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Explore the hardware support and higher levels of software required for interrupt-driven IO. Learn about strategies for IO handling, device drivers, error handling, buffering strategies, and the functional layers of IO systems.
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UQC113S2 Interrupt driven IO
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
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
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.
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
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 can’t ie write to a mouse • Actual I/O errors • Hard disk sector corrupted, disk not in drive, etc
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!
