add rp6502 target

libsrc/rp6502/xreg.s

@@ -0,0 +1,40 @@
+; Copyright (c) 2023 Rumbledethumps
+;
+; SPDX-License-Identifier: Zlib
+; SPDX-License-Identifier: BSD-3-Clause
+; SPDX-License-Identifier: Unlicense
+
+; CC65 will promote variadic char arguments to int. It will not demote longs.
+; int __cdecl__ xreg(char device, char channel, unsigned char address, ...);
+
+.export _xreg
+.importzp sp
+.import addysp, _ria_call_int_errno
+
+.include "rp6502.inc"
+
+.code
+
+.proc _xreg
+
+    ; save variadic size in X
+    tya
+    tax
+
+@copy: ; copy stack
+    dey
+    lda (sp),y
+    sta RIA_XSTACK
+    tya
+    bne @copy
+
+    ; recover variadic size and move sp
+    txa
+    tay
+    jsr addysp
+
+    ; run RIA operation
+    lda #RIA_OP_XREG
+    jmp _ria_call_int_errno
+
+.endproc