- 2069 Bhadra · 8 marks
Arbitration and memory-mapped vs standard I/O
Short answer
Arbitration methods: priority arbitration uses separate Req/Ack lines, granting highest priority; daisy-chain shares Req and passes Grant serially, priority by position; network-oriented (e.g., I2C) uses wired-AND. Memory-mapped I/O places peripheral registers in memory space, accessed with normal instructions; standard I/O uses a separate I/O space and special IN/OUT instructions.
Explain arbitration systems that are implemented to communicate with peripheral devices from the microprocessor. Differentiate between memory mapped I/O and standard I/O.
Arbitration systems
When more than one peripheral wants to be bus master (or to use the bus through the microprocessor) at the same time, an arbiter decides which one gets the bus.
1. Priority arbitration: the arbiter has a Req/Ack pair for each peripheral and grants the bus to the highest-priority requester (fixed priority, or rotating round-robin priority).
Req1,2,3 --> [Arbiter] --> Ack1,2,3
2. Daisy-chain arbitration: the devices share a common Req line and the Grant signal is passed from device to device along a chain; the first requesting device in the chain keeps the grant, so priority depends on position.
Arbiter --Grant--> Dev1 --> Dev2 --> Dev3
3. Network-oriented arbitration: used when masters share a network bus (e.g. CAN or Ethernet). Each device checks that the bus is free before sending; collisions are resolved by priority or by retrying.
Memory-mapped I/O versus standard I/O
| Point | Memory-mapped I/O | Standard (isolated) I/O |
|---|---|---|
| Address space | Shared with memory | Separate I/O space |
| Instructions | Normal data-transfer instructions | Special IN/OUT instructions |
| Control signals | MEMR/MEMW | IOR/IOW |
| Memory available | Reduced | Full |
| Decoding | Must decode the full memory address width | Simpler; fewer address bits for the small I/O space |
| Number of ports | Large | Limited (e.g. 256) |
| Examples | ARM, MIPS | 8085, 8086 |
✓ Answer checked by Claude Opus 5.5 (high effort)
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