diff --git a/sys/src/9/pc64/l.s b/sys/src/9/pc64/l.s
index 40233a768..fe3de017b 100644
--- a/sys/src/9/pc64/l.s
+++ b/sys/src/9/pc64/l.s
@@ -686,15 +686,20 @@ f3:
 TEXT touser(SB), 1, $-4
 	CLI
 	SWAPGS
-	MOVQ	$UDSEL, AX
+
+	MOVW	$UDSEL, AX
 	MOVW	AX, DS
 	MOVW	AX, ES
+
+	MOVW	$NULLSEL, AX
 	MOVW	AX, FS
 	MOVW	AX, GS
 
-	MOVQ	$(UTZERO+0x28), CX		/* ip */
-	MOVQ	$0x200, R11			/* flags */
+	MOVL	$0, RMACH
+	MOVL	$0, RUSER
 
+	MOVQ	$(UTZERO+0x28), CX		/* ip */
+	MOVL	$0x200, R11			/* flags */
 	MOVQ	RARG, SP			/* sp */
 
 	BYTE $0x48; SYSRET			/* SYSRETQ */
@@ -703,45 +708,55 @@ TEXT touser(SB), 1, $-4
  */
 TEXT syscallentry(SB), 1, $-4
 	SWAPGS
-	BYTE $0x65; MOVQ 0, RMACH		/* m-> (MOVQ GS:0x0, R15) */
-	MOVQ	16(RMACH), RUSER		/* m->proc */
+	BYTE $0x65; MOVQ 0, AX			/* m-> (MOVQ GS:0x0, AX) */
+	MOVQ	16(AX), BX			/* m->proc */
 	MOVQ	SP, R13
-	MOVQ	16(RUSER), SP			/* m->proc->kstack */
+	MOVQ	16(BX), SP			/* m->proc->kstack */
 	ADDQ	$KSTACK, SP
+
 	PUSHQ	$UDSEL				/* old stack segment */
 	PUSHQ	R13				/* old sp */
 	PUSHQ	R11				/* old flags */
 	PUSHQ	$UESEL				/* old code segment */
 	PUSHQ	CX				/* old ip */
 
-	SUBQ	$(17*8), SP			/* unsaved registers */
-	PUSHQ	RARG				/* system call number */
+	SUBQ	$(16+16*8), SP			/* unsaved registers */
 
 	MOVW	$UDSEL, (15*8+0)(SP)
 	MOVW	ES, (15*8+2)(SP)
 	MOVW	FS, (15*8+4)(SP)
 	MOVW	GS, (15*8+6)(SP)
 
+	MOVQ	RMACH, (14*8)(SP)
+	MOVQ	RUSER, (13*8)(SP)
+
+	MOVQ	RARG, (6*8)(SP)			/* system call number */
+
+	MOVQ	AX, RMACH			/* m */
+	MOVQ	BX, RUSER			/* up */
+
 	MOVQ	SP, RARG
-	PUSHQ	SP				/* Ureg* */
+	PUSHQ	SP
 	CALL	syscall(SB)
 
 TEXT forkret(SB), 1, $-4
-	MOVQ	8(SP), AX			/* Ureg.ax */
-	MOVQ	(8+6*8)(SP), BP			/* Ureg.bp */
-	ADDQ	$(16*8), SP			/* registers + arguments */
+	MOVQ	8(SP), AX
+	ADDQ	$(8+13*8), SP			/* unsaved registers */
 
 	CLI
 	SWAPGS
-	MOVW	0(SP), DS
-	MOVW	2(SP), ES
-	MOVW	4(SP), FS
-	MOVW	6(SP), GS
 
-	MOVQ	24(SP), CX			/* ip */
-	MOVQ	40(SP), R11			/* flags */
+	MOVW	22(SP), GS
+	MOVW	20(SP), FS
+	MOVW	18(SP), ES
+	MOVW	16(SP), DS
 
-	MOVQ	48(SP), SP			/* sp */
+	MOVQ	8(SP), RMACH
+	MOVQ	0(SP), RUSER
+
+	MOVQ	40(SP), CX			/* ip */
+	MOVQ	56(SP), R11			/* flags */
+	MOVQ	64(SP), SP			/* sp */
 
 	BYTE $0x48; SYSRET			/* SYSRETQ */
 
diff --git a/sys/src/9/pc64/trap.c b/sys/src/9/pc64/trap.c
index 2c2001115..6f6f95f96 100644
--- a/sys/src/9/pc64/trap.c
+++ b/sys/src/9/pc64/trap.c
@@ -686,7 +686,7 @@ syscall(Ureg* ureg)
 	up->dbgreg = ureg;
 
 	sp = ureg->sp;
-	scallnr = ureg->ax;
+	scallnr = ureg->bp;	/* RARG */
 	up->scallnr = scallnr;
 
 	spllo();
@@ -735,13 +735,6 @@ syscall(Ureg* ureg)
 				up->errlab[i].sp, up->errlab[i].pc);
 		panic("error stack");
 	}
-
-	/*
-	 *  Put return value in frame.  On the x86 the syscall is
-	 *  just another trap and the return value from syscall is
-	 *  ignored.  On other machines the return value is put into
-	 *  the results register by caller of syscall.
-	 */
 	ureg->ax = ret;
 
 	if(0){
@@ -902,12 +895,8 @@ noted(Ureg* ureg, ulong arg0)
 		pexit("Suicide", 0);
 	}
 
-	/* don't let user change system flags */
-	nureg->flags = (ureg->flags & ~0xCD5) | (nureg->flags & 0xCD5);
-	nureg->cs |= 3;
-	nureg->ss |= 3;
-
-	memmove(ureg, nureg, sizeof(Ureg));
+	/* don't let user change system flags or segment registers */
+	setregisters(ureg, (char*)ureg, (char*)nureg, sizeof(Ureg));
 
 	switch(arg0){
 	case NCONT:
@@ -965,6 +954,7 @@ execregs(uintptr entry, ulong ssize, ulong nargs)
 	ureg->cs = UESEL;
 	ureg->ss = ureg->ds = ureg->es = UDSEL;
 	ureg->fs = ureg->gs = NULLSEL;
+	ureg->r14 = ureg->r15 = 0;	/* extern user registers */
 	return (uintptr)USTKTOP-sizeof(Tos);		/* address of kernel/user shared data */
 }
 
@@ -981,7 +971,7 @@ userpc(void)
 }
 
 /* This routine must save the values of registers the user is not permitted
- * to write from devproc and then restore the saved values before returning.
+ * to write from devproc and noted() and then restore the saved values before returning.
  */
 void
 setregisters(Ureg* ureg, char* pureg, char* uva, int n)
@@ -995,7 +985,7 @@ setregisters(Ureg* ureg, char* pureg, char* uva, int n)
 	if(ureg->fs != UDSEL)
 		ureg->fs = NULLSEL;
 	if(ureg->gs != UDSEL)
-		ureg->gs = 0;
+		ureg->gs = NULLSEL;
 	ureg->flags = (ureg->flags & 0x00ff) | (flags & 0xff00);
 	ureg->pc &= UADDRMASK;
 }
@@ -1063,8 +1053,7 @@ dbgpc(Proc *p)
 	Ureg *ureg;
 
 	ureg = p->dbgreg;
-	if(ureg == 0)
+	if(ureg == nil)
 		return 0;
-
 	return ureg->pc;
 }