PC Hardware in a Nutshell, 3rd Edition

Chip Density

Memory modules are constructed from individual memory chips, which vary in their capacity. Chip capacities are specified in megabits (Mb or Mbit) rather than megabytes (MB). Because there are eight bits per byte, a memory module that uses eight chips can store the number of megabytes specified by the megabit size of the chip. For example, a memory module that uses eight 128-Mbit chips is a 128 MB memory module.

Recent memory modules may use 16-, 64-, 128-, 256-, or 512-Mbit chips. A memory module of a specified capacity may use fewer high-capacity chips or more low-capacity chips. For example, a 256 MB DIMM might use eight 256-Mbit chips or sixteen 128-Mbit chips. Although both DIMMs have the same 256 MB capacity, the eight-chip DIMM is a single-sided DIMM and the 16-chip DIMM is a double-sided DIMM. For a given capacity, it’s generally better to install a single-sided DIMM when possible because chipsets support only so many rows of memory, and a double-sided DIMM “uses up” two rows.

However, you must not install memory that uses higher-capacity chips than your chipset supports. For example, the Intel 815 chipset supports 16-, 64-, 128-, and 256-Mbit chips, so an 815-based motherboard could use either the eight-chip or the 16-chip, 256 MB DIMM. The Intel 440BX chipset, however, supports only 16-, 64-, and 128-Mbit chips, so a 440BX-based motherboard could use only the 16-chip 256 MB DIMM. Attempting to install a DIMM that uses higher-capacity chips than the chipset supports ...