Digital Electronics: Principles, Devices and Applications

634 Digital Electronics15.7.2 Memory Location ExpansionFigure 15.23 shows how more than one memory chip can be used to expand the number of memorylocations. Let us consider the use of two 16 × 8 chips to get a 32 × 8 chip. A 32 × 8 chip would needfive address input lines. Four of the five address inputs, other than the MSB address bit, are commonto both 16 × 8 chips. The MSB bit feeds the input of one chip directly and the input of the other chipafter inversion. The inputs to the two chips are common.Now, for first half of the memory locations corresponding to address inputs 00000 to 01111 (atotal of 16 locations), the MSB bit of the address is ‘0’, with the result that RAM-1 is selectedAB4AddressInputAB3AB2AB1AB0R/WA 3 A 2 A 1 A 0A 3 A 2 A 1 A 0CSR/WRAM-116 × 8CSR/WRAM-216 × 8I/O 7 I/O 0I/O 7 I/O 0DB7DB6DB5DataBusDB4DB3DB2DB1DB0Figure 15.23Memory location expansion.