INT 15 - BIOS - WAIT (AT,PS)
        AH = 86h
        CX:DX = interval in microseconds
Return: CF clear if successful (wait interval elapsed)
        CF set on error or AH=83h wait already in progress
            AH = status (see AH=84h)
Note:   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
SeeAlso: INT 70