diff --git a/src/pkg/runtime/darwin/386/sys.s b/src/pkg/runtime/darwin/386/sys.s
index 79628a463f0..242800a5f5c 100644
--- a/src/pkg/runtime/darwin/386/sys.s
+++ b/src/pkg/runtime/darwin/386/sys.s
@@ -74,11 +74,6 @@ TEXT sigaction(SB),7,$0
 //	16(FP)	siginfo
 //	20(FP)	context
 TEXT sigtramp(SB),7,$40
-	// Darwin sets GS to 0x37 on entry.
-	// The original GS is at 0x70(FP).
-	MOVL	oldgs+0x70(FP), BX
-	MOVW	BX, GS
-
 	// g = m->gsignal
 	get_tls(CX)
 	MOVL	m(CX), BP
@@ -186,9 +181,9 @@ TEXT bsdthread_register(SB),7,$40
 	MOVL	$bsdthread_start(SB), 4(SP)	// threadstart
 	MOVL	$0, 8(SP)	// wqthread, not used by us
 	MOVL	$0, 12(SP)	// pthsize, not used by us
-	MOVL	$0, 16(SP)	// paranoia
-	MOVL	$0, 20(SP)
-	MOVL	$0, 24(SP)
+	MOVL	$0, 16(SP)	// dummy_value [sic]
+	MOVL	$0, 20(SP)	// targetconc_ptr
+	MOVL	$0, 24(SP)	// dispatchqueue_offset
 	INT	$0x80
 	JAE	2(PC)
 	CALL	notok(SB)
@@ -253,26 +248,10 @@ TEXT mach_semaphore_signal_all(SB),7,$0
 	CALL	sysenter(SB)
 	RET
 
-/*
-descriptor entry format for system call
-is the native machine format, ugly as it is:
-
-	2-byte limit
-	3-byte base
-	1-byte: 0x80=present, 0x60=dpl<<5, 0x1F=type
-	1-byte: 0x80=limit is *4k, 0x40=32-bit operand size,
-		0x0F=4 more bits of limit
-	1 byte: 8 more bits of base
-
-int i386_get_ldt(int, union ldt_entry *, int);
-int i386_set_ldt(int, const union ldt_entry *, int);
-
-*/
-
 // setldt(int entry, int address, int limit)
+// entry and limit are ignored.
 TEXT setldt(SB),7,$32
 	MOVL	address+4(FP), BX	// aka base
-	MOVL	limit+8(FP), CX
 
 	/*
 	 * When linking against the system libraries,
@@ -288,44 +267,20 @@ TEXT setldt(SB),7,$32
 	 * of the constant.
 	 */
 	SUBL	$0x468, BX
-	ADDL	$0x468, CX
 
-	// set up data_desc
-	LEAL	16(SP), AX	// struct data_desc
-	MOVL	$0, 0(AX)
-	MOVL	$0, 4(AX)
+	/*
+	 * Must set up as USER_CTHREAD segment because
+	 * Darwin forces that value into %gs for signal handlers,
+	 * and if we don't set one up, we'll get a recursive
+	 * fault trying to get into the signal handler.
+	 * Since we have to set one up anyway, it might as
+	 * well be the value we want.  So don't bother with
+	 * i386_set_ldt.
+	 */
+	MOVL	BX, 4(SP)
+	MOVL	$3, AX	// thread_fast_set_cthread_self - machdep call #3
+	INT	$0x82	// sic: 0x82, not 0x80, for machdep call
 
-	MOVW	BX, 2(AX)
-	SHRL	$16, BX
-	MOVB	BX, 4(AX)
-	SHRL	$8, BX
-	MOVB	BX, 7(AX)
-
-	MOVW	CX, 0(AX)
-	SHRL	$16, CX
-	ANDL	$0x0F, CX
-	ORL	$0x40, CX		// 32-bit operand size
-	MOVB	CX, 6(AX)
-
-	MOVB	$0xF2, 5(AX)	// r/w data descriptor, dpl=3, present
-
-	// call i386_set_ldt(entry, desc, 1)
-	MOVL	$0xffffffff, 0(SP)	// auto-allocate entry and return in AX
-	MOVL	AX, 4(SP)
-	MOVL	$1, 8(SP)
-	CALL	i386_set_ldt(SB)
-
-	// compute segment selector - (entry*8+7)
-	SHLL	$3, AX
-	ADDL	$7, AX
-	MOVW	AX, GS
+	XORL	AX, AX
+	MOVW	GS, AX
 	RET
-
-TEXT i386_set_ldt(SB),7,$0
-	MOVL	$5, AX
-	INT	$0x82	// sic
-	JAE	2(PC)
-	CALL	notok(SB)
-	RET
-
-GLOBL tlsoffset(SB),$4
diff --git a/src/pkg/runtime/darwin/amd64/sys.s b/src/pkg/runtime/darwin/amd64/sys.s
index 0b85d27db7e..db922f7a030 100644
--- a/src/pkg/runtime/darwin/amd64/sys.s
+++ b/src/pkg/runtime/darwin/amd64/sys.s
@@ -113,6 +113,7 @@ TEXT bsdthread_create(SB),7,$0
 	MOVQ	gg+24(SP), R10	// "pthread"
 // TODO(rsc): why do we get away with 0 flags here but not on 386?
 	MOVQ	$0, R8	// flags
+	MOVQ	$0, R9	// paranoia
 	MOVQ	$(0x2000000+360), AX	// bsdthread_create
 	SYSCALL
 	JCC 2(PC)
@@ -146,6 +147,9 @@ TEXT bsdthread_register(SB),7,$0
 	MOVQ	$bsdthread_start(SB), DI	// threadstart
 	MOVQ	$0, SI	// wqthread, not used by us
 	MOVQ	$0, DX	// pthsize, not used by us
+	MOVQ	$0, R10	// dummy_value [sic]
+	MOVQ	$0, R8	// targetconc_ptr
+	MOVQ	$0, R9	// dispatchqueue_offset
 	MOVQ	$(0x2000000+366), AX	// bsdthread_register
 	SYSCALL
 	JCC 2(PC)