Page "SIMM" Paragraph 9

Due to the differing data bus widths of the memory modules and some processors, sometimes several modules must be installed in identical pairs or in identical groups of four to fill a memory bank.

The general rule of thumb is a 286, 386SX, 68000 or low-end 68020 / 68030 ( e. g. Atari, Mac LC ) system ( using a 16 bit wide data bus ) would require two 30-pin SIMMs for a memory bank.

On 386DX, 486, and full-spec 68020 through 68060 ( e. g. Mac II ) systems ( 32 bit data bus ), either four 30-pin SIMMs or one 72-pin SIMM are required for one memory bank.

On Pentium systems ( data bus width of 64 bits ), two 72-pin SIMMs are required.

However, some Pentium systems have support for a " half bank mode ", in which the data bus would be shortened to only 32-bits to allow operation of a single SIMM.

Conversely, some 386 and 486 systems use what is known as " memory interleaving ", which requires twice as many SIMMs and effectively doubles the bandwidth.