The input-status function applies only to character devices, and in block-device drivers it should do nothing except set the done flag. This function returns the current input status for the device, allowing MS-DOS to test whether characters are waiting in a type-ahead buffer. If the character device does not have a type-ahead buffer, the input-status routine should always return the busy bit equal to zero, so that MS-DOS will not wait forever to call the read (04H) or nondestructive read (05H) function.
The input-status function is called with
RH + 2 BYTE Command code = 6
It returns
RH + 3 WORD Status:
If busy bit = 1, read request goes to
physical device.
If busy bit = 0, characters already
in device buffer and read request
returns quickly.