;;-----------------------------------------
;;-- testLink-1.asm
;;-- Below, "EXTERNAL foo," signals to a linker that "foo" is in another file.
;;-- .obj files need symbol tables to keep track of these references.
;;-- We use the trick of having our non-local address memory at run time,
;;-- and jumping via register. Linker would fill in the ".FILL" with the address
;;-- needed when it was combining this code with "external" code. We don't have
;;-- a linker; so, we will have to do our linking by hand. The value can be
;;-- filled in by editing the .obj file, which is how the linker would do it. 
;;-- Instead, we will edit the source code by hand.
;;-----------------------------------------

;;-- EXTERNAL foo

.ORIG x0200
	br next
        foo: .FILL x0000   ;;-- address to jump to, we fix this by hand.
  next: ld r4, foo
        jsrr r4
	
	;;-- The only problem with this is it uses absolute addresses, making
        ;;-- our code non-relocatable. Alternatively, we could use PC-relative
        ;;-- addressing, replacing the above code with this:

	jsr bar     ;;-- address to jump to, linker should fix this.

        ;;-- That line will not assemble: the symbol "bar" would be undefined.
        ;;-- "EXTERNAL bar" would tell the assembler that "bar" is
        ;;-- in another file, and to put "bar" in the symbol table with
        ;;-- dummy value flag it to be fixed up by the linker. The linker
        ;;-- fixes the offset of the jsr instruction. We can fake this:
        ;;-- put "bar" at the end of this file, and again, do hand linking.
        ;;-- The problem with the jsr instruction is the limited range of
        ;;-- the 11-bit offset (+ or - 1024). The same is true for
        ;;-- the other PC-relative mode (LD, LDI, ST, STI, BR, LEA), which
        ;;-- are restrive (9-bit offset: + or - 256). Except for LEA, these all
        ;;-- have register-indirect versions (LDR, STR, JMP), and the above
        ;;-- can be used. We could do the 

        halt

    bar:   .FILL 0000  ;;--EXTERNAL (We would remove this when linking by hand.)

.END