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Multitasking

    Multitasking is a technique that manages a computer system's work
    when that work consists of multiple activities such as editing a
    file, compiling a program, or performing inter-system transfers.
    Individual tasks execute as if they run on dedicated processors
    and share a common memory. It appears that, except for pauses to
    communicate or synchronize with other tasks, each task runs in
    parallel with all other tasks. The 80386 (80286+) contains
    hardware to support multitasking.

    The 80386 uses no special instructions to control multitasking.
    Instead, it interprets ordinary control-transfer instructions
    differently when they refer to the special data structures. The
    registers and data structures that support multitasking are:

        - Task State Segment (TSS)
        - TSS descriptor
        - Task Register (TR)
        - Task Gate descriptor

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    In addition to the simple task switch, the 80386 offers two other
    task-management features:

      . With each task switch, the processor can also switch to
        another LDT and to another page directory. Thus each task can
        have a different logical-to-linear mapping and a different
        linear-to-physical mapping. Using this feature, tasks can be
        isolated and prevented from interfering with one another.
      . Interrupts and exceptions can cause task switches if needed in
        the system design. The 80386 not only switches to the task
        that handles the interrupt or exception, but it automatically
        switches back to the interrupted task when the interrupt or
        exception has been serviced.

    In reality, the multitasking simulates multiple processors by
    providing each task with a virtual processor. That is, at any one
    instant, the operating system assigns the real processor to any
    one of the virtual processors, which then runs that virtual
    processor's task. To do this, the 80386 uses Task State Segments
    (TSS) and instructions that switch tasks.