diff --git a/src/pkg/runtime/386/asm.s b/src/pkg/runtime/386/asm.s
index a7265b2b2d..67b72f73f3 100644
--- a/src/pkg/runtime/386/asm.s
+++ b/src/pkg/runtime/386/asm.s
@@ -13,12 +13,9 @@ TEXT _rt0_386(SB),7,$0
 	MOVL	AX, 120(SP)		// save argc, argv away
 	MOVL	BX, 124(SP)
 
+	// set up %gs
 	CALL	ldt0setup(SB)
 
-	// set up %gs to refer to that ldt entry
-	MOVL	$(7*8+7), AX
-	MOVW	AX, GS
-
 	// store through it, to make sure it works
 	MOVL	$0x123, 0(GS)
 	MOVL	tls0(SB), AX
@@ -80,7 +77,7 @@ TEXT mainstart(SB),7,$0
 	RET
 
 TEXT	breakpoint(SB),7,$0
-	BYTE $0xcc
+	INT $3
 	RET
 
 /*
@@ -280,6 +277,7 @@ TEXT	sys·setcallerpc+0(SB),7,$0
 TEXT ldt0setup(SB),7,$16
 	// set up ldt 7 to point at tls0
 	// ldt 1 would be fine on Linux, but on OS X, 7 is as low as we can go.
+	// the entry number is just a hint.  setldt will set up GS with what it used.
 	MOVL	$7, 0(SP)
 	LEAL	tls0(SB), AX
 	MOVL	AX, 4(SP)
@@ -297,4 +295,3 @@ TEXT emptyfunc(SB),0,$0
 
 TEXT	abort(SB),7,$0
 	INT $0x3
-
diff --git a/src/pkg/runtime/386/closure.c b/src/pkg/runtime/386/closure.c
index 1a211bd1f3..763fc45da3 100644
--- a/src/pkg/runtime/386/closure.c
+++ b/src/pkg/runtime/386/closure.c
@@ -15,6 +15,9 @@ sys·closure(int32 siz, byte *fn, byte *arg0)
 	int32 i, n;
 	int32 pcrel;
 
+	if(goos != nil && strcmp((uint8*)goos, (uint8*)"nacl") == 0)
+		throw("no closures in native client yet");
+
 	if(siz < 0 || siz%4 != 0)
 		throw("bad closure size");
 
diff --git a/src/pkg/runtime/darwin/386/sys.s b/src/pkg/runtime/darwin/386/sys.s
index 49743359fd..bded7e4211 100644
--- a/src/pkg/runtime/darwin/386/sys.s
+++ b/src/pkg/runtime/darwin/386/sys.s
@@ -270,6 +270,12 @@ TEXT setldt(SB),7,$32
 	MOVL	AX, 4(SP)
 	MOVL	$1, 8(SP)
 	CALL	i386_set_ldt(SB)
+
+	// compute segment selector - (entry*8+7)
+	MOVL	entry+0(FP), AX
+	SHLL	$3, AX
+	ADDL	$7, AX
+	MOVW	AX, GS
 	RET
 
 TEXT i386_set_ldt(SB),7,$0
diff --git a/src/pkg/runtime/linux/386/sys.s b/src/pkg/runtime/linux/386/sys.s
index 7f4787700b..fe07ddd54f 100755
--- a/src/pkg/runtime/linux/386/sys.s
+++ b/src/pkg/runtime/linux/386/sys.s
@@ -219,5 +219,12 @@ TEXT setldt(SB),7,$32
 	CMPL AX, $0xfffff001
 	JLS 2(PC)
 	INT $3
+
+	// compute segment selector - (entry*8+7)
+	MOVL	entry+0(FP), AX
+	SHLL	$3, AX
+	ADDL	$7, AX
+	MOVW	AX, GS
+
 	RET
 
diff --git a/src/pkg/runtime/mheap.c b/src/pkg/runtime/mheap.c
index d0cf2237bd..8f85b5e091 100644
--- a/src/pkg/runtime/mheap.c
+++ b/src/pkg/runtime/mheap.c
@@ -170,6 +170,8 @@ MHeap_Grow(MHeap *h, uintptr npage)
 	// Ask for a big chunk, to reduce the number of mappings
 	// the operating system needs to track; also amortizes
 	// the overhead of an operating system mapping.
+	// For Native Client, allocate a multiple of 64kB (16 pages).
+	npage = (npage+15)&~15;
 	ask = npage<<PageShift;
 	if(ask < HeapAllocChunk)
 		ask = HeapAllocChunk;
diff --git a/src/pkg/runtime/mkasmh.sh b/src/pkg/runtime/mkasmh.sh
index bc99a26255..30df166652 100755
--- a/src/pkg/runtime/mkasmh.sh
+++ b/src/pkg/runtime/mkasmh.sh
@@ -13,6 +13,7 @@ EOF
 
 case "$GOARCH" in
 386)
+	# The offsets 0 and 4 are known to nacl/thread.c:/^newosproc too.
 	echo '#define	g	0(GS)'
 	echo '#define	m	4(GS)'
 	;;
diff --git a/src/pkg/runtime/nacl/386/defs.h b/src/pkg/runtime/nacl/386/defs.h
new file mode 100755
index 0000000000..420b6910b2
--- /dev/null
+++ b/src/pkg/runtime/nacl/386/defs.h
@@ -0,0 +1,17 @@
+// godefs -f-m32 -f-I/home/rsc/pub/nacl/native_client/src/third_party/nacl_sdk/linux/sdk/nacl-sdk/nacl/include -f-I/home/rsc/pub/nacl/native_client defs.c
+
+// MACHINE GENERATED - DO NOT EDIT.
+
+// Constants
+enum {
+	PROT_NONE = 0,
+	PROT_READ = 0x1,
+	PROT_WRITE = 0x2,
+	PROT_EXEC = 0x4,
+	MAP_ANON = 0x20,
+	MAP_PRIVATE = 0x2,
+};
+
+// Types
+#pragma pack on
+#pragma pack off
diff --git a/src/pkg/runtime/nacl/386/rt0.s b/src/pkg/runtime/nacl/386/rt0.s
new file mode 100755
index 0000000000..d967bafd42
--- /dev/null
+++ b/src/pkg/runtime/nacl/386/rt0.s
@@ -0,0 +1,8 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Native Client and Linux use the same linkage to main
+
+TEXT	_rt0_386_nacl(SB),7,$0
+	JMP	_rt0_386(SB)
diff --git a/src/pkg/runtime/nacl/386/signal.c b/src/pkg/runtime/nacl/386/signal.c
new file mode 100644
index 0000000000..79a760a360
--- /dev/null
+++ b/src/pkg/runtime/nacl/386/signal.c
@@ -0,0 +1,14 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "runtime.h"
+#include "defs.h"
+#include "signals.h"
+#include "os.h"
+
+void
+initsig(void)
+{
+}
+
diff --git a/src/pkg/runtime/nacl/386/sys.s b/src/pkg/runtime/nacl/386/sys.s
new file mode 100755
index 0000000000..5f8a1c0ac2
--- /dev/null
+++ b/src/pkg/runtime/nacl/386/sys.s
@@ -0,0 +1,110 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//
+// System calls and other sys.stuff for 386, Linux
+//
+
+#include "386/asm.h"
+
+// http://code.google.com/p/nativeclient/source/browse/trunk/src/native_client/src/trusted/service_runtime/include/bits/nacl_syscalls.h
+#define SYS_exit 30
+#define SYS_mmap 21
+#define SYS_thread_create 80
+#define SYS_thread_exit 81
+#define SYS_tls_init 82
+#define SYS_write 13
+#define SYS_close 11
+#define SYS_mutex_create 70
+#define SYS_mutex_lock  71
+#define SYS_mutex_unlock 73
+
+#define SYSCALL(x)	$(0x10000+SYS_/**/x * 32)
+
+TEXT exit(SB),7,$4
+	MOVL	code+0(FP), AX
+	MOVL	AX, 0(SP)
+	CALL	SYSCALL(exit)
+	INT $3	// not reached
+	RET
+
+TEXT exit1(SB),7,$4
+	MOVL	code+0(FP), AX
+	MOVL	AX, 0(SP)
+	CALL	SYSCALL(thread_exit)
+	INT $3	// not reached
+	RET
+
+TEXT write(SB),7,$0
+	JMP	SYSCALL(write)
+
+TEXT close(SB),7,$0
+	JMP	SYSCALL(close)
+
+TEXT mutex_create(SB),7,$0
+	JMP	SYSCALL(mutex_create)
+
+TEXT mutex_lock(SB),7,$0
+	JMP	SYSCALL(mutex_lock)
+
+TEXT	mutex_unlock(SB),7,$0
+	JMP	SYSCALL(mutex_unlock)
+
+TEXT thread_create(SB),7,$0
+	JMP	SYSCALL(thread_create)
+
+TEXT sys·mmap(SB),7,$24
+	MOVL	a1+0(FP), BX
+	MOVL	a2+4(FP), CX	// round up to 64 kB boundary; silences nacl warning
+	ADDL	$(64*1024-1), CX
+	ANDL	$~(64*1024-1), CX
+	MOVL	a3+8(FP), DX
+	MOVL	a4+12(FP), SI
+	MOVL	a5+16(FP), DI
+	MOVL	a6+20(FP), BP
+	MOVL	BX, 0(SP)
+	MOVL	CX, 4(SP)
+	MOVL	DX, 8(SP)
+	MOVL	SI, 12(SP)
+	MOVL	DI, 16(SP)
+	MOVL	BP, 20(SP)
+	CALL	SYSCALL(mmap)
+	CMPL	AX, $0xfffff001
+	JLS	6(PC)
+	MOVL	$1, 0(SP)
+	MOVL	$mmap_failed(SB), 4(SP)
+	MOVL	$12, 8(SP)	// "mmap failed\n"
+	CALL	SYSCALL(write)
+	INT $3
+	RET
+
+// setldt(int entry, int address, int limit)
+TEXT setldt(SB),7,$32
+	// entry is ignored - nacl tells us the
+	// segment selector to use and stores it in GS.
+	MOVL	address+4(FP), BX
+	MOVL	limit+8(FP), CX
+	MOVL	BX, 0(SP)
+	MOVL	CX, 4(SP)
+	CALL	SYSCALL(tls_init)
+	CMPL	AX, $0xfffff001
+	JLS	6(PC)
+	MOVL	$1, 0(SP)
+	MOVL	$tls_init_failed(SB), 4(SP)
+	MOVL	$16, 8(SP)	// "tls_init failed\n"
+	CALL	SYSCALL(write)
+	INT $3
+	RET
+
+// There's no good way (yet?) to get stack traces out of a
+// broken NaCl process, so if something goes wrong,
+// print an error string before dying.
+
+DATA mmap_failed(SB)/8, $"mmap fai"
+DATA mmap_failed+8(SB)/4, $"led\n"
+GLOBL mmap_failed(SB), $12
+
+DATA tls_init_failed(SB)/8, $"tls_init"
+DATA tls_init_failed+8(SB)/8, $" failed\n"
+GLOBL tls_init_failed(SB), $16
diff --git a/src/pkg/runtime/nacl/os.h b/src/pkg/runtime/nacl/os.h
new file mode 100644
index 0000000000..eb4af57b2d
--- /dev/null
+++ b/src/pkg/runtime/nacl/os.h
@@ -0,0 +1,9 @@
+// Copyright 2009 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+int32 thread_create(void(*fn)(void), void *stk, void *tls, int32 tlssize);
+void close(int32);
+int32 mutex_create(void);
+int32 mutex_lock(int32);
+int32 mutex_unlock(int32);
diff --git a/src/pkg/runtime/nacl/signals.h b/src/pkg/runtime/nacl/signals.h
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/src/pkg/runtime/nacl/thread.c b/src/pkg/runtime/nacl/thread.c
new file mode 100644
index 0000000000..906f0bbeee
--- /dev/null
+++ b/src/pkg/runtime/nacl/thread.c
@@ -0,0 +1,160 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "runtime.h"
+#include "defs.h"
+#include "os.h"
+
+int8 *goos = "nacl";
+
+// Thread-safe allocation of a mutex.
+// (The name sema is left over from the Darwin implementation.
+// Native Client implements semaphores too, but it is just a shim
+// over the host implementation, which on some hosts imposes a very
+// low limit on how many semaphores can be created.)
+//
+// Psema points at a mutex descriptor.
+// It starts out zero, meaning no mutex.
+// Fill it in, being careful of others calling initsema
+// simultaneously.
+static void
+initsema(uint32 *psema)
+{
+	uint32 sema;
+
+	if(*psema != 0)	// already have one
+		return;
+
+	sema = mutex_create();
+	if((int32)sema < 0) {
+		printf("mutex_create failed\n");
+		breakpoint();
+	}
+	// mutex_create returns a file descriptor;
+	// shift it up and add the 1 bit so that can
+	// distinguish unintialized from fd 0.
+	sema = (sema<<1) | 1;
+	if(!cas(psema, 0, sema)){
+		// Someone else filled it in.  Use theirs.
+		close(sema);
+		return;
+	}
+}
+
+// Lock and unlock.
+// Defer entirely to Native Client.
+// The expense of a call into Native Client is more like
+// a function call than a system call, so as long as the
+// Native Client lock implementation is good, we can't
+// do better ourselves.
+
+static void
+xlock(int32 fd)
+{
+	if(mutex_lock(fd) < 0) {
+		printf("mutex_lock failed\n");
+		breakpoint();
+	}
+}
+
+static void
+xunlock(int32 fd)
+{
+	if(mutex_unlock(fd) < 0) {
+		printf("mutex_lock failed\n");
+		breakpoint();
+	}
+}
+
+void
+lock(Lock *l)
+{
+	if(m->locks < 0)
+		throw("lock count");
+	m->locks++;
+	if(l->sema == 0)
+		initsema(&l->sema);
+	xlock(l->sema>>1);
+}
+
+void
+unlock(Lock *l)
+{
+	m->locks--;
+	if(m->locks < 0)
+		throw("lock count");
+	xunlock(l->sema>>1);
+}
+
+
+// One-time notifications.
+//
+// Since the lock/unlock implementation already
+// takes care of sleeping in the kernel, we just reuse it.
+// (But it's a weird use, so it gets its own interface.)
+//
+// We use a lock to represent the event:
+// unlocked == event has happened.
+// Thus the lock starts out locked, and to wait for the
+// event you try to lock the lock.  To signal the event,
+// you unlock the lock.
+//
+// Native Client does not require that the thread acquiring
+// a lock be the thread that releases the lock, so this is safe.
+
+void
+noteclear(Note *n)
+{
+	if(n->lock.sema == 0)
+		initsema(&n->lock.sema);
+	xlock(n->lock.sema>>1);
+}
+
+void
+notewakeup(Note *n)
+{
+	if(n->lock.sema == 0) {
+		printf("notewakeup without noteclear");
+		breakpoint();
+	}
+	xunlock(n->lock.sema>>1);
+}
+
+void
+notesleep(Note *n)
+{
+	if(n->lock.sema == 0) {
+		printf("notesleep without noteclear");
+		breakpoint();
+	}
+	xlock(n->lock.sema>>1);
+	xunlock(n->lock.sema>>1);	// Let other sleepers find out too.
+}
+
+void
+newosproc(M *m, G *g, void *stk, void (*fn)(void))
+{
+	void **vstk;
+
+	// I wish every OS made thread creation this easy.
+	m->tls[0] = (uint32)g;
+	m->tls[1] = (uint32)m;
+	vstk = stk;
+	*--vstk = nil;
+	if(thread_create(fn, vstk, m->tls, sizeof m->tls) < 0) {
+		printf("thread_create failed\n");
+		breakpoint();
+	}
+}
+
+void
+osinit(void)
+{
+}
+
+// Called to initialize a new m (including the bootstrap m).
+void
+minit(void)
+{
+}
diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 58839f97f0..34bc262525 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -308,6 +308,7 @@ int32	goidgen;
 extern	int32	gomaxprocs;
 extern	int32	panicking;
 extern	int32	maxround;
+int8*	goos;
 
 /*
  * common functions and data
diff --git a/src/pkg/runtime/symtab.c b/src/pkg/runtime/symtab.c
index 97501623d3..a082a7615d 100644
--- a/src/pkg/runtime/symtab.c
+++ b/src/pkg/runtime/symtab.c
@@ -32,6 +32,15 @@ sys·symdat(Slice *symtab, Slice *pclntab)
 	Slice *a;
 	int32 *v;
 
+	// TODO(rsc): Remove once TODO at top of file is done.
+	if(goos != nil && strcmp((uint8*)goos, (uint8*)"nacl") == 0) {
+		symtab = mal(sizeof *a);
+		pclntab = mal(sizeof *a);
+		FLUSH(&symtab);
+		FLUSH(&pclntab);
+		return;
+	}
+
 	v = SYMCOUNTS;
 
 	a = mal(sizeof *a);
@@ -66,6 +75,10 @@ walksymtab(void (*fn)(Sym*))
 	byte *p, *ep, *q;
 	Sym s;
 
+	// TODO(rsc): Remove once TODO at top of file is done.
+	if(goos != nil && strcmp((uint8*)goos, (uint8*)"nacl") == 0)
+		return;
+
 	v = SYMCOUNTS;
 	p = SYMDATA;
 	ep = p + v[0];
@@ -260,6 +273,10 @@ splitpcln(void)
 	Func *f, *ef;
 	int32 *v;
 
+	// TODO(rsc): Remove once TODO at top of file is done.
+	if(goos != nil && strcmp((uint8*)goos, (uint8*)"nacl") == 0)
+		return;
+
 	// pc/ln table bounds
 	v = SYMCOUNTS;
 	p = SYMDATA;