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Comp archch scsi bus

Sarika Khope
Sarika Khope

The SCSI (Small Computer System Interface) bus is a parallel interface standard used to connect hard disks and other peripherals to computers. It uses an 8- or 16-bit wide data bus plus a parity bit for error detection, and 10 or more control signals. SCSI originally provided a bandwidth of 5 MB/s but newer extensions allow up to 320 MB/s. The SCSI controller arbitration policy prioritizes access to the highest priority device, but this can cause starvation issues for lower priority devices waiting long periods to access the bus.

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Lesson 16: Standard I/O bus SCSI Chapter 11: Input/Output Organisation
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 2 Objective Familiar with a standard I/O interface parallel bus called SCSI Learn that SCSI data bus has data bus plus parity check bit, has 8- or in recent extensions 16- or 32-bits communicate data as well as address and 10 or in recent extensions more control signals Learn that SCSI Communicate with the hard disk with bandwidth of 5 MB/s, in recent extensions 160 or 320 MB/s bandwidth
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 3 SCSI (Small Computer System Interface) Bus
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 4 SCSI bus SCSI bus controller Communicate 8- or in recent extensions 16- or 32-bits communicate data as well as address and 10 or in recent extensions more control signals Communicate with the hard disk with bandwidth of 5 MB/s In recent extensions 160 or 320 MB/s bandwidth
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 5 Bus protocols Allow any device on the bus to request the use of the bus Have a policy for deciding which device gets to use the bus if more than one device wants to access it at the same time
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 6 SCSI Data bus Data bus has data bus plus parity check bit Enables single bit-error detection in a data transfer
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 7 SCSI bus signals
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 8 SCSI ten Control Signals 1. RSTthe reset signal activates to bring all devices with SCSI controller to start up state. 2. SELwhen bus is used for selecting an SCSI target (It differs from Poll count, instead of poll count, the controller activates, SEL, the SCSI controller examines SCSI ID and always select the highest ID device Each SCSI device has a distinct bus request line BRqi
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 9 SCSI ten Control Signals 3. BSY (for Busy)when SCSI bus is not available to other device (controller) 4. ATNactivates when a device draws attention before sending a message to a target
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 10 SCSI ten Control Signals 5. REQlike DRQ in sequence 1 Section 11.13 requests a data transfer by a device with the SCSI controller 6. ACKacknowledgement from the SCSI controller that data transfer completes, like DACK 7. MSGwhen a message or information is being transferred through the SCSI bus
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 11 SCSI ten Control Signals 8. C/D1 or 0 when a control (command or status) or data word is being transferred, respectively 9. I/O1 or 0 when a data is input to SCSI controller or output from the controller, respectively 10. Parity
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 12 High-speed SCSI versions Use double edge detection of the signals
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 13 SCSI bus signal sequences
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 14 Sequences of SCSI Signals 1. Sequence 0: Arbitration when Busy is not active 2. Sequence 1: Selection using SELthe selection of signals is done
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 15 Sequences of SCSI Signals 3. Sequence 2: Command or status or data transfer 4. Sequence 3: Reselection
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 16 SCSI bus Arbitration policy
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 17 SCSI controller arbitration policy Access to highest priority device first Advantage that it is easy to decide which device gets access to the bus
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 18 SCSI controller arbitration policy Disadvantage Starvation Starvation when a device with a high SCSI ID can prevent a device with a lower ID from ever getting a chance to use the bus by making repeated requests for the bus Can occur any time an arbitration policy always gives one device priority over another
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 19 Starvation problem Can occur any time an arbitration policy always gives one device priority over another Some arbitration policies prevent starvation by giving more priority to the devices that have been waiting for the bus longest
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 20 Summary
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 21 We learnt Data bus has data bus plus parity check bit SCSI has 8- or in recent extensions 16- or 32- bits communicate data as well as address and 10 or in recent extensions more control signals Communicate with the hard disk with bandwidth of 5 MB/s, in recent extensions 160 or 320 MB/s bandwidth Access to highest priority device first arbitration policy
Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 22 End of Lesson 16 on Standard I/O bus SCSI

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Comp archch scsi bus

  • 1. Lesson 16: Standard I/O bus SCSI Chapter 11: Input/Output Organisation
  • 2. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 2 Objective Familiar with a standard I/O interface parallel bus called SCSI Learn that SCSI data bus has data bus plus parity check bit, has 8- or in recent extensions 16- or 32-bits communicate data as well as address and 10 or in recent extensions more control signals Learn that SCSI Communicate with the hard disk with bandwidth of 5 MB/s, in recent extensions 160 or 320 MB/s bandwidth
  • 3. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 3 SCSI (Small Computer System Interface) Bus
  • 4. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 4 SCSI bus SCSI bus controller Communicate 8- or in recent extensions 16- or 32-bits communicate data as well as address and 10 or in recent extensions more control signals Communicate with the hard disk with bandwidth of 5 MB/s In recent extensions 160 or 320 MB/s bandwidth
  • 5. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 5 Bus protocols Allow any device on the bus to request the use of the bus Have a policy for deciding which device gets to use the bus if more than one device wants to access it at the same time
  • 6. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 6 SCSI Data bus Data bus has data bus plus parity check bit Enables single bit-error detection in a data transfer
  • 7. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 7 SCSI bus signals
  • 8. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 8 SCSI ten Control Signals 1. RSTthe reset signal activates to bring all devices with SCSI controller to start up state. 2. SELwhen bus is used for selecting an SCSI target (It differs from Poll count, instead of poll count, the controller activates, SEL, the SCSI controller examines SCSI ID and always select the highest ID device Each SCSI device has a distinct bus request line BRqi
  • 9. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 9 SCSI ten Control Signals 3. BSY (for Busy)when SCSI bus is not available to other device (controller) 4. ATNactivates when a device draws attention before sending a message to a target
  • 10. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 10 SCSI ten Control Signals 5. REQlike DRQ in sequence 1 Section 11.13 requests a data transfer by a device with the SCSI controller 6. ACKacknowledgement from the SCSI controller that data transfer completes, like DACK 7. MSGwhen a message or information is being transferred through the SCSI bus
  • 11. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 11 SCSI ten Control Signals 8. C/D1 or 0 when a control (command or status) or data word is being transferred, respectively 9. I/O1 or 0 when a data is input to SCSI controller or output from the controller, respectively 10. Parity
  • 12. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 12 High-speed SCSI versions Use double edge detection of the signals
  • 13. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 13 SCSI bus signal sequences
  • 14. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 14 Sequences of SCSI Signals 1. Sequence 0: Arbitration when Busy is not active 2. Sequence 1: Selection using SELthe selection of signals is done
  • 15. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 15 Sequences of SCSI Signals 3. Sequence 2: Command or status or data transfer 4. Sequence 3: Reselection
  • 16. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 16 SCSI bus Arbitration policy
  • 17. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 17 SCSI controller arbitration policy Access to highest priority device first Advantage that it is easy to decide which device gets access to the bus
  • 18. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 18 SCSI controller arbitration policy Disadvantage Starvation Starvation when a device with a high SCSI ID can prevent a device with a lower ID from ever getting a chance to use the bus by making repeated requests for the bus Can occur any time an arbitration policy always gives one device priority over another
  • 19. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 19 Starvation problem Can occur any time an arbitration policy always gives one device priority over another Some arbitration policies prevent starvation by giving more priority to the devices that have been waiting for the bus longest
  • 20. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 20 Summary
  • 21. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 21 We learnt Data bus has data bus plus parity check bit SCSI has 8- or in recent extensions 16- or 32- bits communicate data as well as address and 10 or in recent extensions more control signals Communicate with the hard disk with bandwidth of 5 MB/s, in recent extensions 160 or 320 MB/s bandwidth Access to highest priority device first arbitration policy
  • 22. Schaums Outline of Theory and Problems of Computer Architecture Copyright 息 The McGraw-Hill Companies Inc. Indian Special Edition 2009 22 End of Lesson 16 on Standard I/O bus SCSI