Mailbox is a multitasking environment written to encourage exploration and applications in concurrent processing. Unfortunately, multitasking is usually not available to the personal computer user, as most personal computer operating systems only represent a subset of many mainframe functions. It must be made clear, however, that Mailbox is multitasking environment, not an operating system. It does not provide file-, memory-, and other resource management functions required of a complete operating system. However, it does provide a multitasking environment within a program that you write with it. And because the code for Mailbox is easily ported and its interface is the same on all machines, your own multitasking code can also be portable.
Traditionally, multitasking, when available, has been a function of the operating system and usually machine specific. The Macintosh Multifinder provides limited multitasking support, but its programs which use it will be limited to the Macintosh. Microsoft's OS/2, when it is released, promises an impressive library of multitasking functions, but OS/2 is highly IBM AT specific. Though UNIX does provide portable multiprocessing, the operating system is still usually found only on workstations and mainframes, and its common BSD distribution processes are big and ``expensive'' and cannot share memory.