- 2072 Chaitra · 4 marks
Unique vs non-unique address decoding
Short answer
Unique decoding uses all higher address lines to generate CS', giving each location a single address. Non-unique decoding uses only some lines, causing foldback images. Unique decoding avoids bus conflicts and wastes no address space, but requires more hardware.
Differentiate unique vs non unique address decoding.
In unique (full / absolute) decoding all the higher address lines (those not used inside the chip) take part in generating the chip select, so every memory location or port has exactly one address. In non-unique (partial / linear) decoding only some of the higher lines are decoded, so the same location responds to several addresses (foldback or image addresses).
| Point | Unique (full) decoding | Non-unique (partial) decoding |
|---|---|---|
| Address lines decoded | All remaining lines | Only some lines |
| Address of a location | One unique address | Several addresses (images) |
| Hardware | More gates / larger decoder | Less hardware, lower cost |
| Address space | Fully usable, no waste | Wastes address space; limits expansion |
| Future expansion | Easy | Difficult (conflicts) |
| Bus conflict risk | None | Possible if overlapping devices |
| Example (8085, 2K RAM) | A15-A11 = 00000 all decoded: 0000H-07FFH only | Only A15 decoded: 0000H-07FFH, and images at 0800H-0FFFH, ... up to 7FFFH |
| Use | Large systems, memory | Small systems with few devices |
✓ Answer checked by Claude Sonnet 5.5
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