LISP Linkage Feature: Incorporating MIDAS into PDP-6 LISP
Some PDP6 LISP users have felt a need for a way to incorporate MIDAS subroutines into LISP. LISP has been changed to let you do this, using files found on the LISP SYSTEM microtape. You write a routine for LISP in much the same way that you write any other MIDAS relocatable subroutine. You must, however, observe the constraints imposed by LISP's allocation and use of accumulators, and its methods of handling input, output, and interrupts. In addition, you require linkage to LISP before your routine can operate properly: The entry point(s) of the subroutine must be put on the property list(s) of the appropriate atom(s), and the address fields of the instructions pointing to other routines, to list structure, or the other LISP data structures must be set properly. This is done when LISP begins operation??er allocation, but before going into its listen loop. We provide eight macros to ease the job of creating such linkages: SUBR, FSUBR, MACRO, QUOTE, E, SPECIAL, and SYM. If you write "SUBR name" at a location a in your routine, LISP will subsequently ascribe the property SUBR to the atom name, with entry location a. Similar remarks apply to the use of FSBUR, LSBUR, and MACRO. The significance and use of other four macros is perhaps best communicated through examples: 1. An instruction like "MOVEI A,QUOTE(X Y Z)" will be assembled as "MOVEI A,O". At link time, however, LISP will insert the location of list (X Y Z) into the address field of the instruction. 2. 2. Suppose that the atom FOO has the properties shown in Figure 1. Then the instructions "MOVEI A QUOTE FOO", "MOVEM B, SPECIAL FOO", "PUSHJ P, SYM FOO", and "CALL E FOO" will each be assembled with a zero address field, which will be modified at link time to be b, c, 106, and 101, respectively.