ROM-Read Only Memory (ROM) is permanent memory used to store Basic Input Output System code shipped with a PC or PC components. Unlike Random Access Memory, ROM retains its contents even when the machine is not powered. For economic reasons, ROM is generally implemented as a (Ultraviolet) Erasable Programmable ROM (EPROM) or as Electronically Erasable, (Rewritable) Non-Volatile RAM (NVRAM). ROMs used in PCs are generally slow -- 100ns or longer -- and only 8 or 16 bits wide requiring multiple memory reads to obtain instructions on 32 bit wide CPUs. In the case of ROM on disk or video controller cards, it must be accessed through relatively slow I/O buses. In order to improve ROM access speeds, ROM is generally "shadowed" into DRAM on 386 or above machines. ROMs on PCs are always located between addresses C0000 and FFFFF. The only fixed location is FFFFF0 which is the address at which the PC will start running upon reset. Data stored in EPROM is only moderately permanent. Many EPROMs guarantee storage for only 10 years. EPROM failures due to "forgetting" of contents in older machines are theoretically possible, but are not being widely reported.
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Copyright 1994-2002 by Donald Kenney.