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  This service lets a program set a specified time interval and lets the
  program check a flag to show when the interval expires. The program should
  call this service with AL = 00H, with the address of a flag byte in
  registers ES and BX, and with the time interval in microseconds in
  registers CX and DX. The high-order 16 bits of the interval should be in
  CX; the low-order 16 bits in DX.

  Initially, the flag byte should be 00H. When the time interval elapses,
  the ROM BIOS sets this byte to 80H. The program can thus inspect the flag
  byte at its own convenience to determine when the time interval has
  elapsed:

  Clear the flag byte
  Call service 83H to start the interval timer
  WHILE   (flag byte = 00H)
          BEGIN
          (do something useful)
          END

  The ROM BIOS interval timer uses the system time-of-day clock, which ticks
  about 1024 times per second, so the timer's resolution is approximately
  976 microseconds.