Allow "sp" as an alias for "c_sp" for backwards compatibility. Using it will

work but generates a linker warning. Added a test to check for this warning.

asminc/zeropage.inc

@@ -13,6 +13,12 @@
         .globalzp       tmp1, tmp2, tmp3, tmp4
         .globalzp       regbank
 
+        ; The following symbol is supplied for compatibility reasons only, it
+        ; will get removed in future versions. Using it will cause a linker
+        ; warning.
+        .globalzp       sp
+
+
 ; The size of the register bank
 regbanksize     = 6
 

libsrc/runtime/sp-compat.s

@@ -0,0 +1,11 @@
+;
+; Kugelfuhr, 2025-06-26
+;
+; Add "sp" as an alias for "c_sp" so we don't break old code but emit a
+; linker warning if it is used. Added after renaming "sp" to "c_sp".
+;
+
+.include        "zeropage.inc"
+.export         sp := c_sp
+.assert         0, ldwarning, "Symbol 'sp' is deprecated - please use 'c_sp' instead"
+

test/asm/listing/ref/sp-compat.ld65err2-ref

@@ -0,0 +1 @@
+ld65: Warning: runtime/sp-compat.s:10: Symbol 'sp' is deprecated - please use 'c_sp' instead

test/asm/listing/sp-compat.s

@@ -0,0 +1,7 @@
+.include "zeropage.inc"
+
+.proc   _func
+        ldy     #0
+        lda     (sp),y
+        rts
+.endproc