The parameter list for this callable service is intended to be variable length to allow for future expansion. The mapping macro IRRPCOMP is for 31 bit callers and IRRPCOMX is for 64 bit callers. To allow for this, the last word in the parameter list , for 31 bit callers, must have a 1 in the high-order (sign) bit. Num_Parms takes care of 64 bit callers.