ISRO CSE 2018 | Question: 65

Question

Of the following, which best characterizes computers that use memory-mapped $\text{I/O}?$

1. The computer provides special instructions for manipulating $\text{I/O}$ ports
2. $\text{I/O}$ ports are placed at addresses on the bus and are accessed just like other memory locations
3. To perform $\text{I/O}$ operations. it is sufficient to place the data in an address register and call channel to perform the operation
4. $\text{I/O}$ can be performed only when memory management hardware is turned on

Answer 1

In memory some of the address of the Memory are assigned to IO port. so whenever those address are placed we refer to the IO devices.

option B

Comments

1. can u explain why C is wrong

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https://www.geeksforgeeks.org/memory-mapped-i-o-and-isolated-i-o/

Answer 2

→ Memory-mapped I/O uses the same address space to address both memory and I/O devices.
→ The memory and registers of the I/O devices are mapped to (associated with) address values.
→ So when an address is accessed by the CPU, it may refer to a portion of physical RAM, or it can instead refer to the memory of the I/O device. Thus, the CPU instructions used to access the memory can also be used for accessing devices.
→ Each I/O device monitors the CPU's address bus and responds to any CPU access of an address assigned to that device, connecting the data bus to the desired device's hardware register.
→ To accommodate the I/O devices, areas of the addresses used by the CPU must be reserved for I/O and must not be available for normal physical memory.
→ The reservation may be permanent, or temporary (as achieved via bank switching).

Correct option - (b)

Answer 3

Correct Answer: B I/O ports are placed at addresses on the bus and are accessed just like other memory locations.

A: That’s I/O-mapped I/O (isolated I/O) — not memory-mapped. → Uses special instructions like IN and OUT.

C is false for memory-mapped I/O because: Memory-mapped I/O does not involve a channel or calling a channel; it is directly handled by CPU read/write instructions.

D: Irrelevant — I/O does not depend on turning memory management hardware on.

Answer 4

Option B  is correct  because i/o and main memory both use the same address space. so once an address is generated based on the location either the memory or i/o interface gets activated but not the both . 

My doubt was why option C is incorrect? 

I mean i have an explanation that as cpu is the one who is controlling the address space also ,so whatever the operations have to be done , just keep the address on address register and call channel to perform the operation . But i think "calling channel"  is not needed i think so and that's why option c is wrong. Correct if im wrong.