The final complication of the floppy disk driver is motor control Diskettes

The final complication of the floppy disk driver is

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The final complication of the floppy disk driver is motor control. Diskettes cannot be read or written unless they are revolving. Hard disks are designed to run for thousands of hours on end without wearing out, but leaving the motors on all the time causes a floppy drive and diskette to wear out quickly. If the motor is not already on when a drive is accessed, it is necessary to issue a command to start the drive and then to wait about a half second before attempting to read or write data. Turning the motors on or off is slow, so MINIX 3 leaves a drive motor on for a few seconds after a drive is used. If the drive is used again within this interval, the timer is extended for another few seconds. If the drive is not used in this interval, the motor is turned off 3.3. Deadlocks Computer systems are full of resources that can only be used by one process at a time. Common examples include printers, tape drives, and slots in the system's internal tables. Having two processes simultaneously writing to the printer leads to gibberish. Having two processes using the same file system table slot will invariably lead to a corrupted file
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system. Consequently, all operating systems have the ability to (temporarily) grant a process exclusive access to certain resources, both hardware and software. [Page 238] For many applications, a process needs exclusive access to not one resource, but several. Suppose, for example, two processes each want to record a scanned document on a CD. Process A requests permission to use the scanner and is granted it. Process B is programmed differently and requests the CD recorder first and is also granted it. Now A asks for the CD recorder, but the request is denied until B releases it. Unfortunately, instead of releasing the CD recorder B asks for the scanner. At this point both processes are blocked and will remain so forever. This situation is called a deadlock. Deadlocks can occur in a variety of situations besides requesting dedicated I/O devices. In a database system, for example, a program may have to lock several records it is using, to avoid race conditions. If process A locks record R1 and process B locks record R2, and then each process tries to lock the other one's record, we also have a deadlock. Thus deadlocks can occur on hardware resources or on software resources. In this section, we will look at deadlocks more closely, see how they arise, and study some ways of preventing or avoiding them. Although this material is about deadlocks in the context of operating systems, they also occur in database systems and many other contexts in computer science, so this material is actually applicable to a wide variety of multiprocess systems. 3.3.1. Resources Deadlocks can occur when processes have been granted exclusive access to devices, files, and so forth. To make the discussion of deadlocks as general as possible, we will refer to the objects granted as resources. A resource can be a hardware device (e.g., a tape drive) or a piece of information (e.g., a locked record in a database). A computer will normally
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