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From: ART100@psuvm.psu.edu (Andy Tefft)
Newsgroups: comp.sys.apple2
Subject: relocatable code
Message-ID: <90315.194340ART100@psuvm.psu.edu>
Date: 12 Nov 90 00:43:40 GMT
Organization: Penn State University
Lines: 40

I'm trying to write something small in assembly that will be
completely relocatable. The problem is that the code
contains a lookup table and I need to know where that
table is in memory.

I can do a neat trick to find where the program is
in memory, but the problem is it requires a jsr. This
means at least a small subroutine (ok, so it's only about
20 bytes long) be at a fixed point in memory. Here it is:

start   jsr storit   ; this is in the main program

        ...

storit  pla          ; obviously this has to be at a fixed loc.
        sta begin
        pla
        sta begin+1
        pha          ; put it back for the rts
        lda begin
        pha
        rts

where begin is a pair of convenient 0-page or otherwise fixed locations.
This routine returns start+2 in begin and begin+1. Then all i have to do
is add a fixed offset from the 'start' location to get the location
of my table.

I guess I can force the user of the routine to store the
starting location of the program at some fixed point and
have the program use that, but obviously it isn't foolproof.
Is there any other way a program can get the contents of the PC
besides pulling them off the stack after a jmp or jsr?
The program will not necessarily be brun from basic.system,
so I can't use any prodos tricks. The table is about 48 bytes,
too long to manually store in memory with lda #3, sta $1000, lda #76,
sta $1001, etc. Same deal with the storit subroutine. Think I've
covered all the important difficulties there.

ideas? ideas?