--- title: "Bus hierarchy and bus arbitration" subject: "Computer Organization and Architecture" chapter: 1 key: "semester_5/computer_organization_and_architecture:bus-hierarchy-arbitration" source: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/bus-hierarchy-arbitration site: BCT Engineering --- # Bus hierarchy and bus arbitration Computer Organization and Architecture · Chapter 1: Introduction - Asked in: 2069 Asar, 6 marks - Video: [COMPUTER ORGANIZATION | Part-26 | Bus Arbitration](https://www.youtube.com/watch?v=d0lw6FB_QAU) — Engineering Drive - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/computer_organization_and_architecture/bus-hierarchy-arbitration - Whole chapter: https://www.bctengineer.com/md/semester_5/computer_organization_and_architecture/chapter-1 ## Question Explain bus hierarchy and why it is required. Describe the method of bus arbitration. ## Short answer A bus hierarchy uses local, system, and expansion buses with bridges to avoid a single-bus bottleneck, letting fast and slow devices operate at their own speeds. Bus arbitration decides which master gets the bus; centralized methods include daisy chaining, polling, and independent requests, while distributed arbitration lets each module compete by priority number. ## Answer #### Bus hierarchy A bus hierarchy uses several buses at different speeds, joined by bridges, instead of one shared bus. ```text +-----+ Local bus +-------+ | CPU |-------------| Cache | +-----+ +---+---+ | ===+====================+=======+==== System bus | | +--+-----+ +-----------+--------+ | Main | | Expansion bus | | memory | | interface (bridge) | +--------+ +-----------+--------+ | ====+==========+===========+====+==== Expansion bus | | | +--+--+ +---+--+ +----+---+ | LAN | | SCSI | | Serial | +-----+ +------+ +--------+ ``` - **Local bus:** connects the CPU, the cache and sometimes memory. It is the fastest. - **System bus:** connects the cache to main memory and to the expansion bus interface. - **Expansion bus:** connects I/O devices such as the network, SCSI and serial ports. **Why it is needed:** a single bus becomes a bottleneck with many devices; fast CPU and memory traffic is kept separate from slow I/O; devices of different speeds can work at their own speeds; each bus is shorter, so delay is less; and the CPU can work from the cache while I/O goes on. #### Bus arbitration Several modules (masters) may want the bus at once, but only one can control it. Arbitration is the method of choosing which one gets it. - **Centralised arbitration:** one arbiter (bus controller) decides. Methods: daisy chaining, polling and independent requests. - *Daisy chaining:* a Bus Grant signal passes through the devices in order; the first requesting device takes it. Priority depends on position in the chain. - *Polling:* the controller polls the devices in turn using a counter; the requesting device whose number matches gets the bus. - *Independent requests:* each device has its own request and grant lines; the arbiter picks one by priority. - **Distributed arbitration:** there is no central controller. Each module has its own arbitration logic; each requesting module puts its priority number on shared lines, and the highest one wins. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a reason for a bus hierarchy? A. Fast CPU and memory traffic is isolated from slow I/O B. It lets all devices share one clock C. It removes the need for memory D. It lengthens the bus Answer: A. Isolation prevents slow devices from holding up fast ones. (Key point: Why hierarchy) 2. In daisy chain arbitration, who gets priority? A. The device farthest from the arbiter B. The device with the largest data C. The device nearest the arbiter in the chain D. A randomly chosen device Answer: C. The Bus Grant reaches the nearest requester first. (Key point: Daisy chaining) 3. How does polling arbitration work? A. A counter sends device numbers and the matching requesting device gets the bus B. All devices compare priority numbers C. Each device has own grant line D. The first device to send data wins Answer: A. The controller polls devices in turn. (Key point: Polling) 4. How does distributed arbitration choose a winner? A. A central arbiter chooses by position B. The lowest number wins by a counter C. Requesting modules place their priority numbers on shared lines and the highest wins D. The slowest device wins Answer: C. There is no central controller. 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