In a microcomputer, the microprocessor unit (MPU) is always communicating with one of the following: 

(1) Random-access memory (RAM), which stores programs and data that can be readily changed; 

(2) Read only memory (ROM), which stores programs and data that never change; 

(3) External input/output (I/O) devices such as keyboards, video displays, printers, and disk drives. 

As it is executing a program, the MPU will generate an address code that selects which type of device (RAM, ROM, or I/O) it wants to communicate with. Figure 4-82 shows a typical arrangement where the MPU outputs an eight-bit address code A₁₅ through A₈. Actually, the MPU outputs a 16-bit address code, but the low-order bits A₇ through A₀ are not used in the device selection process. The address code is applied to a logic circuit that uses it to generate the device select signals:

Analyze this circuit and determine the following.

(a) The range of addresses A₁₅ through A₈ that will activate R̅A̅M̅

(b) The range of addresses that activate I̅/O̅

(c) The range of addresses that activate R̅O̅M̅

Express the addresses in binary and hexadecimal. For example, the answer to (a) is A₁₅ to A₈ = 00000000₂ to 11101111₂ = 00₁₆ to EF₁₆.

Figure 4-82

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RAM, ROM, and I/O.