When you need both a wider word and more words, you build in two steps. First, put enough chips side by side to make one full-width word: that group is a bank. Then stack several banks, each covering its own block of addresses.
Inside a bank:
- all chips share the same address lines
- all chips share one chip select
- each chip supplies its own slice of the data bits
Across banks:
- a decoder on the high address bits produces one select per bank
- each decoder output drives the chip selects of every chip in that bank
So:
- chips per bank = wanted word size ÷ chip word size (width expansion)
- banks = wanted words ÷ chip words (depth expansion)
- decoder outputs needed = number of banks, not number of chips
In a 64K × 16 system made of 16K × 4 chips there are 4 banks of 4 chips. A15 A14 choose the bank, and the chosen bank's four chips each return 4 bits of the 16-bit word.