Exercise 5.47: This section described how to
modify the explicit-control evaluator so that interpreted code can call
compiled procedures.  Show how to modify the compiler so that compiled
procedures can call not only primitive procedures and compiled procedures, but
interpreted procedures as well.  This requires modifying
compile-procedure-call to handle the case of compound (interpreted)
procedures.  Be sure to handle all the same target and linkage
combinations as in compile-proc-appl.  To do the actual procedure
application, the code needs to jump to the evaluator’s compound-apply
entry point.  This label cannot be directly referenced in object code (since
the assembler requires that all labels referenced by the code it is assembling
be defined there), so we will add a register called compapp to the
evaluator machine to hold this entry point, and add an instruction to
initialize it:


  (assign compapp (label compound-apply))
  ;; branches if flag is set:
  (branch (label external-entry))
read-eval-print-loop …


To test your code, start by defining a procedure f that calls a
procedure g.  Use compile-and-go to compile the definition of
f and start the evaluator.  Now, typing at the evaluator, define
g and try to call f.