OS_Fwm, File write miscellaneous

IN:
     A = reason code
          FA_PTR ($01)   sequential pointer
          FA_EXT ($02)   extent (size) of file
		  FA_EOF ($03) 	 goto end of file
		  FA_RWD ($04)   rewind to beginning of file
     HL = pointer to a 4 byte (32bit) vector (only for FA_PTR and FA_EXT)
          (contains value to be written, low byte first)
     IX = file handle (previously obtained using GN_Opf)

OUT, if call successful:
     Fc = 0
OUT, if call failed:
     Fc = 1
     A = error code:
          RC_HAND ($08), bad handle
          RC_BAD ($04), bad reason code

Registers changed after return:
     ..BCDEHL/IXIY same
     AF....../.... different