Width expansion makes the word longer. When each chip stores too few bits per word, put several chips side by side:
- Every chip gets the same address lines.
- Every chip gets the same chip select and R/W, so they always act together.
- Each chip handles its own slice of the data bits.
On every access, all the chips look at the same address at the same moment, and each reads or writes its slice. To the outside, they behave as one memory with a wider word size.
Chips needed = wanted word size ÷ chip word size.
What doesn't change: the number of words, so the address bits and the address range stay exactly as for one chip. No decoder is needed.
With 4-bit chips building a 16-bit word, chip i holds data bits 4i to 4i + 3. A group of chips wired like this is called a bank.
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2147483648
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1073741824
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536870912
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134217728
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2097152
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262144
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131072
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65536
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16384
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