Memory devices in computers are used for storing different forms of information such as data, programs, addresses, textual files and status information on processor and other computer devices. Information stored in memory devices can be divided into bits, bytes, words, blocks, segments, pages and other larger data structures, which have their own identifiers.
In main memory, information is stored in memory cells or memory locations. Memory locations contain information to which an access can take place.
To read or write information in a memory location, a single memory access operation has to be
executed, which requires independent control signals to be supplied to the memory.
executed, which requires independent control signals to be supplied to the memory.
We can distinguish different parameters which determine properties of different kinds of memories. The most important parameters will be discussed below:
- Memory capacity or memory volume is the number of locations that exist in a given memory. Memory capacity is measured in bits, byte or words. When words are used, the length of a word in bits or bytes has to be given.
- Memory access time is the time that separates sending a memory access request and the reception of the requested information. The access time determines unitary speed of a memory (the reception time of unitary data). The access time is small for fast memories
- Memory cycle time is the shortest time that has to elapse between consecutive access requests to the same memory location. The memory cycle time is another parameter that characterizes the overall speed of the memory. The speed is big when the cycle time is small
- Memory transfer rate is the speed of reading or writing data in the given memory, measured in bits/sec or bytes/sec.
It is easy to see that the following rules, concerning memory parameters, hold:
- Growing memory volume makes the memory cost per bit of stored information decreased,
- Lower access time makes the memory cost per bit increased,
- Growing memory volume frequently corresponds to larger access time.
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