DRAM Form Factors

When you buy RAM for your PC, you need to be aware of the size and shape it comes in. Just like a 5.25" floppy won't fit in a 3.5" drive, RAM shapes also differs from type to type. This shape is referred to as the "form factor".

However, it isn't as hard as finding light bulbs for your stove. Each form factor is related to a certain age of computing. 30 pin SIMMs are relics from the 286 / 386 era, 72 pin SIMMs from the 486 / Pentium era, and DIMMs / RIMMs from the Pentium 2 era. There is also some overlap many 486 boards have both 72 and 30 pin sockets, while many Pentium boards have both DIMM and SIMM sockets.

There are currently 4 form factors of RAM for the PC, although the first type stopped use after the Pentium was brought to market, and the last type has yet to be released.

SIMM: Single Inline Memory Module (30 pin)

The mainstay SIMM for the 286/386 field was the 30 pin SIMM. It only has a 8-bit pathway, but managed to be about half as wide as the 72-pin SIMM. The wide data paths needed for subsequent CPUs demanded the upgrade to higher density form factors.

SIMM: Single Inline Memory Module (72 pin)

By far, the most commonly used form factor is the 72 pin SIMM. It is the defacto standard for all Pentium Class motherboards and many 486 or Pentium Pro boards. The PCB is notched almost directly in the middle to separate the 72 pins into two groups. It is used for flavors of DRAM and EDO DRAM. SIMMs have 32-bit wide paths.

DIMM: Dual Inline Memory Module

The DIMM is quickly becoming the most commonly used form factor for DRAM today, since it is required to accommodate a 64-bit memory path. They have a split section pin array with a third notch on the left side to accommodate the 168 pins. DIMMs are structured to accept all flavors of SDRAM or occasionally, EDO DRAM.

This Corsair 256 MB module has a few more chips than most, but still shows off the form factor nicely.

RIMM: Rambus Inline Memory Module

The RIMM is a new socket form factor built to sustain the upcoming Direct RDRAM chips. It is physically the same form factor as a DIMM, but has a radically different pin configuration.

The new form factor is required to support a Direct Rambus Channel at 400 MHz with DDR doubling the potential data clock speed. Only 3 RIMM slots are available per Rambus Channel (The pathway through which the RAM communicates with the computer).

The pre-production model shown by Rambus has approximately 83 pins - 40 on the left, 43 on the right, with a mysteriously empty middle section.

[ Back ] [ Next ]

If you have any questions or comments about our articles post them to our Message Boards