System DRAM Form
Factors
DRAM
Form Factors
When
you buy RAM for 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
Check this site for more information about DDR Memory RAM
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.
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