diff --git a/src/rp2_common/boot_stage2/pad_checksum b/src/rp2_common/boot_stage2/pad_checksum
index 4d7905a..356227d 100755
--- a/src/rp2_common/boot_stage2/pad_checksum
+++ b/src/rp2_common/boot_stage2/pad_checksum
@@ -45,7 +45,9 @@ odata = idata_padded + struct.pack("<L", checksum)
 try:
     with open(args.ofile, "w") as ofile:
         ofile.write("// Padded and checksummed version of: {}\n\n".format(args.ifile))
-        ofile.write(".section .boot2, \"a\"\n\n")
+        ofile.write(".cpu cortex-m0plus\n")
+        ofile.write(".thumb\n\n")
+        ofile.write(".section .boot2, \"ax\"\n\n")
         for offs in range(0, len(odata), 16):
             chunk = odata[offs:min(offs + 16, len(odata))]
             ofile.write(".byte {}\n".format(", ".join("0x{:02x}".format(b) for b in chunk)))