3.3.2 Selection Considerations

		Main memory is implemented in one of the DRAM technologies. Beowulf nodes will usually include between 128 MBytes and 512 MBytes of SDRAM memory.

		3.4.5 Package Styles

		The packaging of memory has evolved along with the personal computers in which they were installed and has converged on a few industry-wide standards. Single Inline Memory Modules (SIMM) and Dual Inline Memory Modules (DIMM) are the primary means of packaging DRAMs and most modern motherboards use one or more of these forms. Both are short but wide printed circuit cards with flat thin memory chips mounted on one or both sides. The lower edge of the DIMM or SIMM has a sequence of pins. On a SIMM, the adjacent pins on opposite sides of the board are connected together to form a single joint pin. On a DIMM, these pins are separate. The most common form factors are the 72 pin SIMM and the 168 pin DIMM.

		3.5 BIOS

		Even with effective industry wide standardization, hardware components will differ in detail. In order to avoid the necessity of customizing a different operating system for each new hardware system, a set of low level service routines are provided, incorporated into read only memory (ROM) on the mother board. This basic I/O system (BIOS) software is a logical interface to the hardware, giving a layer of abstraction which facilitates and makes robust higher level support software. Besides the system BIOS that is hardwired to the motherboard, additional BIOS ROMs may be provided with specific hardware peripherals. These include the video BIOS, the drive controller BIOS, the network interface controller BIOS, and the SCSI drive controller BIOS. The BIOS contains a large number of small routines. These are organized in three groups: Startup referred to as POST (for Power-On Self-Test), Setup, and System Services.

		The POST startup BIOS routines manage initialization activities including running diagnostics, setting up the motherboard chip set, organizing scratch pad memory for the BIOS Data Area or BDA, identifying optional equipment and their respective BIOS ROMs, and then bootstrapping the operating system. The CMOS setup routine provides access to the system configuration information which is stored in a small CMOS RAM. The system services routines are called through interrupts directly from hardware on the motherboard, from the processor itself, or from software. They allow access to low-level services provided by the system