- 2071 Magh · 4 marks
Short note: I/O mapped and memory mapped I/O
Short answer
I/O mapped I/O uses a separate address space with IN/OUT instructions and IOR/IOW signals, typically 256 ports. Memory mapped I/O treats devices as memory locations, using all memory instructions and MEMR/MEMW signals, but reduces memory space and needs more decoding.
Write a short note on I/O mapped and memory mapped I/O.
There are two ways to assign addresses to I/O devices.
I/O mapped I/O (isolated I/O): I/O devices have a separate address space. The processor uses special instructions IN and OUT and the control signal IO/M = 1 (8085) or M/IO = 0 (8086). 8085/8086 give 8-bit port addresses (256 ports; 8086 can also use DX for 64K ports). Data transfer is only through the accumulator. Example: OUT 80H.
Memory mapped I/O: an I/O device is treated as a memory location; the device gets an address in the normal memory map, and all memory instructions (MOV, LDA, STA, ADD, etc.) can be used. The control signals are MEMR/MEMW, and the 16-bit address is fully decoded. Example: STA 8000H writes to a port at 8000H.
| Point | I/O mapped | Memory mapped |
|---|---|---|
| Address space | Separate (256 ports in 8085) | Shares memory space |
| Address lines | 8 (A0–A7) | 16 (A0–A15) |
| Instructions | IN, OUT only | All memory instructions |
| Control signals | IOR, IOW | MEMR, MEMW |
| Data transfer | Only through accumulator | Any register |
| Decoding hardware | Simpler (fewer lines) | More complex |
| Memory available | Full 64 KB for memory | Reduced by the I/O addresses |
| Speed | Faster in general | Slower (longer instructions) |
| Arithmetic on I/O data | Not possible directly | Possible |
✓ Answer checked by Claude Opus 5.5 (high effort)
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