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ROM, Read Only Memory, , —

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Introduction, , » Read-only memory (ROM) is a class of storage, medium used in computers and other electronic, devices. Data stored in ROM cannot be modified, or, can be modified only slowly or with difficulty., , » Read Only Memories (ROM) or Programmable, Read Only Memories (PROM) have:, , N input lines,, M output lines, and, 2% decoded minterms., , » Fixed AND array with 2" outputs implementing all, , N-literal minterms.

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» Programmable OR Array with M outputs, lines to form up to M sum of minterm, expressions., , » A program fora ROM or PROM is simply a, multiple-output truth table, , » Ifa 1 entry, a connection is made to the, corresponding minterm for the corresponding, output., , > Ifa 0, no connection is made, , Can be viewed as a memory with the inputs as, , addresses of data (output values), hence, ROM or PROM names!

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Read only memory (ROM), , Y ROM holds programs and data permanently even when computer, is switched off, , Y Data can be read by the CPU in any order so ROM is also direct, access, , Y The contents of ROM are fixed at the time of manufacture, , Y Stores a program called the bootstrap loader that helps start up, the computer, , Y Access time of between 10 and 50 nanoseconds

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ROMS VS. RAMS, , There are some important differences between ROM and RAM., ROMs are “non-volatile’—data is preserved even without, power. On the other hand, RAM contents disappear once, power is lost., , ROMs require special (and slower) techniques for writing, so, they’re considered to be “read-only” devices., , Some newer types of ROMs do allow for easier writing, although, , the speeds still don’t compare with regular RAMs., , MP3 players, digital cameras and other toys use, CompactFlash, Secure Digital, or Memory Stick cards for, non-volatile storage., , Many devices allow you to upgrade programs stored in “flash, ROM.”