Memory Packages

Whether you're purchasing or installing RAM, understanding the different types of memory packages available is important. The following sections identify different memory packages used in desktop computers and laptop systems.

SIMMs

Single Inline Memory Modules (SIMMs) used to be one of the most popular types of memory modules, but they have been replaced by DIMMs (see the next section) in recent years. A SIMM is a card that holds a number of memory chips and has an edge connector containing a number of pins that make contact with the motherboard. This design makes it quite a bit easier to install memory than it was many years ago. In the past, you had to take a dual inline package (DIP) chip out of the system board and reinsert a new chip. Today, you purchase a card of chips (a SIMM) and install the SIMM into one of the SIMM sockets.

SIMMs come in two flavors, 30-pin and 72-pin, which describe the number of connectors that make contact with the motherboard. Before buying a SIMM to install in a computer, review the documentation for the computer or look at the system board to determine what size SIMM module you need.

The 30-pin SIMMs have an 8-bit data path, meaning they supply information in 8-bit blocks. When installing memory into a system, you must install enough SIMMs to fill a memory bank. A memory bank is the number of SIMMs it takes to fill the data path of the processor. For example, if you have a system with a 486 processor, the processor is a 32-bit processor. Therefore, the processor wants to deal with information in 32-bit chunks. When using 30-pin SIMMs, you need to install four of them at a time to fill a memory bank because each 30-pin SIMM only supplies 8 bits of data (8 bits x 4 SIMMs = 32-bit chunks).

The 72-pin SIMMs supply information in 32-bit chunks. Therefore, if you are installing 72-pin SIMMs on a system using a 32-bit 486 chip, you need just one SIMM to fill a memory bank and the data path. If you're installing 72-pin SIMMs in a Pentium system, you must install SIMMs in pairs because the Pentium data path is 64-bit; to fill a bank on these systems, you need two 32-bit modules (72-pin SIMMs).

In this day and age, you most likely will not see SIMMs in a system unless you are supporting older computers. If you see a system that uses SIMMs, it most likely conforms to the 72-pin format.

You can easily distinguish what size SIMM a system uses, even if you don't have the documentation for that system. The 72-pin SIMMs have a notch close to the center of the module. If there are SIMMs already installed in the system, you can take them out and examine them. They usually have a label with a 1 or a 72, representing the pin numbers, at either end of the module - so if you see a number 72, you know you have a 72-pin SIMM.