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2.2 Memories

Memories are characterized by their function, capacity, and response times. Operations on memories are called reads and writes, defined from the perspective of a processor or other device that uses a memory: a read transfers information from the memory to the other device, and a write transfers information into the memory. A memory that performs both reads and writes is often just called a RAM, for random access memory. The term ``random access'' means that if location M[x] is accessed at time t, there are no restrictions on the address of the item accessed at time t + 1. Other types of memories commonly used in systems are read-only memory, or ROM, and programmable read-only memory, or PROM (information in a ROM is set when the chips are designed; information in a PROM can be written later, one time only, usually just before the chips are inserted into the system). For example, the Apple Macintosh, shown in Figure 1, had a PROM called the ``toolbox'' that contained code for commonly used operating system functions.