A memory is organized as a number of addressable words, each with a chosen width. Address bits select a word; data bits carry its contents. Capacity equals the number of words multiplied by the number of bits per word. Address space and physical memory capacity are related but distinct.
Worked example
A memory organized as 1024 × 8 contains 8192 bits, or 1024 bytes. It needs ten address bits to select one of 1024 words, because 2^10 = 1024. Its eight-bit data interface carries one selected word at a time.
What to watch for
A 16-bit-wide word is two bytes, not one. State whether a quoted capacity uses bits, bytes or words, and whether prefixes are decimal or binary.
Try it yourself
What capacity does a 256 × 16 memory contain?
Answer: 4096 bits, which equals 512 bytes. Eight address bits select its 256 words.
How to study this topic
Separate the ideal logical model from the physical device interface. Write units beside capacities, voltages and timing values. The architecture explains the idea; the chosen part's documentation supplies the operating limits.
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Digital Electronics