Memory Types and Forms

Physical forms of RAM have changed greatly over the years since the first IBM PC was introduced. Throughout the 1980s, most systems used individual memory chips in capacities ranging from 64KB to 1MB. In the late 1980s, memory modules (multiple memory chips on a miniature board that we know today as SIMMs, DIMMs, and RIMMs) became popular, and this type of memory is still the standard today, although specific forms and capacities are changing rapidly .

The same physical form (chip or module) can be used for RAM with different speeds, types, and sizes. Memory also can be purchased in forms that enable the system to detect or even correct memory errors.

To specify the correct memory for any given system, you must choose the correct options for that system, including the following (all of which are explained in more detail later in the chapter):

• Form factor ” Whether DIMM, SIMM, or another standard or proprietary type, the memory must be designed to fit into the memory upgrade sockets in the system.

• Memory type ” You must install memory modules that contain memory chips of the types specified by the system. Normally, all memory in the system should use the same types of chips (EDO, SDRAM, DDR, RAMBUS, and so forth).

• Memory speed ” Memory chips (and the modules created with those chips) have different speed ratings; you must choose modules that meet the speed requirements of the system you are upgrading. For best results, all memory should be the same speed.

• Memory capacity (MB ) ” You must choose modules that meet the capacity requirements of the system you are upgrading.

• Socket and module metals ” Whereas the latest RAMBUS modules and DIMM modules and sockets are all gold plated, earlier SIMM modules and sockets could be either tin plated or gold plated. Mixing metals can cause long- term corrosion, leading to memory failures.

All of these factors must be specified correctly to get the correct memory for any given system.