This document discusses programmed input/output (I/O) mode for data transfer. It explains that programmed I/O involves (1) the program waiting for the device to signal ready status by repeatedly testing status bits before each transfer, and (2) data is only transferred once the other end is ready. While this ensures the processor is dedicated to I/O, it can result in prolonged wait states which are inappropriate for asynchronous devices that can trigger events unpredictably, like keyboards. The document provides examples of reading and writing data using programmed I/O mode.

1. Lesson 06:
Programmed IO
Chapter 11: Input/Output Organisation

2. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
2
Objective
• Understand the programmed IO mode of data
transfer
• Learn that the program waits for the ready status
by repeatedly testing the status bit(s) and data
transfer is only when other end ready

3. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Three modes of transfer of device data,
commands and status
(i) Programmed IO
(ii) Interrupt driven IO
(iii) Direct memory access (DMA)

4. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Programmed IO mode input data transfer

5. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Read Input in Programmed IO mode
(i) Each input is read after first testing whether
the device is ready with the input
(a state reflected by a bit in a status register) or
whether the device input buffer is not empty

6. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Input read by the processor in programmed
I/O mode

7. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Input read in Programmed IO mode
• The program waits for the ready status by
repeatedly testing the status bit(s) and till all
targeted bytes are read from the input device
• The program is in busy (non-waiting) state only
after the device gets ready else in wait state

8. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
8
Programmed IO mode output data transfer

9. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Output write in Programmed IO mode
(ii) Each output written after first testing whether
the device is ready to accept the byte(s) at its
output register or output buffer is empty

10. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Output write in Programmed IO mode

11. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Output write in Programmed IO mode
• The program waits for the ready status by
repeatedly testing the status bit(s) and till all the
targeted bytes are written to the device
• The program in busy (non-waiting) state only
after the device gets ready else wait state

12. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Advantage and Disadvantage Programmed
IO mode data transfer

13. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Advantage of programmed I/O mode
• A program and processor dedicated to wait and
repeatedly tests the status and for IO data
transfer till the IO operation completes

14. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Disadvantage of programmed I/O mode
• A program has to wait and repeatedly tests the
status; Waiting period for an asynchronous event
can be too large
• Many I/O devices generate asynchronous
events— events that occur at times that the
processor cannot predict or control, but which
the processor must respond to reasonably
quickly to provide acceptable performance

15. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Example of Unpredictable wait events
• Keyboard on a workstation or PC
• The processor cannot predict when the user will
press a key but must react to the key-press in
well under a second or the response time will be
noticeable to the user
• The programmed I/O mode therefore not
appropriate due to prolonged wait states

16. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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Summary

17. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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• Programmed IO method of IO operations
• The program waits for the ready status by
repeatedly testing the status bit(s) and data
transfer is only when other end ready
We Learnt

18. Schaum’s Outline of Theory and Problems of Computer Architecture
Copyright © The McGraw-Hill Companies Inc. Indian Special Edition 2009
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End of Lesson 06 on
Programmed IO