You have a microprocessor chip with a 16?bit data bus. The computer accesses words; that is, it performs only 16?bit reads and writes. You are going to build a minimal computer which requires the processor and two 8? bit memory components. Unfortunately, you have only one 8?bit memory component which means that you can’t build a system. Or can you? Explain how you could construct a system with a 16?bit processor and an 8? bit memory.

Question

What will be an ideal response?

Answer 1

Of course, you can’t directly interface an 8?bit memory to a processor with a 16?bit data bus. The 8?bit memory has to be converted into a 16?bit memory. Control and timing logic is necessary to convert a 16?bit operation into two 8?bit operations. Consider a read cycle. The processor supplies an address. This can be
connected to the memory’s address inputs A01, A02, … The least significant bit of the memory’s address, A00, is used to access an 8?bit word at the address with A00 = 0. These 8 bits are latched into the lower?half of a 16? bit register. Then, A00 is set to 1 and a second read performed. The 8?bits read are loaded into the upper 8
bits of the register. Finally, the 16 bits in the register are supplied to the CPU along with a suitable data acknowledge strobe. Clearly, the logic has to take the read cycle trigger from the CPU and create the timing of two read cycles.

Similarly, in a write cycle the latch would capture a 16?bit word from the CPU and then provide two consecutive 8?bit writes to the memory.