Description
This item is from the PC Hardware
FAQ, by Willie Lim and Ralph Valentino with numerous
contributions by others. (v1.25).
2.19 What are "single-sided" and "double-sided" 72-pin SIMMs?
[From: rbean@execpc.com (Ron Bean)]
All 72-pin SIMMs are 32 bits wide (36 with parity), but
double-sided SIMMs have four RAS (Row Address Strobe) lines instead of
two. This can be thought of as two single-sided SIMMs wired in
parallel. But since there is only one set of data lines, you can only
access one "side" at a time.
Usually, 1Mb, 4Mb, and 16Mb 72-pin SIMMs are single-sided, and 2Mb,
8Mb, and 32Mb SIMMs are double-sided. This only refers to how the
chips are wired-- SIMMs that are electrically "single-sided" may have
chips on both sides of the board.
Most 486 motherboards use memory in banks of 32 bits (plus parity),
and may treat a double-sided SIMM as "two banks" (see your
motherboard's manual for details). Some can take four SIMMs if they're
single-sided, but only two if they're double-sided. Others can take
four of either type.
Pentium (and some 486) motherboards use pairs of 72-pin SIMMs for
64-bit memory. Since double-sided SIMMs can only access 32 bits at a
time, you still need to use them in pairs to make 64 bits.
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