo and ELI Aps., Denmark, Europe.

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    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.


    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


    SD RAM, PC100800 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:

  • Avoid mixing RAM from various suppliers and with various specifications in the same PC – even if others say it is fine to do so.

  • Note that the RAM chips are produced at one factory, and the RAM modules may be produced at another.

  • Buy standard RAM from a supplier you trust. You need to know who manufactured the RAM modules and the seller needs to have sold them over a longer period of time. Good brands are Samsung, Kingston and Corsair.

  • The modules have to match the motherboard. Ensure that they have been tested at the speed you need to use them at.

  • The best thing is to buy the motherboard and RAM together. It’s just not always the cheapest.

  • Avoid modules with more than 8 chips on each side.

    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


    Windows 98





    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):

  • The operating system (Windows).

  • The chipset and motherboard.

  • The processor.

    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 1˝-3 GB RAM, and that is more than adequate for most people. Server motherboards with special chipsets can address much more.

    Figur 143. This window shows how much RAM each program is using (Windows XP).

    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:


    Address bus
    width (bits)

    System RAM

    8088, 8086


    1 MB

    80286, 80386SX


    16 MB

    80386DX, 80486, Pentium, Pentium MMX, K5, K6 etc.


    4 GB

    Pentium Pro, Pentium II, III
    Pentium 4


    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:

    ”640K ought to be enough for anybody.”
    Bill Gates, 1981.

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