Computer Bus Design

Bus Design

• Bus design involves bus width, bus clocking scheme, operation, and arbitration method
• Bus width determines the size of the addressable memory
• Increasing bus width increases bus capacity, but also creates physical connection problems
• Multiplexing data and addresses in different phases can reduce the number of lines, but also reduces bus performance

Bus Clocking Scheme

• Synchronous bus: includes a clock in the control lines, fixed protocol for communication, and can run fast
• Synchronous bus disadvantages: all devices on the bus must run at the same clock rate, and buses cannot be long if they are fast
• Asynchronous bus: not clocked, can accommodate a wide range of devices, and can be lengthened without caring about clock skew
• Asynchronous bus requires a handshake protocol

Operation

• Master-slave scheme: avoids chaos, with the bus master controlling access to the bus and handling all bus requests
• Slave responds to read/write requests
• Simplest system: processor is the only bus master, and all bus requests must be controlled by the processor
• Big drawback: processor is involved in everything

Arbitration Method

• Centralized arbitration with a bus arbiter: authorization given centrally
• Daisy chain arbitration method: simple, but cannot assure fairness, and a low-priority device may be locked out forever
• Four possible arbitration classes: distributed arbitration by self-selection, distributed arbitration by collision detection, authorization given in sequence, and centralized arbitration

Examples of Bus Designs

• Peripheral Component Interconnect (PCI Express or PCI-e): created by Intel, Dell, HP, IBM, connects HDD, SSD, Ethernet, graphics, and other cards in personal computers, point-to-point topology, and replaces other standards
• InfiniBand (IB): originated by Compaq, Dell, Hewlett-Packard, IBM, Intel, Microsoft, and Sun, typically used in clusters/racks, very high throughput, and very low latency
• MIL-STD-1553: avionic data bus for use with military avionics, also used in spacecrafts, dual technology, bus controller handling multiple remote terminals connected with redundant links
• ARINC 429: predominant avionics data bus, used on most commercial aircraft, pair of wires accommodates one transmitter and up to 20 receivers
• Avionics Full-Duplex Switched Ethernet (AFDX): implementation of deterministic Ethernet, defined by ARINC, addresses real-time issues