UQC113S2

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UQC113S2

• Interrupt driven IO

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Interrupt driven IO

• We have already seen the hardware support
  required to facilitate interrupts
• We will now look at the higher levels of software
  required to support interrupt driven IO
• Much of what we will be looking at is really
  operating system material

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Strategies for I/O handling

• Can be programmed or polled io
• Is wasteful of cpu resources
• Can be interrupt driven
• Is difficult for some devices
• Can use DMA
• Is a mixture of polled and interrupt
• Can be slow

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Layers of Software for IO
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Device Drivers

• We have already looked at the interrupt routines
• All the other hardware specific code is placed in
  the device driver
• These are normally either block or character
  orientated devices
• Can be generic for specific devices
• ie all SCSI devices, disks, cd-roms.

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Device Drivers

• Device Driver should offer
• Uniform interface, all should be able to be read,
  written, opened and closed
• Buffering capacity this will vary per device
• Error handling and reporting should be graceful
• Often should be hot swappable

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Error Handling Reporting

• Errors are much more common with IO than with
  anything else (except users!)
• There can be
• programming errors
• Asking a device to do something it cant ie write
  to a mouse
• Actual I/O errors
• Hard disk sector corrupted, disk not in drive, etc

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Buffering Strategies
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Buffering Strategies

• A unbuffered input
• Inefficient, must re-start user process
  constantly
• B Buffering in user space
• Users may not have correct control over buffer
• C Buffering in kernel copy to user space
• Efficient but what about overflow?
• D Double buffering in the kernel
• The best!

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I/O Functional layers
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The structure of I/O systems