Both RAM and ROM memories are organized in the same way, as an ordered collection of words. A word is the smallest unit of memory that is read or written, and is usually a multiple of 2, such as 8 bits, 16 bits, 32 bits, or 64 bits. In the past, 36-bit memories and 60-bit memories were used on a few famous computers, such as the DEC PDP and the CDC Cyber. But as the 8-bit unit gained popularity, it came to be called a byte and a memory whose smallest addressable unit is the byte is called byte-addressable. Most computers today are byte-addressable, whereas the older computers were word-addressable. We shall use the term word in the following discussion in order to be totally generic about memory schemes. |