Q2Microprocessor
Question
Q.2. What is the drawback of Memory Mapped I/O?
Answer
The main drawback of memory-mapped I/O is that it consumes part of the available memory address space for I/O devices, reducing memory available for program/data, and typically requires longer/slower memory-reference instructions instead of dedicated, shorter I/O instructions.
In memory-mapped I/O, I/O device registers occupy addresses within the same address space as memory, requiring ordinary memory-access instructions to communicate with them. The primary drawback is that this reduces the total memory address space available for actual program code and data storage, which can be significant in systems with a limited overall address range; additionally, since there is no dedicated, distinct I/O instruction class in this scheme, accessing I/O devices requires the same (often longer, more execution-cycle-intensive) memory-reference instructions used for ordinary memory access, rather than shorter, dedicated I/O instructions such as the 8085's IN/OUT, potentially reducing I/O access efficiency compared to an isolated (I/O-mapped) I/O scheme.