Library routines are addressed through index register XR3. (Fortran subprograms use index register XR1 for the addresses of parameters and the return address, but register XR2 is unused.) XR3 points to a sequence of three-word transfer vectors such that the first entry is -128 words from XR3's value. The programmer calls the library routine using the codice_14 pseudo-operation, which assembles not a direct codice_15 to the routine but a one-word indexed branch instruction (codice_16) whose displacement (-128, -125, and so on) identifies the start of the routine's transfer vector.