An Operating Systems Vade Mecum

chapter 5TRANSPUTThus far we have treated operating systems as resource-allocation schemes, as suggestedby the Resource Principle. We will now turn to the view of operating systems suggestedby the Beautification Principle introduced in Chapter 1.Beautification PrincipleAn operating system is a set of algorithms that hide the details of the hardware and provide a more pleasant environment. From this point of view, storage management is not an attempt to allocate the main-storeresource but rather is meant to hide address translation and its implementation so thatprocesses can live in a more pleasant, virtual world. All a process needs to know for segmentationis how to get and release segments.Controlling transput is another way the kernel beautifies the process interface.Computer devices (sometimes called ‘‘peripherals’’) are often quite unpleasant to usedirectly. Processes are interested in moving data between their virtual space and the device.They are not interested in the peculiarities of line printers, for example. Nor do theycare to know that different brands and models of tape drive require completely differentcontrolling programs. In fact, they may not care to know whether output is beingdirected to a tape drive, to a printer, or even to another process. All such details can behidden, in which case we say that transput is transparent with respect to brand, model,or device type.Just as processes enjoy the benefits of virtual store, they also enjoy the benefits ofvirtual devices. These devices are simulated by the kernel, with data actually kept eitherin main store or on other devices, typically large disks. For example, the VM operatingsystem has the concept of ‘‘minidisks,’’ which are meant to simulate small disks. Theyare actually implemented by allocating cylinders on much larger disks. Another example147