In order to access the devices hardware meaning the controllers registers the

In order to access the devices hardware meaning the

This preview shows page 70 - 72 out of 140 pages.

In order to access the device's hardware, meaning the controller's registers, the device driver traditionally has been part of the system kernel. This approach gives the best performance and the worst reliability since a bug in any device driver can crash the entire system. MINIX 3 departs from this model in order to enhance reliability. As we shall see, in MINIX 3 each device driver is now a separate user-mode process. As we mentioned earlier, operating systems usually classify drivers as block devices, such as disks, or character devices, such as keyboards and printers. Most operating systems define a standard interface that all block drivers must support and a second standard interface that all character drivers must support. These interfaces consist of a number of procedures that the rest of the operating system can call to get the driver to do work for it. In general terms, the job of a device driver is to accept abstract requests from the device- independent software above it and see to it that the request is executed. A typical request to a disk driver is to read block n. If the driver is idle at the time a request comes in, it starts carrying out the request immediately. If, however, it is already busy with a request, it will normally enter the new request into a queue of pending requests to be dealt with as soon as possible. The first step in actually carrying out an I/O request is to check that the input parameters are valid and to return an error if they are not. If the request is valid the next step is to translate it from abstract to concrete terms. For a disk driver, this means figuring out where on the disk the requested block actually is, checking to see if the drive's motor is running, determining if the arm is positioned on the proper cylinder, and so on. In short, the driver must decide which controller operations are required and in what sequence. Once the driver has determined which commands to issue to the controller, it starts issuing them by writing into the controller's device registers. Simple controllers can handle only one command at a time. More sophisticated controllers are willing to accept a linked list of commands, which they then carry out by themselves without further help from the operating system.
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After the command or commands have been issued, one of two situations will apply. In many cases the device driver must wait until the controller does some work for it, so it blocks itself until the interrupt comes in to unblock it. In other cases, however, the operation finishes without delay, so the driver need not block. As an example of the latter situation, scrolling the screen on some graphics cards requires just writing a few bytes into the controller's registers. No mechanical motion is needed, so the entire operation can be completed in a few microseconds.
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