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Memory mapped versus isolated I/O
Short answer
In memory-mapped I/O, peripheral registers share memory addresses and use ordinary load/store instructions, reducing available memory. In isolated I/O, a separate I/O space uses special IN/OUT instructions, preserving full memory but needing extra instructions and simpler decoding for a limited port range.
What are the differences between memory-mapped I/O and isolated (standard) I/O?
In both schemes the processor talks to peripherals through registers; they differ in how those registers are addressed.
| Point | Memory-mapped I/O | Standard (isolated) I/O |
|---|---|---|
| Address space | Peripheral registers share the same address space as memory | Separate I/O address space from memory |
| Instructions | Ordinary memory instructions (MOV, LOAD, STORE) | Special instructions (IN, OUT) |
| Control signals | Same MEMR/MEMW | Separate IOR/IOW (or an M/IO' signal) |
| Memory available | Reduced, since some addresses are used for I/O | Full memory space available |
| Address decoding | Uses the full memory address width, so peripherals need wider decoders | Simpler; the small I/O space needs fewer address bits to decode |
| Instruction set | Simple; no extra instructions | Needs extra I/O instructions |
| Flexibility | Arithmetic and logic can be applied directly to I/O | Data must pass through the accumulator/register |
| Number of I/O addresses | Many | Limited (e.g. 256 ports in the 8085 with an 8-bit port address) |
| Example | ARM, 68000, MIPS | 8085, 8086 (IN/OUT) |
✓ Answer checked by Claude Opus 5.5 (high effort)
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