It was used by Microsoft OS/2 1.0 to initiate task switching. IBM added the key to the PC AT, the first PC to use the Intel 80286 chip, which made pre-emptive multitasking practical. IBM felt that a hardware-level interrupt that could be initiated by the user would be handy. Oddly, when IBM later took over OS/2, they didn't make much use of the key. --LDC
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