On 11 Dec 98 at 16:14, Eric Greenberg wrote:

> Would someone here be kind enough to explain the different types
> of DIMM's on the market today?  I am aware of 100Mhz vs. 66Mhz
> modules, but am only now starting to hear of EDO, FPM and so on.
> Are these only for older DIMM slots or are they just coming out
> now.
>
> Forgive my ignorance but after years of dealing with SIMM's, they
> went and changed the scheme of things while my back was turned 8-)

30-pin SIMMs
  These came with or without parity, in 256K, 1M and 4M capacities.
There might have been some "double-sided" capacities as well, but I
don't recall any.

72-pin SIMMs
  These came with parity, without parity, or with circuitry on the
SIMM to "fake" parity.  When EDO (Extended Data Out) appeared, older
chips started being called "Fast Page Mode" to distinguish them.
  I recall long discussions from about late 1995 or early 1996, about
whether a system using EDO without L2 cache could be faster than one
using FPM and L2 cache.
  72-pin SIMMs have come in 4M, 16M and 64M capacities; double-sided
in 8M and 32M.

DIMMs
  DIMMs have two notches in their lower edge.  One differentiates
between different voltage ratings -- check your motherboard manual to
see what it prefers.
  The other notch differentiates between Unbuffered EDO (was common),
Buffered EDO (needed for SMP (multiple-CPU) systems, and SDRAM, the
current favourite.
  Because DIMMs are 64 data bits wide, one bit per byte is enough to
do ECC (Error-Correcting Code) instead of settling for parity.  SO
the same two variations are available, only they're called "ECC" and
"non-ECC" instead of parity and non-parity.

  Some SDRAM DIMMs have an EEPROM on them, providing the SPD (Serial
Presence Detect) info that some motherboards require.
  Some SDRAM DIMMs are certified "PC100" for operation on
motherboards with a 100 MHz FSB (Front-Side Bus).


  The most flexible current motherboards accept (FPM and/or EDO
SIMMs) and/or (EDO and/or SDRAM DIMMs) -- possibly all four at once.

David G

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