Q4Microprocessor
Question
Q.4. 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 the memory available for actual program and data storage, and typically requires more complex/longer address-decoding instructions since only memory-reference instructions (not dedicated, shorter I/O instructions) can access the devices.
In memory-mapped I/O, I/O device registers are assigned addresses within the same address space used for memory, and are accessed using ordinary memory-read/write instructions. The main drawback of this scheme is that it consumes a portion of the total available address space for I/O devices, correspondingly reducing the amount of address space available for actual program code and data memory, which can be a significant limitation in microprocessors with a relatively small total address space. It also generally requires using full memory-reference instructions (which are often longer and take more execution cycles) to access I/O devices, rather than shorter, dedicated I/O instructions (such as 8085's IN/OUT), since there is no separate, distinct I/O instruction class in a memory-mapped I/O scheme.