;;; -*- Mode:LISP; Package:LISP-INTERNALS; Base:10; Readtable:CL -*-

;; User provides a piece of code which is in two parts, each of which
;; resides on one page (probably using up less than half a page)
;; This code will be written using defafuns which can jump between
;; each other, finally jumping to a constant location, the copy routine.

;; The copy routine will work from an image of the code in low memory
;; and copy it to two pages at offsets which are different and change
;; each time the routine is called.  All the other virtual pages should
;; map to a page of all halt instructions.  The unused instructions on
;; the copied pages are halt instuctions.

Questions:

We need to make a type of defafun which is an alignment within a page.
Defafuns need to provide us with program length.

The user program must guarantee that it does not use A6-A15 ever. These
are reserved for the copy program so that we can both call and jump to
the user program.

We do not handle dispatch instructions.  To do this, the relocator will need to
be smarter.

Set Up.
Memory layout
Page 0
 Trap code
 Copy code
Page 2
 User1
Page 3
 User2
Page 4
 Halt instructions.
Remaining pages are possible candidates for copying to.

Begin Loop
select two new physical pages
select two new virtual pages
Map virtual pages to the physical pages
Fill the virtual pages with halt bit (using the function boundary marker)
select two new offsets in the pages
Copy each of the instructions
  Relocate the jump instructions
    scan function looking for conditional jump opcode, add offset to low 12 bits
    also, scan function looking for calls/long jumps to other function and fix them
Jump to the first Copied user function
...
Map the used virtual pages back to the halt page.
Loop.