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Types of Memory

Memory chips are soldered to small circuit boards to form a package called a memory module. In the old days, before memory chips were soldered to modules as shown in Figure 6.1, you had to install each individual chip onto the motherboard. Today, memory modules make memory upgrades easy.

Figure 6.1Figure 6.1 Memory "chips" are mounted onto memory modules.

The size and shape of the module, as well as other characteristics, make up its form factor. Starting from the top of Figure 6.2, each memory module's form factor is described below. In the next few sections, you'll use these illustrations to identify the type of memory your system has.

Figure 6.2Figure 6.2 Types of memory modules. From top: 72-pin SIMM, 168-pin DIMM, 184-pin DDR DIMM, 184-pin RIMM.

  • 72-pin SIMM (single inline memory module). This is an older style module that was used on early Pavilions (series 50xx, 51xx, 52xx, 53xx, 70xx, 71xx, 72xx, 73xx, 74xx, 81xx).

  • 168-pin DIMM (dual inline memory module). This is the most common memory module on HP Pavilions. It was used extensively on models from 1997 to the present.

  • 184-pin DDR DIMM (double-data rate DIMM). This module uses a form factor similar to the DIMM and is often confused for standard DIMMs, but the memory chips are capable of transferring data at double the data rate of traditional DIMMs.

  • 184-pin RIMM (Rambus inline memory module). This is a special memory type developed by Rambus that Intel tried to popularize. It works with Intel Pentium 4 processors, but is used on only two of HP's motherboards at this writing.

One of the differences between the older style SIMM and the new DIMM and RIMM formats is the mounting technique. I mention this so you can identify what your system has. Figure 6.3 shows the 72-pin SIMM socket. There is a small metal clip at each end that holds the SIMM in place. You push the clip outward and then tilt the SIMM out of the socket, as shown. Compare this to the DIMM slot shown in Figure 6.4. When a DIMM is pushed into place, the hammer-type lock engages the notch on the side of the DIMM. To remove a DIMM, you push down and outward on the locks at both ends, which pushes the DIMM out of its socket. Installation and removal methods are discussed later.

Figure 6.3Figure 6.3 SIMM slot with metal mounting clip.

Figure 6.4Figure 6.4 DIMM slot with hammer-type locks.

Note the other identifying characteristics of the different memory modules. There are small notches near the metal contacts at the bottom of the modules. These are "keys" that ensure the module is inserted correctly and that only the right type of module is inserted in a memory socket.

Interesting Memory Characteristics

PC memory has gone through a lot of changes over the last 20 years. As mentioned, in the early days, you had to push each individual chip into the motherboard. Now that they are mounted on small circuit boards, installation is much easier and you don't need to hire a technician to install your memory.

The biggest change in PC memory occurred in 1997 when Intel created synchronous dynamic RAM, or SDRAM. SDRAM is synchronized with the CPU so that data transfers can be more efficiently coordinated. A side benefit is that memory speeds could be increased dramatically. Almost all memory for PCs is a form of SDRAM. The packaging (form factor) is what differs—and of course the speed and access methods.

The new SDRAMs were placed on a circuit board with 168 pins, and the entire package was called a DIMM (dual inline memory module). DIMMs use a 64-bit data path to the CPU as compared to the 32-bit data path of the older SIMMs. Most were designed to work in the early Pentium-class system, which had a 66 MHz CPU bus. The memory specification for these systems was called PC66. Later, Intel developed the PC100 specification, which defines a 100 MHz CPU-to-memory bus. Later, this bus was increased to 133 MHz and the PC133 memory specification was defined.

If you look through Table 6.1 in the next section, you'll see a variety of speeds for memory. This is the speed at which data transfers across the CPU-memory bus (front side bus). New types of memory called double-data rate (DDR) SDRAM and Rambus DRAM or RDRAM have emerged to improve the data transfer rate.

DDR memory doubles the amount of data that is sent during each clock cycle. So if it is attached to a 100 MHz bus, DDR memory essentially transfers data at 200 MHz (100 x 2). On a 133 MHz bus, DDR memory transfers at 266 MHz (133 x 2). Below is a list of standard and DDR memory types that are used in current Pavilion models. Other memory types are available, but they are not used in Pavilion models.

  • PC66. This is standard 66 MHz SDRAM on a DIMM-format memory module.

  • PC100. Standard 100 MHz SDRAM on a DIMM-format memory module. The motherboard and CPU must support a 100 MHz CPU-memory bus.

  • PC133. Standard 133 MHz SDRAM on a DIMM-format memory module. The motherboard and CPU must support a 133 MHz CPU-memory bus.

  • DDR 200. This memory implements double data rate transfers on a 100 MHz bus. Also called PC1600 because it transfers 1,600 MB/sec.

  • DDR 266. This memory implements double data rate transfers on a 133 MHz bus. Also called PC2100 because it transfers 2,100 MB/sec.

RDRAM is a high-performance memory technology developed by Rambus Corporation and adopted by Intel. At one point, Intel was trying to move the entire market to adopt RDRAM and its licensing agreements, but DDR memory leaped ahead in performance and cost savings. Only two HP Pavilion motherboards support RDRAM, while most new boards support DDR memory. RDRAM is described later in this chapter.

Tips and Techniques

Here are few things to know about memory if you are going to upgrade your system.

  • Some motherboard support multiple CPU bus (front side bus) speeds such as 66/100 MHz or 100/133 MHz. The CPU installed will determine at what speed this bus runs. If a Celeron CPU is installed with a 66 MHz CPU bus, PC66 memory modules should be installed. If a CPU with a 100 MHz front side bus is supported, then use PC100 memory. Likewise, if a CPU with a 133 MHz front side bus is supported, then use PC133 memory.

  • Some HP Pavilions have integrated video graphics controllers that use part of RAM for video memory. By upgrading memory, you're also able to expand video memory, which means you can use higher resolutions and more colors. This is discussed in the next chapter.

  • You are wasting money if you buy a low-capacity memory module and then need to expand in the future. Remember, there are only two or three memory slots. To expand, you might need to remove a module in a slot to make way for a higher capacity module.

  • Recycle your memory modules by moving them to another computer that needs a boost, or give them to someone who will really appreciate them.

  • If you have two DIMM modules with different memory sizes, put the largest module in the first slot.

  • Never mix modules of different speeds (i.e., don't mix 100 MHz and 133 MHz modules).

  • If you have problems after installing memory, make sure the module is properly seated and check the contacts for dirt.

  • If you are not sure whether your system supports 100 MHz or 133 MHz DIMMs, go ahead and get the faster DIMMs. They cost just a little more, and will run on a 100 MHz CPU bus. And you'll have the faster DIMMs to put in a new motherboard if necessary.

  • If you have problems after installing a module, it may be defective or it may not work with an existing module. Try removing it and see if your system still starts with the original module, then try starting with just the new module. You can also try switching the slot locations of the modules.

  • Motherboards that use the Intel 430 series chipsets have a caching problem when over 64 MB of memory is installed. If you have a 31xx, 32xx, 5030, 5040, 5120, 5215, 5250, 5315, 5315D, 5320, 7010, 7020, 7030, 7050, 7070, 7090, 7055, 7110Z, 7125, 7130P, 7145, 7160, 7170, 7222, 7260, 7275, 7310, 8140, 8160, 8201, 8240, 8242, or 8246 model, don't install more than 64 MB of memory.

  • In systems that use the Type 1 case, the height of the DIMM modules must not exceed 1 inch. Taller modules will not clear the drive bay when the motherboard panel is swung back into place. In fact, you practically have to wedge the motherboard into the case even with the right size of DIMM.

  • Windows 98 may run slowly on systems with over 512 MB of memory. Search the Microsoft Knowledge Base for Q253912 to learn more about this problem if you plan on installing that much memory and running Windows 98.

  • You can buy memory from the big names like PNY (http://www.pny.com) and Crucial (http://www.crucial.com), or you can check prices with a whole group of memory vendors at Price Watch (http://www.pricewatch.com).


Hewlett-Packard tests memory from various vendors for compatibility in its systems. It recommends you use only memory that it has tested and approved. While other memory will work, HP may ask you to pull the memory before it will provide any warranty service.

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