INT 15 - BIOS - SET EVENT WAIT INTERVAL (AT,PS50+)
        AH = 83h
        AL = subfunction
            00h set interval
                CX:DX = microseconds to delay
                ES:BX -> byte whose high bit is to be set at end of interval
            01h cancel wait interval
Return: CF set on error or function already busy
            AH = status
                80h invalid command (PC,PCjr)
                86h function not supported (XT and later)
        CF clear if successful
Notes:  the resolution of the wait period is 977 microseconds on most systems
          because most BIOSes use the 1/1024 second fast interrupt from the AT
          real-time clock chip which is available on INT 70
        IBM AT 1/10/84 BIOS ignores AL and always performs subfunction 00h
SeeAlso: AH=41h,AH=86h,INT 70

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