computer hardware, consumer electronics, electronic components  

Read only memory (ROM)

Read only memories (ROMs) are used in computer systems to provide a permanent storage of program instructions. A read only memory (ROM) structure comprises a matrix of intersecting bit lines and word lines with memory cells at select intersections. A read only memory (ROM) consists of an array of semiconductor devices (diodes, bipolar or field-effect transistors), which interconnect to store an array of binary data. A ROM basically consists of a memory array of programmed data and a decoder to select the data located at a desired address in the memory array. A ROM array of memory cells is defined by a number of transistors generally arranged in a grid pattern having a plurality of rows and columns. Each individual transistor of each memory cell of the ROM array is placed between a column of the series of columns and a voltage bus. A resistive ROM typically includes a planar array of parallel word lines, which is perpendicular to and insulated from a planar array of parallel bit lines. A designated number of the memory cells in the ROM have a resistive, element connecting a node of one word line with a node of one bit line. Each memory cell, consisting of a single transistor per bit of storage, is hardware preprogrammed during the integrated circuit (IC) fabrication process and is capable of maintaining the stored data indefinitely. ROM memory is used to hold and make available data or code that will not be altered after IC manufacture. Data or code is programmed into ROM memory during fabrication. The values stored within the ROM are "read" (i.e., output) by measuring a sense current flowing through each bit line from the memory cells of successive word lines. Three basic types of ROMs are mask-programmable ROM, erasable programmable ROM (EPROM) and field-programmable ROM (PROM).

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