COBOL programs communicate with IMS through `CALL 'CBLTDLI'` -- the DL/I call interface. - Every call takes a function code, a PCB mask, an I/O area, and optional Segment Search Arguments (SSAs). - Function codes are 4-byte values: GU, GN, GNP, GHU, GHN, GHNP, ISRT, REPL, DLET, CHKP, XRST.