diff --git a/lib/BUILD.bazel b/lib/BUILD.bazel
index 163584d..25027e1 100644
--- a/lib/BUILD.bazel
+++ b/lib/BUILD.bazel
@@ -1,4 +1,4 @@
-load("@rules_cc//cc:defs.bzl", "cc_library")
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load("//bazel:copts.bzl", "COPTS")
 
 cc_library(
@@ -8,8 +8,20 @@ cc_library(
     ],
     hdrs = [
         "defs.h",
+        "endian.h",
         "test.h",
     ],
     copts = COPTS,
     visibility = ["//visibility:public"],
 )
+
+cc_test(
+    name = "endian_test",
+    srcs = [
+        "endian_test.c",
+    ],
+    copts = COPTS,
+    deps = [
+        ":lib",
+    ],
+)
diff --git a/lib/defs.h b/lib/defs.h
index 4cb7717..45df4dd 100644
--- a/lib/defs.h
+++ b/lib/defs.h
@@ -1,13 +1,59 @@
-#ifndef defs_h
-#define defs_h
+#ifndef lib_defs_h
+#define lib_defs_h
 /* defs.h - common definitions. */
 
 /*==============================================================================
-Basic definitions
+Target information
 ==============================================================================*/
 
+/*
+  Note that this code does not need to be especially portable. It just runs on
+  Mac OS and development systems (for testing). We can assume that the
+  development system has GCC.
+
+  Macros we care about:
+
+  TARGET_OS_MAC             OS is some Macintosh variant (broadly speaking)
+  TARGET_API_MAC_OS8        Targeting classic Mac OS (9.x and earlier)
+
+  TARGET_RT_LITTLE_ENDIAN   Little-endian byte order
+  TARGET_RT_BIG_ENDIAN      Big-endian byte order
+*/
 #if macintosh
 
+/* Classic Mac OS. Header is part of Universal Interfaces & Carbon. */
+#include <ConditionalMacros.h>
+
+#elif __APPLE__
+
+/* Newer apple systems, including macOS >= 10. Header is in /usr/include, or
+   within /usr/include inside the SDK. */
+#include <TargetConditionals.h>
+
+#else
+
+#if __BYTE_ORDER__
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#define TARGET_RT_LITTLE_ENDIAN 1
+#define TARGET_RT_BIG_ENDIAN 0
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#define TARGET_RT_LITTLE_ENDIAN 0
+#define TARGET_RT_BIG_ENDIAN 1
+#else
+#error "unknown endian"
+#endif
+#else
+#error "could not determine byte order"
+#endif
+
+#endif
+
+/*==============================================================================
+Basic types
+==============================================================================*/
+
+#if TARGET_API_MAC_OS8
+
 #include <MacTypes.h>
 
 #else
@@ -65,7 +111,7 @@ typedef enum
 Memory allocation
 ==============================================================================*/
 
-#if macintosh
+#if TARGET_API_MAC_OS8
 
 #include <MacMemory.h>
 
diff --git a/lib/endian.h b/lib/endian.h
new file mode 100644
index 0000000..e95b133
--- /dev/null
+++ b/lib/endian.h
@@ -0,0 +1,75 @@
+#ifndef lib_endian_h
+#define lib_endian_h
+/* endian.h - byte order and byte swapping */
+
+/*
+  Defines macros for swapping to and from big-endian:
+
+  EndianU16_BtoN
+  EndianU16_NtoB
+  EndianU32_BtoN
+  EndianU32_NtoB
+*/
+
+#include "lib/defs.h"
+
+#if TARGET_API_MAC_OS8
+
+/* Use the legacy Mac OS header, if available. */
+#include <Endian.h>
+
+#else
+
+/* Use the compiler built-in, if available. */
+#if __clang__
+
+#if __has_builtin(__builtin_bswap16)
+#define Endian16_Swap(x) __builtin_bswap16(x)
+#endif
+
+#if __has_builtin(__builtin_bswap32)
+#define Endian32_Swap(x) __builtin_bswap32(x)
+#endif
+
+#elif __GNUC__
+
+#if __GNUC__ && (__GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ >= 3)
+#define Endian16_Swap(x) __builtin_bswap16(x)
+#define Endian32_Swap(x) __builtin_bswap32(x)
+#endif
+
+#endif
+
+/* Fallback implementations. */
+
+/* clang-format off */
+#ifndef Endian16_Swap
+#define Endian16_Swap(x)             \
+	((((UInt16)(x) << 8) & 0xff00) | \
+	 (((UInt16)(x) >> 8) & 0x00ff))
+#endif
+
+#ifndef Endian32_Swap
+#define Endian32_Swap(x)                  \
+	((((UInt32)(x) << 24) & 0xff000000) | \
+	 (((UInt32)(x) <<  8) & 0x00ff0000) | \
+	 (((UInt32)(x) >>  8) & 0x0000ff00) | \
+	 (((UInt32)(x) >> 24) & 0x000000ff))
+#endif
+/* clang-format on */
+
+#if TARGET_RT_BIG_ENDIAN
+#define EndianU16_BtoN(x) (x)
+#define EndianU16_NtoB(x) (x)
+#define EndianU32_BtoN(x) (x)
+#define EndianU32_NtoB(x) (x)
+#else
+#define EndianU16_BtoN(x) Endian16_Swap(x)
+#define EndianU16_NtoB(x) Endian16_Swap(x)
+#define EndianU32_BtoN(x) Endian32_Swap(x)
+#define EndianU32_NtoB(x) Endian32_Swap(x)
+#endif
+
+#endif /* TARGET_API_MAC_OS8 */
+
+#endif /* lib_endian_h */
diff --git a/lib/endian_test.c b/lib/endian_test.c
new file mode 100644
index 0000000..98e824f
--- /dev/null
+++ b/lib/endian_test.c
@@ -0,0 +1,62 @@
+#include "lib/endian.h"
+
+#include <inttypes.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+static int gDidFail;
+
+static void Fail(const char *name, UInt32 got, UInt32 expect)
+{
+	fprintf(stderr, "%s = 0x%" PRIx32 ", expect 0x%" PRIx32 "\n", name, got,
+	        expect);
+	gDidFail = 1;
+}
+
+#define CHECK(fn, expect)      \
+	be = fn(u.i);              \
+	if (be != expect) {        \
+		Fail(#fn, be, expect); \
+	}
+
+static void Test16(void)
+{
+	union {
+		UInt16 i;
+		UInt8 b[2];
+	} u;
+	UInt16 be;
+
+	u.b[0] = 0x12;
+	u.b[1] = 0x34;
+
+	CHECK(EndianU16_BtoN, 0x1234)
+	CHECK(EndianU16_NtoB, 0x1234)
+}
+
+static void Test32(void)
+{
+	union {
+		UInt32 i;
+		UInt8 b[2];
+	} u;
+	UInt32 be;
+
+	u.b[0] = 0x12;
+	u.b[1] = 0x34;
+	u.b[2] = 0x56;
+	u.b[3] = 0x78;
+
+	CHECK(EndianU32_BtoN, 0x12345678)
+	CHECK(EndianU32_NtoB, 0x12345678)
+}
+
+int main(int argc, char **argv)
+{
+	(void)argc;
+	(void)argv;
+
+	Test16();
+	Test32();
+	return gDidFail;
+}