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diff --git a/src/asm.h b/src/asm.h
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+/*
+ * Copyright (c) 2018 Richard Braun.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#ifndef _ASM_H
+#define _ASM_H
+
+#ifndef __ASSEMBLER__
+#error "this header may only be included from assembly source files"
+#endif
+
+/*
+ * Enable the assembler to generate Thumb-2 instructions.
+ *
+ * See https://sourceware.org/binutils/docs/as/ARM_002dInstruction_002dSet.html#ARM_002dInstruction_002dSet
+ */
+.syntax unified
+
+/*
+ * The .thumb_func directive tells the assembler to export the symbol as
+ * a thumb function, with bit 0 set.
+ *
+ * The .global directive tells the assembler to make the symbol global,
+ * i.e. to make it visible to other compilation units.
+ */
+#define ASM_FUNC(x)     \
+.thumb_func;            \
+.global x;              \
+x:
+
+#endif /* _ASM_H */