The programmer would call the library routine using the LIBF pseudo-operation, which assembled not a direct BSI to the routine but a one-word indexed branch instruction ( BSI 3 " disp " ) whose displacement (-128,-125, and so on ) identified the start of the routine's transfer vector.
12.
Library routines were addressed through index register XR3 . ( Fortran programs would use index register XR1 for the addresses of parameters and the return address, but register XR2 was unused . ) XR3 would be pointed to a sequence of three-word "'transfer vectors "'such that the first entry would be-128 words from XR3's value.
13.
Then the indirect LDX instruction points XR1 not at the transfer vector, but through it to the return address, or to any parameters stored in-line after the BSI . SIMPL then does whatever it was written to do, gaining access to any in-line parameters through XR1 ( in which case it must increment XR1 for the return address ), and returns as follows:
14.
The complexity of LIBF saves memory for subprograms that are frequently called . : The LIBF linkage requires one word per invocation, plus three words for the transfer vector entry and extra code in the routine itself, whereas the CALL linkage requires two words per invocation because most CALLs will be to an address beyond the-128 to + 127 word reach of the one-word opcode.
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