add rp6502 xregn

libsrc/rp6502/xregn.c

@@ -0,0 +1,19 @@
+#include <rp6502.h>
+#include <stdarg.h>
+
+int __cdecl__ xregn (char device, char channel, unsigned char address, unsigned count,
+    ...)
+{
+    va_list args;
+    va_start (args, count);
+    RIA.xstack = device;
+    RIA.xstack = channel;
+    RIA.xstack = address;
+    while (count--) {
+        unsigned v = va_arg (args, unsigned);
+        RIA.xstack = v >> 8;
+        RIA.xstack = v;
+    }
+    va_end (args);
+    return ria_call_int_errno (RIA_OP_XREG);
+}