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Computer Architectures - Digital Circuits - Read-only memories

Since the contents cannot be altered, we don't have a r/w signal. Except for the enable signal, a ROM is thus like an ordinary combinatorial circuit with m inputs and n outputs.

If a ROM is like an ordinary sequential circuit, then why do we treat it separately? The answer is that ROMs are usually programmable. They are often sold with a contents of all 0s or all 1s. The user can then stick it in a special machine and fill it with the desired contents, i.e. the ROM can be programmed. In that case, we sometimes call it a PROM (programmable ROM).

Some varieties of PROMS can be erased and re-programmed. The way they are erased is typically with ultra-violet light. When the PROM can be erased, we sometimes call it EPROM (erasable PROM).

 


 

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