64-Megabit-DRAM

A 64-megabit DRAM contains 67,108,864 storage bits, equivalent to 8 MiB. The capacity is four times that of a 16-megabit device. It is a density description, not a guarantee of speed, voltage, or module compatibility. A historical memory specification should state both its bit capacity and its external organization.

A 4M × 16 device illustrates how density and width combine. Multiplying 4,194,304 words by 16 bits gives 67,108,864 bits. ISSI’s IS42S16400 datasheet provides a concrete 64Mb SDRAM example. The sixteen data pins define the transfer width, while the larger address space and internal organization account for the total stored capacity.

Banking helps organize accesses without changing the total bit count. A controller may exploit independent bank states to schedule useful work while another bank is busy. It still must obey each device’s timing requirements. Banking is therefore a scheduling opportunity, not a promise that every arbitrary access can proceed without delay.

The fourfold capacity increase raises integration demands beyond cell shrink. Larger arrays require suitable decoding, sensing, refresh organization, redundancy, and test coverage. The peripheral circuits do not disappear when the memory cell becomes smaller. Manufacturing success depends on delivering enough working bits with the specified retention and access behavior.

Chip density and installed memory should be reported separately. Eight 64-megabit chips provide 64 MiB of raw capacity, but module wiring, ranks, error-correction overhead, and controller support determine the usable configuration. A label such as 64M on a device or board must therefore be read in its documented units and context.

Sources and further reading. [ISSI IS42S16400 SDRAM datasheet](https://www.issi.com/WW/pdf/42S16400.pdf).

Read 64-megabit DRAM as an eight-mebibyte-per-chip density whose practical behavior depends on organization, banking, and interface timing.