INT 21,3F - Read From File or Device Using Handle

	AH = 3F
	BX = file handle
	CX = number of bytes to read
	DS:DX = pointer to read buffer

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

	- read specified number of bytes from file into buffer DS:DX
	- when AX is not equal to CX then a partial read occurred due
	  to end of file
	- if AX is zero, no data was read, and EOF occurred before read