INT 21,40 - Write To File or Device Using Handle

	AH = 40h
	BX = file handle
	CX = number of bytes to write, a zero value truncates/extends
	     the file to the current file position
	DS:DX = pointer to write buffer


	on return:
	AX = number of bytes written if CF not set
	   = error code if CF set  (see DOS ERROR CODES)


	- if AX is not equal to CX on return, a partial write occurred
	- this function can be used to truncate a file to the current
	  file position by writing zero bytes