OS_Op, Internal open

 IN:
     A = OP_IN ($01), open file for input
          IX = (file) DOR handle

     A = OP_OUT ($02), open (create) file for output
          IX = (directory) DOR handle
          HL = pointer to ASCII filename (null-terminated)

     A = OP_UP ($03), open for update
          IX = (file) DOR handle

     A = OP_MEM ($04), open memory
          BHL = pointer to memory
          C = size (max. 255 bytes)

     A = OP_DIR ($05), create directory
          IX = (directory) DOR handle
          HL = pointer to ASCII filename (null-terminated)

	 A = OP_DOR ($06), create node (OZ 5.0 and newer)
          IX = parent DOR handle
	   	  B = minor DOR type 
          HL = pointer to ASCII filename (null-terminated)

OUT, if call successful:
     Fc = 0
     A(in) = OP_IN:
          IX = file handle
     A(in) = OP_OUT:
          IX = file handle
     A(in) = OP_UP:
          IX = file handle
     A(in) = OP_MEM:
          IX = file handle
     A(in) = OP_DIR:
          IX = DOR handle
     A(in) = OP_DOR:
          IX = DOR handle

Registers changed after return:
     ..BCDEHL/..IY same
     AF....../IX.. different