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2.2 Memories     continued...

The smallest unit of information is a single bit, which can have one of two values. The capacity of an individual memory chip is often given in terms of bits. For example one might have a memory built from 64Kb (64 kilobit) chips. When discussing the capacity of an entire memory system, however, the preferred unit is a byte, which is commonly accepted to be 8 bits of information. Memory sizes in modern systems range from 4MB (megabytes) in small personal computers up to several billion bytes (gigabytes, or GB) in large high-performance systems. Note the convention that lower case b is the abbreviation for bit and upper case B is the symbol for bytes.

The performance of a memory system is defined by two different measures, the access time and the cycle time. Access time, also known as response time or latency, refers to how quickly the memory can respond to a read or write request. Several factors contribute to the access time of a memory system. The main factor is the physical organization of the memory chips used in the system. This time varies from about 80 ns in the chips used in personal computers to 10 ns or less for chips used in caches and buffers (small, fast memories used for temporary storage, described in more detail below). Other factors are harder to measure. They include the overhead involved in selecting the right chips (a complete memory system will have hundreds of individual chips), the time required to forward a request from the processor over the bus to the memory system, and the time spent waiting for the bus to finish a previous transaction before initiating the processor's request. The bottom line is that the response time for a memory system is usually much longer than the access time of the individual chips.