diff --git a/usr.bin/truss/aarch64-cloudabi64.c b/usr.bin/truss/aarch64-cloudabi64.c
index 6a1694a965df..ee81a5321a81 100644
--- a/usr.bin/truss/aarch64-cloudabi64.c
+++ b/usr.bin/truss/aarch64-cloudabi64.c
@@ -80,7 +80,9 @@ static struct procabi aarch64_cloudabi64 = {
 	"CloudABI ELF64",
 	SYSDECODE_ABI_CLOUDABI64,
 	aarch64_cloudabi64_fetch_args,
-	aarch64_cloudabi64_fetch_retval
+	aarch64_cloudabi64_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(aarch64_cloudabi64.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(aarch64_cloudabi64);
diff --git a/usr.bin/truss/aarch64-freebsd.c b/usr.bin/truss/aarch64-freebsd.c
index a9944f52f4fd..b47d0f8e1011 100644
--- a/usr.bin/truss/aarch64-freebsd.c
+++ b/usr.bin/truss/aarch64-freebsd.c
@@ -102,7 +102,9 @@ static struct procabi aarch64_freebsd = {
 	"FreeBSD ELF64",
 	SYSDECODE_ABI_FREEBSD,
 	aarch64_fetch_args,
-	aarch64_fetch_retval
+	aarch64_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(aarch64_freebsd.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(aarch64_freebsd);
diff --git a/usr.bin/truss/amd64-cloudabi64.c b/usr.bin/truss/amd64-cloudabi64.c
index 06de94474c1e..d0caaec67679 100644
--- a/usr.bin/truss/amd64-cloudabi64.c
+++ b/usr.bin/truss/amd64-cloudabi64.c
@@ -89,7 +89,9 @@ static struct procabi amd64_cloudabi64 = {
 	"CloudABI ELF64",
 	SYSDECODE_ABI_CLOUDABI64,
 	amd64_cloudabi64_fetch_args,
-	amd64_cloudabi64_fetch_retval
+	amd64_cloudabi64_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(amd64_cloudabi64.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(amd64_cloudabi64);
diff --git a/usr.bin/truss/amd64-freebsd.c b/usr.bin/truss/amd64-freebsd.c
index fa844fa09340..7e2ed160a90d 100644
--- a/usr.bin/truss/amd64-freebsd.c
+++ b/usr.bin/truss/amd64-freebsd.c
@@ -124,7 +124,9 @@ static struct procabi amd64_freebsd = {
 	"FreeBSD ELF64",
 	SYSDECODE_ABI_FREEBSD,
 	amd64_fetch_args,
-	amd64_fetch_retval
+	amd64_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(amd64_freebsd.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(amd64_freebsd);
diff --git a/usr.bin/truss/amd64-freebsd32.c b/usr.bin/truss/amd64-freebsd32.c
index adbc3e74c0c5..66ca417e9495 100644
--- a/usr.bin/truss/amd64-freebsd32.c
+++ b/usr.bin/truss/amd64-freebsd32.c
@@ -120,7 +120,9 @@ static struct procabi amd64_freebsd32 = {
 	"FreeBSD ELF32",
 	SYSDECODE_ABI_FREEBSD32,
 	amd64_freebsd32_fetch_args,
-	amd64_freebsd32_fetch_retval
+	amd64_freebsd32_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(amd64_freebsd32.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(amd64_freebsd32);
@@ -129,7 +131,9 @@ static struct procabi amd64_freebsd32_aout = {
 	"FreeBSD a.out",
 	SYSDECODE_ABI_FREEBSD32,
 	amd64_freebsd32_fetch_args,
-	amd64_freebsd32_fetch_retval
+	amd64_freebsd32_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(amd64_freebsd32.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(amd64_freebsd32_aout);
diff --git a/usr.bin/truss/amd64-linux.c b/usr.bin/truss/amd64-linux.c
index 94a3d6abce94..00eb06ff75f8 100644
--- a/usr.bin/truss/amd64-linux.c
+++ b/usr.bin/truss/amd64-linux.c
@@ -99,7 +99,9 @@ static struct procabi amd64_linux = {
 	"Linux ELF64",
 	SYSDECODE_ABI_LINUX,
 	amd64_linux_fetch_args,
-	amd64_linux_fetch_retval
+	amd64_linux_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(amd64_linux.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(amd64_linux);
diff --git a/usr.bin/truss/amd64-linux32.c b/usr.bin/truss/amd64-linux32.c
index 3f32c8473f37..70e201adfe91 100644
--- a/usr.bin/truss/amd64-linux32.c
+++ b/usr.bin/truss/amd64-linux32.c
@@ -109,7 +109,9 @@ static struct procabi amd64_linux32 = {
 	"Linux ELF32",
 	SYSDECODE_ABI_LINUX32,
 	amd64_linux32_fetch_args,
-	amd64_linux32_fetch_retval
+	amd64_linux32_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(amd64_linux32.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(amd64_linux32);
diff --git a/usr.bin/truss/arm-freebsd.c b/usr.bin/truss/arm-freebsd.c
index 826a8d838294..753829005946 100644
--- a/usr.bin/truss/arm-freebsd.c
+++ b/usr.bin/truss/arm-freebsd.c
@@ -131,7 +131,9 @@ static struct procabi arm_freebsd = {
 	"FreeBSD ELF32",
 	SYSDECODE_ABI_FREEBSD,
 	arm_fetch_args,
-	arm_fetch_retval
+	arm_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(arm_freebsd.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(arm_freebsd);
diff --git a/usr.bin/truss/i386-freebsd.c b/usr.bin/truss/i386-freebsd.c
index 469ab7de9b7a..8103ca7beead 100644
--- a/usr.bin/truss/i386-freebsd.c
+++ b/usr.bin/truss/i386-freebsd.c
@@ -113,7 +113,9 @@ static struct procabi i386_freebsd = {
 	"FreeBSD ELF32",
 	SYSDECODE_ABI_FREEBSD,
 	i386_fetch_args,
-	i386_fetch_retval
+	i386_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(i386_freebsd.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(i386_freebsd);
@@ -122,7 +124,9 @@ static struct procabi i386_freebsd_aout = {
 	"FreeBSD a.out",
 	SYSDECODE_ABI_FREEBSD,
 	i386_fetch_args,
-	i386_fetch_retval
+	i386_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(i386_freebsd_aout.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(i386_freebsd_aout);
diff --git a/usr.bin/truss/i386-linux.c b/usr.bin/truss/i386-linux.c
index 56fab65cefb7..e96db16c7a91 100644
--- a/usr.bin/truss/i386-linux.c
+++ b/usr.bin/truss/i386-linux.c
@@ -106,7 +106,9 @@ static struct procabi i386_linux = {
 	"Linux ELF",
 	SYSDECODE_ABI_LINUX,
 	i386_linux_fetch_args,
-	i386_linux_fetch_retval
+	i386_linux_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(i386_linux.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(i386_linux);
diff --git a/usr.bin/truss/mips-freebsd.c b/usr.bin/truss/mips-freebsd.c
index ac803b942f27..e3f2a04e97fb 100644
--- a/usr.bin/truss/mips-freebsd.c
+++ b/usr.bin/truss/mips-freebsd.c
@@ -134,7 +134,9 @@ static struct procabi mips_freebsd = {
 #endif
 	SYSDECODE_ABI_FREEBSD,
 	mips_fetch_args,
-	mips_fetch_retval
+	mips_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(mips_freebsd.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(mips_freebsd);
diff --git a/usr.bin/truss/powerpc-freebsd.c b/usr.bin/truss/powerpc-freebsd.c
index dbeba2b36baf..f60c3339fdd1 100644
--- a/usr.bin/truss/powerpc-freebsd.c
+++ b/usr.bin/truss/powerpc-freebsd.c
@@ -115,7 +115,9 @@ static struct procabi powerpc_freebsd = {
 	"FreeBSD ELF32",
 	SYSDECODE_ABI_FREEBSD,
 	powerpc_fetch_args,
-	powerpc_fetch_retval
+	powerpc_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(powerpc_freebsd.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(powerpc_freebsd);
diff --git a/usr.bin/truss/powerpc64-freebsd.c b/usr.bin/truss/powerpc64-freebsd.c
index a8927bb5cc7c..ca2e108a2a82 100644
--- a/usr.bin/truss/powerpc64-freebsd.c
+++ b/usr.bin/truss/powerpc64-freebsd.c
@@ -111,7 +111,9 @@ static struct procabi powerpc64_freebsd = {
 	"FreeBSD ELF64",
 	SYSDECODE_ABI_FREEBSD,
 	powerpc64_fetch_args,
-	powerpc64_fetch_retval
+	powerpc64_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(powerpc64_freebsd.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(powerpc64_freebsd);
diff --git a/usr.bin/truss/powerpc64-freebsd32.c b/usr.bin/truss/powerpc64-freebsd32.c
index bc8d2fc97a7f..cddf492acd6a 100644
--- a/usr.bin/truss/powerpc64-freebsd32.c
+++ b/usr.bin/truss/powerpc64-freebsd32.c
@@ -120,7 +120,9 @@ static struct procabi powerpc64_freebsd32 = {
 	"FreeBSD ELF32",
 	SYSDECODE_ABI_FREEBSD32,
 	powerpc64_freebsd32_fetch_args,
-	powerpc64_freebsd32_fetch_retval
+	powerpc64_freebsd32_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(powerpc64_freebsd32.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(powerpc64_freebsd32);
diff --git a/usr.bin/truss/setup.c b/usr.bin/truss/setup.c
index 036cb8f40ccc..24e028a98186 100644
--- a/usr.bin/truss/setup.c
+++ b/usr.bin/truss/setup.c
@@ -344,7 +344,7 @@ alloc_syscall(struct threadinfo *t, struct ptrace_lwpinfo *pl)
 
 	assert(t->in_syscall == 0);
 	assert(t->cs.number == 0);
-	assert(t->cs.name == NULL);
+	assert(t->cs.sc == NULL);
 	assert(t->cs.nargs == 0);
 	for (i = 0; i < nitems(t->cs.s_args); i++)
 		assert(t->cs.s_args[i] == NULL);
@@ -378,12 +378,11 @@ enter_syscall(struct trussinfo *info, struct threadinfo *t,
 		return;
 	}
 
-	t->cs.name = sysdecode_syscallname(t->proc->abi->abi, t->cs.number);
-	if (t->cs.name == NULL)
+	sc = get_syscall(t, t->cs.number, narg);
+	if (sc->unknown)
 		fprintf(info->outfile, "-- UNKNOWN %s SYSCALL %d --\n",
 		    t->proc->abi->type, t->cs.number);
 
-	sc = get_syscall(t->cs.name, narg);
 	t->cs.nargs = sc->nargs;
 	assert(sc->nargs <= nitems(t->cs.s_args));
 
@@ -396,25 +395,22 @@ enter_syscall(struct trussinfo *info, struct threadinfo *t,
 	 * now.	This doesn't currently support arguments that are
 	 * passed in *and* out, however.
 	 */
-	if (t->cs.name != NULL) {
 #if DEBUG
-		fprintf(stderr, "syscall %s(", t->cs.name);
+	fprintf(stderr, "syscall %s(", sc->name);
 #endif
-		for (i = 0; i < t->cs.nargs; i++) {
+	for (i = 0; i < t->cs.nargs; i++) {
 #if DEBUG
-			fprintf(stderr, "0x%lx%s", sc ?
-			    t->cs.args[sc->args[i].offset] : t->cs.args[i],
-			    i < (t->cs.nargs - 1) ? "," : "");
+		fprintf(stderr, "0x%lx%s", t->cs.args[sc->args[i].offset],
+		    i < (t->cs.nargs - 1) ? "," : "");
 #endif
-			if (!(sc->args[i].type & OUT)) {
-				t->cs.s_args[i] = print_arg(&sc->args[i],
-				    t->cs.args, 0, info);
-			}
+		if (!(sc->args[i].type & OUT)) {
+			t->cs.s_args[i] = print_arg(&sc->args[i],
+			    t->cs.args, 0, info);
 		}
-#if DEBUG
-		fprintf(stderr, ")\n");
-#endif
 	}
+#if DEBUG
+	fprintf(stderr, ")\n");
+#endif
 
 	clock_gettime(CLOCK_REALTIME, &t->before);
 }
diff --git a/usr.bin/truss/sparc64-freebsd.c b/usr.bin/truss/sparc64-freebsd.c
index 3cbad50df70b..d1ae89685de7 100644
--- a/usr.bin/truss/sparc64-freebsd.c
+++ b/usr.bin/truss/sparc64-freebsd.c
@@ -118,7 +118,9 @@ static struct procabi sparc64_freebsd = {
 	"FreeBSD ELF64",
 	SYSDECODE_ABI_FREEBSD,
 	sparc64_fetch_args,
-	sparc64_fetch_retval
+	sparc64_fetch_retval,
+	STAILQ_HEAD_INITIALIZER(sparc64_freebsd.extra_syscalls),
+	{ NULL }
 };
 
 PROCABI(sparc64_freebsd);
diff --git a/usr.bin/truss/syscall.h b/usr.bin/truss/syscall.h
index 0cd752d77a03..90b0e8aa94cd 100644
--- a/usr.bin/truss/syscall.h
+++ b/usr.bin/truss/syscall.h
@@ -72,9 +72,10 @@ struct syscall {
 	struct timespec time; /* Time spent for this call */
 	int ncalls;	/* Number of calls */
 	int nerror;	/* Number of calls that returned with error */
+	bool unknown;	/* Unknown system call */
 };
 
-struct syscall *get_syscall(const char *, int nargs);
+struct syscall *get_syscall(struct threadinfo *, u_int, u_int);
 char *print_arg(struct syscall_args *, unsigned long*, long *, struct trussinfo *);
 
 /*
diff --git a/usr.bin/truss/syscalls.c b/usr.bin/truss/syscalls.c
index 46bc87f9575b..2aeafcb224a6 100644
--- a/usr.bin/truss/syscalls.c
+++ b/usr.bin/truss/syscalls.c
@@ -51,6 +51,7 @@ __FBSDID("$FreeBSD$");
 #include <netinet/in.h>
 #include <arpa/inet.h>
 
+#include <assert.h>
 #include <ctype.h>
 #include <err.h>
 #include <fcntl.h>
@@ -819,21 +820,66 @@ init_syscalls(void)
 	for (sc = decoded_syscalls; sc->name != NULL; sc++)
 		STAILQ_INSERT_HEAD(&syscalls, sc, entries);
 }
+
+static struct syscall *
+find_syscall(struct procabi *abi, u_int number)
+{
+	struct extra_syscall *es;
+
+	if (number < nitems(abi->syscalls))
+		return (abi->syscalls[number]);
+	STAILQ_FOREACH(es, &abi->extra_syscalls, entries) {
+		if (es->number == number)
+			return (es->sc);
+	}
+	return (NULL);
+}
+
+static void
+add_syscall(struct procabi *abi, u_int number, struct syscall *sc)
+{
+	struct extra_syscall *es;
+
+	if (number < nitems(abi->syscalls)) {
+		assert(abi->syscalls[number] == NULL);
+		abi->syscalls[number] = sc;
+	} else {
+		es = malloc(sizeof(*es));
+		es->sc = sc;
+		es->number = number;
+		STAILQ_INSERT_TAIL(&abi->extra_syscalls, es, entries);
+	}
+}
+
 /*
  * If/when the list gets big, it might be desirable to do it
  * as a hash table or binary search.
  */
 struct syscall *
-get_syscall(const char *name, int nargs)
+get_syscall(struct threadinfo *t, u_int number, u_int nargs)
 {
 	struct syscall *sc;
-	int i;
+	const char *name;
+	char *new_name;
+	u_int i;
 
-	if (name == NULL)
-		return (NULL);
-	STAILQ_FOREACH(sc, &syscalls, entries)
-		if (strcmp(name, sc->name) == 0)
+	sc = find_syscall(t->proc->abi, number);
+	if (sc != NULL)
+		return (sc);
+
+	name = sysdecode_syscallname(t->proc->abi->abi, number);
+	if (name == NULL) {
+		asprintf(&new_name, "#%d", number);
+		name = new_name;
+	} else
+		new_name = NULL;
+	STAILQ_FOREACH(sc, &syscalls, entries) {
+		if (strcmp(name, sc->name) == 0) {
+			add_syscall(t->proc->abi, number, sc);
+			free(new_name);
 			return (sc);
+		}
+	}
 
 	/* It is unknown.  Add it into the list. */
 #if DEBUG
@@ -842,7 +888,9 @@ get_syscall(const char *name, int nargs)
 #endif
 
 	sc = calloc(1, sizeof(struct syscall));
-	sc->name = strdup(name);
+	sc->name = name;
+	if (new_name != NULL)
+		sc->unknown = true;
 	sc->ret_type = 1;
 	sc->nargs = nargs;
 	for (i = 0; i < nargs; i++) {
@@ -851,6 +899,7 @@ get_syscall(const char *name, int nargs)
 		sc->args[i].type = LongHex;
 	}
 	STAILQ_INSERT_HEAD(&syscalls, sc, entries);
+	add_syscall(t->proc->abi, number, sc);
 
 	return (sc);
 }
@@ -1866,7 +1915,7 @@ print_syscall(struct trussinfo *trussinfo)
 
 	t = trussinfo->curthread;
 
-	name = t->cs.name;
+	name = t->cs.sc->name;
 	nargs = t->cs.nargs;
 	s_args = t->cs.s_args;
 
diff --git a/usr.bin/truss/truss.h b/usr.bin/truss/truss.h
index dee2155dfb22..53282965cc93 100644
--- a/usr.bin/truss/truss.h
+++ b/usr.bin/truss/truss.h
@@ -38,13 +38,29 @@
 #define	DISPLAYTIDS		0x00000080
 
 struct procinfo;
+struct syscall;
 struct trussinfo;
 
+/*
+ * The lookup of normal system calls are optimized by using a fixed
+ * array for the first 1024 system calls that can be indexed directly.
+ * Unknown system calls with other IDs are stored in a linked list.
+ */
+#define	SYSCALL_NORMAL_COUNT	1024
+
+struct extra_syscall {
+	STAILQ_ENTRY(extra_syscall) entries;
+	struct syscall *sc;
+	u_int number;
+};
+
 struct procabi {
 	const char *type;
 	enum sysdecode_abi abi;
 	int (*fetch_args)(struct trussinfo *, u_int);
 	int (*fetch_retval)(struct trussinfo *, long *, int *);
+	STAILQ_HEAD(, extra_syscall) extra_syscalls;
+	struct syscall *syscalls[SYSCALL_NORMAL_COUNT];
 };
 
 #define	PROCABI(abi)	DATA_SET(procabi, abi)
@@ -64,10 +80,9 @@ struct procabi {
  */
 struct current_syscall {
 	struct syscall *sc;
-	const char *name;
-	int number;
-	unsigned long args[10];
+	unsigned int number;
 	unsigned int nargs;
+	unsigned long args[10];
 	char *s_args[10];	/* the printable arguments */
 };