TC: void harderr( int (*fptr)())

	- prototype in dos.h

	- harderr() establishes a hardware error handler for current
	  program, invoked wherever interrupt 0x24 occurs
	- function fptr is called when such an interrupt occurs
	- handler function will be called with the following
	  arguments= handler( int errval, int ax, int bp, int si)
	  where errval is error code in DI register by MS-DOS, and
	  ax, bp, si are values MS-DOS has in AX, BP, and SI regs
	- ax indicates if disk or other device error occurs;  if ax is
	  not negative, then disk error, else device error.  For disk
	  error, ax ANDed with 0x00ff will give bad drive number
	- bp and si together point to device driver header
	- hardresume() may be called with rescode to return
	  to MS-DOS, where rescode is

		2 - for abort
		1 - retry
		0 - ignore

	- hardrtn() may be called to return directly to the application
	- handler must return:

		  0 - ignore
		  1 - retry
		  2 - abort

	- handler may issue DOS calls 1 through 0xC, but no others, and
	  no C standard I/O or UNIX I/O calls may be used
	- MS C uses _harderr()

	- see  hardresume()  hardretn()  INT 24