The nondestructive read function applies only to character devices, and in block devices it should do nothing except set the done flag. It returns the next character that would be obtained with a read function (command code 4), without removing that character from the driver's internal buffer. MS-DOS uses this function to check the console driver for pending Control-C characters during other operations.
The nondestructive read function is called with
RH + 2 BYTE Command code = 5
It returns
RH + 3 WORD Status
If busy bit = 0, at least one
character is waiting
If busy bit = 1, no characters are
waiting
RH + 13 BYTE Character (if busy bit = 0)