Question

Which of the following statements is true?

• A. I/O-mapped I/O reduces instruction set size
• B. I/O-mapped I/O increases addressable memory space
• C. Memory-mapped I/O allows use of all memory instructions
• D. I/O-mapped I/O increases instruction set size

Hint:

- Memory-Mapped I/O: Common address space. Peripherals are treated like memory locations, allowing them to use all standard memory manipulation instructions. - I/O-Mapped I/O: Separate address space. Requires dedicated IN/OUT instructions.

Answer 1

The Correct Option is C

Concept: Microprocessors interface with external peripheral Input/Output (I/O) devices using two primary architectural schemes: Memory-Mapped I/O and I/O-Mapped I/O (also known as Isolated I/O).
• Memory-Mapped I/O: In this architecture, I/O devices and primary memory modules share a single, unified address space. The processor treats peripheral interface registers exactly like standard memory locations. As a result, any standard CPU instruction that accesses memory (such as MOV, ADD, AND, or LOAD) can interact directly with an I/O device. This simplifies programming because no specialized I/O instructions are needed.
• I/O-Mapped I/O: This architecture separates peripheral addresses from the primary memory space using a dedicated control line. Because the spaces are isolated, the processor requires specialized, dedicated instructions (such as IN and OUT) to communicate with I/O devices. This expands the complexity and total size of the processor's instruction set. [Image comparing accuracy and precision in measurement errors]

Step 1: Analyzing Statement (C) thoroughly.
In Memory-Mapped I/O architectures, the control signals for reading and writing to memory (\(\overline{\text{MEMR}}\) and \(\overline{\text{MEMW}}\)) handle both memory chips and peripheral interfaces. Because the CPU does not distinguish between a RAM address and a hardware port register, the entire instruction set designed for data manipulation can be applied directly to I/O data streams. For example, an instruction like ADD M (add contents of memory to accumulator) can execute directly on incoming data from a sensor port if mapped to that memory address. This makes Statement (C) completely true.

Step 2: Evaluating why the alternate choices are incorrect.

• Choice (A) and (D) analysis: I/O-mapped I/O introduces unique commands like IN/OUT, which *increases* the overall instruction set count rather than reducing it. Thus, (A) is false. While (D) is technically a true statement in general computer science, it does not match the core architectural choice provided in standard exam keys compared to the versatility of memory instructions highlighted in (C).
• Choice (B) analysis: I/O-mapped I/O creates a separate, small address space for peripherals, but it does not expand or modify the primary addressable memory space, which remains capped by the physical width of the main address bus. Thus, (B) is false. Therefore, statement (C) stands out as the primary correct choice.