Not sure about the S/38 but IBM i (AS/400) documentation for programming MI (machine interface) is available [1].

I once wrote an XMODEM crc16 routine in MI because I didn't have the C compiler available and I couldn't get RPG to calculate it fast enough to saturate the modem line.

[1]: http://www-01.ibm.com/support/knowledgecenter/ssw_ibm_i_71/a...