Computer Organization - Computer Buses PDF

COMPUTER ORGANIZATION 3. COMPUTER BUSES Instructor: Dr. Abrar Wafa Based on notes from Dr. Fakhry Kellah OVERVIEW CLO4: Describe the organization and architecture of computer memory and buses. Outline: q Computer buses q Bus arbitration q Distributed arbitration GENERIC COMPUTER Stores program...

COMPUTER ORGANIZATION 3. COMPUTER BUSES Instructor: Dr. Abrar Wafa Based on notes from Dr. Fakhry Kellah OVERVIEW CLO4: Describe the organization and architecture of computer memory and buses. Outline: q Computer buses q Bus arbitration q Distributed arbitration GENERIC COMPUTER Stores programs & data (input, output, intermediate). Memory Supervises information flow CPU Inputs: keyboard, mouse, microphone, scanner… Control unit Datapath Input/Output Performs arithmetic & logical operations Outputs: LCD screen, speakers, printer… 3 COMPUTER BUSES Q. What is a bus? A bus is a communication pathway that connects two or more devices. What are the three bus lines groups? A Bus consists of three parts: Ø Data lines: Carries data Ø Address Lines: Carries Address Ø Control Lines: Carries commands from CPU to Memory and I/Os 4 BUS ARBITRATION Bus is a shared medium. Q. What happens if two or more devices want to use the bus at the same time? Use bus arbitration! Ø Bus arbitration: is a scheme the devices use to avoid stepping over each other. Ø Bus arbiter: is a master device used to decide which device will be allowed to use the bus. 5 BUS ARBITRATION Bus Arbitration Types: 1- Centralized Arbitration: there is a central control device device (arbiter) a. A single shared bus request Line (daisy chained). b. Multiple shared bus request lines but with different priority levels c. Independent bus request lines 2- Distributed Arbitration: No central device 6 A- A SINGLE SHARED BUS REQUEST LINE ØUses Single bus request line and a single arbiter. ØWhen the central arbiter detects that the bus request line is one it will issue a grant by setting the bus grant line to 1. ØThe bus grant line is connected to all devices in series (daisy chained). A centralized one-level bus arbiter using daisy chaining A- A SINGLE SHARED BUS REQUEST LINE Ø When the device physically closest to the arbiter sees the grant signal, and wants to use the bus, it will take it and will not propagate the grant signal to the next device if it made a bus request. Ø Otherwise, it will pass the grant signal to its neighbor, which will repeat the same process and so on until some device accepts the grant and takes the bus. & A centralized one-level bus arbiter using daisy chaining 8 A- A SINGLE SHARED BUS REQUEST LINE Advantage Disadvantages Easy to implement Priorities are fixed (device 1>2>3>4). Inexpensive because it needs only two control lines independent of the number of I/O devices connected to the bus. Devices nearer to the arbiter have higher priorities than those which are far away. (Bus starvation may happen). Arbitration time increases as number of devices increases. Not fault-tolerant (if the arbiter failed, the whole system will fail). 9 B- MULTIPLE SHARED BUS REQUEST Ø Multiple shared bus request but with different priority levels (Daisy Chaining) Ø For each priority level there is a bus request line and a bus grant line. Ø Each device is attached to one of the bus request levels, with devices that need higher priorities (time-critical) attached to the higher priority request levels. A centralized two-level bus arbiter using daisy chaining B- MULTIPLE SHARED BUS REQUEST WITH DIFFERENT PRIORITY LEVELS Ø If more than one request is made at the same time, the arbiter issues a grant signal to the highest priority request line only. Ø If more than one request is made on the same priority level, then daisy chaining is used. A centralized two-level bus arbiter using daisy chaining 11 C- INDEPENDENT BUS REQUEST LINES Ø This configuration is used in Peripheral Components Interconnect (PCI) bus. Ø Each device has its own bus request and bus grant lines. Ø Arbiter can apply several priority policies for the devices. A centralized bus arbitration with independent bus request and grant lines 12 C- INDEPENDENT BUS REQUEST LINES Advantages: Faster Arbitration time than the shared bus request Better fault-tolerance than the shared configuration Disadvantages: Complex and expensive to implement Number of control lines is proportional to the number of hosts Arbiter can apply several priority policies for the devices. 13 2- DISTRIBUTED ARBITRATION In this configuration there is no bus arbiter. All devices waiting to use the bus have equal responsibility in carrying out the arbitration process without using a central arbiter. Each device has its own bus request line and is also connected to all bus request lines of other devices in order to read them. Bus request lines are assigned priorities usually from 0 to 15. 14 2- DISTRIBUTED ARBITRATION When a device wants to use the bus, it sets its bus request line to 1. All devices can read the request lines, so when the bus becomes free, each device knows whether it was the highest priority requester, if yes, it can use the bus otherwise it has to wait until all the higher priority requester finish using the bus. This type of arbitration is used in SCSI bus. 15 2- DISTRIBUTED ARBITRATION Advantages Less cost because no arbiter exists Offers higher reliability than centralized arbitration because operation of the bus is not dependent on any single device. (Fault-tolerant). Disadvantages Priorities are fixed Possibility that some devices with low priority levels might not be able to use the bus for long time (Bus starvation). Requires many bus lines. Number of devices is limited to the number of priority request lines in the bus. 16 TUTORIAL Dr. Abrar Wafa QUESTIONS Review questions § What happens if two or more devices all want to use the bus at the same time? § What is Daisy Chained? § What are disadvantages of Single Bus Arbitration? § What is PCI bus? § What type of arbitration is used in SCSI bus? § Which kind of arbitration is more Fault tolerant? § Which of the technique may result in process starvation? 18