Working of read-only memory, Computer Engineering

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Q. Working of Read-Only Memory?

A ROM is fundamentally a memory or storage device in which a fixed set of binary information is stored. A block diagram of ROM is as displayed in Figure. It comprises n input lines and m output lines. Every bit combination of input variables is known as an address. Every bit combination which comes out of output lines is known as a word. Total number of bits per word is equal to number of output lines m. Number of distinct addresses possible with n input variables is 2n.

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Figure: ROM

A ROM is characterised by number of words (2n ) and number of bits (m) per word. For illustration a 32 × 8 ROM that can be written as 25 × 8 comprises 32 words of 8 bit each that means there are 8 output lines and 32 distinct words stored in unit. There are only 5 input lines since 32 = 25and with 5 binary variables we can specify 32 addresses.  

A ROM is fundamentally a combinational circuit and can be constructed as displayed in Figure below. On applying an Input I1 = 0, I2 = 0 the 00 line of decoder is selected and we will get O1 = 0 and O2 = 1 on applying I1 = 0 and I2 = 1 we will obtain O1 = 1 AND O2 = 0. This same logic can be used for creating larger ROMs.


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