diff --git a/src/pkg/runtime/asm_amd64.s b/src/pkg/runtime/asm_amd64.s
index d43eb02835..363e680db9 100644
--- a/src/pkg/runtime/asm_amd64.s
+++ b/src/pkg/runtime/asm_amd64.s
@@ -374,13 +374,11 @@ TEXT runtime·cas(SB), 7, $0
 //		*val = new;
 //		return 1;
 //	} else {
-//		*old = *val
 //		return 0;
 //	}
 TEXT runtime·cas64(SB), 7, $0
 	MOVQ	8(SP), BX
-	MOVQ	16(SP), BP
-	MOVQ	0(BP), AX
+	MOVQ	16(SP), AX
 	MOVQ	24(SP), CX
 	LOCK
 	CMPXCHGQ	CX, 0(BX)
@@ -388,7 +386,6 @@ TEXT runtime·cas64(SB), 7, $0
 	MOVL	$1, AX
 	RET
 cas64_fail:
-	MOVQ	AX, 0(BP)
 	MOVL	$0, AX
 	RET
 
diff --git a/src/pkg/runtime/atomic_386.c b/src/pkg/runtime/atomic_386.c
index 1046eb81e3..fbbe9b58e9 100644
--- a/src/pkg/runtime/atomic_386.c
+++ b/src/pkg/runtime/atomic_386.c
@@ -24,10 +24,10 @@ runtime·xadd64(uint64 volatile* addr, int64 v)
 {
 	uint64 old;
 
-	old = *addr;
-	while(!runtime·cas64(addr, &old, old+v)) {
-		// nothing
-	}
+	do
+		old = *addr;
+	while(!runtime·cas64(addr, old, old+v));
+
 	return old+v;
 }
 
@@ -37,9 +37,9 @@ runtime·xchg64(uint64 volatile* addr, uint64 v)
 {
 	uint64 old;
 
-	old = *addr;
-	while(!runtime·cas64(addr, &old, v)) {
-		// nothing
-	}
+	do
+		old = addr;
+	while(!runtime·cas64(addr, old, v));
+
 	return old;
 }
diff --git a/src/pkg/runtime/atomic_arm.c b/src/pkg/runtime/atomic_arm.c
index a78b1dfe2c..b186d1b704 100644
--- a/src/pkg/runtime/atomic_arm.c
+++ b/src/pkg/runtime/atomic_arm.c
@@ -92,16 +92,15 @@ runtime·atomicstore(uint32 volatile* addr, uint32 v)
 
 #pragma textflag 7
 bool
-runtime·cas64(uint64 volatile *addr, uint64 *old, uint64 new)
+runtime·cas64(uint64 volatile *addr, uint64 old, uint64 new)
 {
 	bool res;
 	
 	runtime·lock(LOCK(addr));
-	if(*addr == *old) {
+	if(*addr == old) {
 		*addr = new;
 		res = true;
 	} else {
-		*old = *addr;
 		res = false;
 	}
 	runtime·unlock(LOCK(addr));
diff --git a/src/pkg/runtime/lfstack.c b/src/pkg/runtime/lfstack.c
index 1d48491aac..140384d3dc 100644
--- a/src/pkg/runtime/lfstack.c
+++ b/src/pkg/runtime/lfstack.c
@@ -29,10 +29,10 @@ runtime·lfstackpush(uint64 *head, LFNode *node)
 
 	node->pushcnt++;
 	new = (uint64)(uintptr)node|(((uint64)node->pushcnt&CNT_MASK)<<PTR_BITS);
-	old = runtime·atomicload64(head);
 	for(;;) {
+		old = runtime·atomicload64(head);
 		node->next = (LFNode*)(uintptr)(old&PTR_MASK);
-		if(runtime·cas64(head, &old, new))
+		if(runtime·cas64(head, old, new))
 			break;
 	}
 }
@@ -43,8 +43,8 @@ runtime·lfstackpop(uint64 *head)
 	LFNode *node, *node2;
 	uint64 old, new;
 
-	old = runtime·atomicload64(head);
 	for(;;) {
+		old = runtime·atomicload64(head);
 		if(old == 0)
 			return nil;
 		node = (LFNode*)(uintptr)(old&PTR_MASK);
@@ -52,7 +52,7 @@ runtime·lfstackpop(uint64 *head)
 		new = 0;
 		if(node2 != nil)
 			new = (uint64)(uintptr)node2|(((uint64)node2->pushcnt&CNT_MASK)<<PTR_BITS);
-		if(runtime·cas64(head, &old, new))
+		if(runtime·cas64(head, old, new))
 			return node;
 	}
 }
diff --git a/src/pkg/runtime/parfor.c b/src/pkg/runtime/parfor.c
index a4468c2afc..ceaac8bc92 100644
--- a/src/pkg/runtime/parfor.c
+++ b/src/pkg/runtime/parfor.c
@@ -144,9 +144,9 @@ runtime·parfordo(ParFor *desc)
 			if(victim >= tid)
 				victim++;
 			victimpos = &desc->thr[victim].pos;
-			pos = runtime·atomicload64(victimpos);
 			for(;;) {
 				// See if it has any work.
+				pos = runtime·atomicload64(victimpos);
 				begin = (uint32)pos;
 				end = (uint32)(pos>>32);
 				if(begin+1 >= end) {
@@ -159,7 +159,7 @@ runtime·parfordo(ParFor *desc)
 				}
 				begin2 = begin + (end-begin)/2;
 				newpos = (uint64)begin | (uint64)begin2<<32;
-				if(runtime·cas64(victimpos, &pos, newpos)) {
+				if(runtime·cas64(victimpos, pos, newpos)) {
 					begin = begin2;
 					break;
 				}
diff --git a/src/pkg/runtime/runtime.c b/src/pkg/runtime/runtime.c
index f59a3f4e80..f0571f1899 100644
--- a/src/pkg/runtime/runtime.c
+++ b/src/pkg/runtime/runtime.c
@@ -157,11 +157,12 @@ TestAtomic64(void)
 	z64 = 42;
 	x64 = 0;
 	PREFETCH(&z64);
-	if(runtime·cas64(&z64, &x64, 1))
+	if(runtime·cas64(&z64, x64, 1))
 		runtime·throw("cas64 failed");
-	if(x64 != 42)
+	if(x64 != 0)
 		runtime·throw("cas64 failed");
-	if(!runtime·cas64(&z64, &x64, 1))
+	x64 = 42;
+	if(!runtime·cas64(&z64, x64, 1))
 		runtime·throw("cas64 failed");
 	if(x64 != 42 || z64 != 1)
 		runtime·throw("cas64 failed");
@@ -193,7 +194,7 @@ runtime·check(void)
 	uint64 h;
 	float32 i, i1;
 	float64 j, j1;
-	void* k;
+	byte *k, *k1;
 	uint16* l;
 	struct x1 {
 		byte x;
@@ -232,6 +233,17 @@ runtime·check(void)
 	if(z != 4)
 		runtime·throw("cas4");
 
+	k = (byte*)0xfedcb123;
+	if(sizeof(void*) == 8)
+		k = (byte*)((uintptr)k<<10);
+	if(runtime·casp((void**)&k, nil, nil))
+		runtime·throw("casp1");
+	k1 = k+1;
+	if(!runtime·casp((void**)&k, k, k1))
+		runtime·throw("casp2");
+	if(k != k1)
+		runtime·throw("casp3");
+
 	*(uint64*)&j = ~0ULL;
 	if(j == j)
 		runtime·throw("float64nan");
diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 6c590be633..49503ab41b 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -761,7 +761,7 @@ int32	runtime·write(int32, void*, int32);
 int32	runtime·close(int32);
 int32	runtime·mincore(void*, uintptr, byte*);
 bool	runtime·cas(uint32*, uint32, uint32);
-bool	runtime·cas64(uint64*, uint64*, uint64);
+bool	runtime·cas64(uint64*, uint64, uint64);
 bool	runtime·casp(void**, void*, void*);
 // Don't confuse with XADD x86 instruction,
 // this one is actually 'addx', that is, add-and-fetch.