I/O software Layers

I/O software Layers

1. interrupt handlers

2. device driver

   • It is device specific code for controlling it.  It is generally written by the device's manufacturer and delivered along with the device.
   • OS usually classify drivers into one of a small number of categories.  The most common categories are the block devices, character devices. 
   • In order to access the device's hardware, actually, meaning the controller's registers, the device driver normally has to be part of the operating system kernel.
   • Since the designer of the OS know that pieces of code written by outsiders will be installed in it, it needs to have an architecture that allows such installation. This means having a well-defined model of what a driver does and how it interacts with the rest of the operating system. 
   • A device driver has several functions. The most obvious one is to accept abstract read and write requests from the device-independent software above it and see that they are carried out.

3. device independent I/O software

   • uniform interfacing for device drivers
     • mapping symbolic device names
       • major device number
         • Is used to locate the appropriate driver.
       • minor device number
         • Is passed as a parameter to the driver in order to specify the unit to be read or written.
   • buffering
     • It drawback, if data get buffered too many times, performance suffers.
     • no buffering
     • buffering in user space
     • buffering in kernel followed by copying to user space
     • Double buffering in the kernel
     • circular buffer
   • error reporting
     • programming errors
       • These occur when a process asks for something impossible, such as writing to input device or reading from output device. Providing invalid buffer address or specifying an invalid device. Report back an error code to the caller.
     • actual I/O errors
       • Trying to write a disk block that has been damaged.   If the driver does not know what to do, it may pass the problem back up to device-independent software  
   • Allocating and releasing dedicated devices
   • Device-independent block size

4. User-space I/O software

   • 1- Make I/O call 2- Format I/O 3- Spooling