PC Hardware —

The 72-Pin SIMM

With the advent of 32- and 64-bit CPUs, the bank began to take up too much space on the motherboard and added to the cost of memory. (The board that houses the chips often costs more than the DRAM chips.) Enter the 72-pin SIMMs, with 72 pins on each card. One of these is four times wider than a 30-pin SIMM, which is 8-bits wide (see Figures 7.5 and 7.6). Therefore, a motherboard requiring four rows of 30-pin SIMMs to fill one bank needs only one 72-pin SIMM. Virtually all Pentium and Pentium Pro systems use 72-pin SIMMs.

Because 72-pin SIMMs are 32 bits wide, the term "x 32" is used to describe them. A 1 MB x 32 SIMM contains 4 MB of RAM because it is 4 bytes wide. (One MB of RAM is 1,048,576 x 32, which equals 4 MB.) Remember, memory is measured in bytes, and chips are measured in bits.

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Figure 7.5 A 72-pin SIMM

There are many varieties of SIMMs on the market. The following table lists some common 72-pin SIMMs.

Configuration Memory
256 KB x 32 1 MB, no parity
256 KB x 36 1 MB, parity
1 MB x 32 4 MB, no parity
1 MB x 36 4 MB, parity
2 MB x 32 8 MB, no parity
2 MB x 36 8 MB, parity
4 MB x 32 16 MB, no parity
4 MB x 36 16 MB, parity
8 MB x 32 32 MB, no parity
8 MB x 36 32 MB, parity
16 MB x 32 64 MB, no parity
16 MB x 36 64 MB, parity