3.4 Preparing the Motherboard for Installation
3.4.6 Installing RAM
There are two types of memory modules used on most PCs. These are 168-pin Dual In-line Memory Module (DIMM) cards and 72-pin Single In-line Memory Module (SIMM) cards. DIMMS and SIMMS both share common edge connectors and fit into slots on the motherboard called RAM sockets. RAM sockets used for DIMM cards are often called DIMM sockets, while those used for SIMM cards are called SIMM sockets. When the DIMM card or SIMM card is inserted into the slot, each edge connector makes contact with a corresponding gold trace on the motherboard. Each gold line represents an individual data "road". Just as the gold lines leading to the CPU make up the processor bus, all these gold lines make up the memory bus. The memory bus data "highway" is used to transfer data between the RAM and the CPU. Bus types are discussed more thoroughly in Chapter 2. For information relating to memory modules that use other access technologies, see a note at the end of the section.

Configuring Memory
The motherboard manual usually will show the permissible combinations of DIMM types that can be installed in the system. New motherboards do not use SIMMs. It may be found, for example, that the DIMM sockets on the motherboard map are grouped into three or four banks of one slot each. Using the information provided in the figure, identify DIMM 1 and DIMM 2 . DIMM 1 and DIMM 2 memory Banks are Bank O and Bank 1. In some cases motherboards have more than two slots for RAM. These slots would be DIMM 2 and DIMM 3 and the memory Banks are Bank 3 and Bank 4. Each bank can have any type of SDRAM (the most commonly used form of RAM). Each bank can have any type of SDRAM (the most commonly used form of RAM). 

It is recommended that the memory banks be filled in the exact combinations shown in the system board (motherboard) manual. For example, the manual might state that the maximum memory size is 512 MB and that the size of each DIMM can be 8 MB, 16 MB, 32 MB, 64 MB, or 128 MB. Any combination of these sizes can be used. Depending on memory needs, there may be four 128 MB DIMMs installed in the memory banks on the motherboard. Or, there may be two 128 MB DIMMs and two 64 MB DIMMs installed. When DIMM sizes are mixed on the motherboard, it is important to remember to put the DIMM with the largest memory size in the first bank. The system automatically reads the size of the first DIMM and records it as the largest. If a smaller DIMM were put in the first bank, the system would read it as the largest and might fail to recognize or use the additional memory capacity of the DIMMs placed in the subsequent banks.

Banking with SIMM modules is slightly different. Each bank of memory for a SIMM has two sockets. Users must fill the first bank before moving onto the next. Additionally, each bank must be filled with RAM modules that have the same access time and size.

Step-by-Step Installation of RAM

Step 1 First, decide on which slots to use and then orient the SIMM or DIMM chip over it. Both SIMMs and DIMMs are keyed, so they can go in only one way.
Step 2 Insert the DIMM module straight into the slot. The SIMM module is inserted at an angle of about 45 degrees.
Step 3 Now, the memory module must be locked into place. With a SIMM, rotate it from the angled position to the vertical position. Some resistance is normal. Do not force it. If difficulty is encountered, the chip might be backwards. Rotate it and try again. When the SIMM is vertical, the little metal or plastic clip should snap in place, securing the SIMM vertically in the memory slot.

With a DIMM, simply close the levers on either side of it. Usually, if the levers do not close, it is because the DIMM is not inserted all the way into the slot or it is installed backwards. In most cases, if the DIMM is inserted properly, the levers will snap in place without further action.

Step 4 Repeat steps one to three for the rest of the memory modules. When finished, check the work to be sure that each module is well seated in the slot on both ends.

Note: This procedure specifically describes the installation of DIMM and SIMM memory modules. When using other types of memory modules such as Rambus Inline Memory Modules (RIMMs) know that other considerations have to be taken into account. Unlike DIMMs and SIMMs, RIMM modules use only the Direct Rambus Memory Chips (RDRAM). Some systems require that RIMM modules be added in identical pairs, and others allow single RIMMs to be installed. Information on specific memory types can be found in their manuals, the motherboard manual or the manufacturer websites.