diff --git a/src/pkg/runtime/arm/cas.s b/src/pkg/runtime/arm/cas.s
index 648fa40c8d..63df1396d7 100644
--- a/src/pkg/runtime/arm/cas.s
+++ b/src/pkg/runtime/arm/cas.s
@@ -10,5 +10,20 @@
 //	}else
 //		return 0;
 
-TEXT	cas+0(SB),0,$12
-	TODO
+TEXT	cas(SB),7,$0
+	MOVW	0(FP), R1	// *val
+	MOVW	4(FP), R2	// old
+	MOVW	8(FP), R3	// new
+l:
+	LDREX	(R1), R0
+	CMP		R0, R2
+	BNE		fail
+	STREX	R3, (R1), R0
+	CMP		$0, R0
+	BNE		l
+	MOVW	$1, R0
+	RET
+fail:
+	MOVW	$0, R0
+	RET
+
diff --git a/src/pkg/sync/asm_arm.s b/src/pkg/sync/asm_arm.s
index a6ad9d949c..077e81410c 100644
--- a/src/pkg/sync/asm_arm.s
+++ b/src/pkg/sync/asm_arm.s
@@ -10,5 +10,21 @@
 //	}else
 //		return false;
 
-TEXT	sync·cas+0(SB),0,$12
-	TODO
+TEXT	sync·cas(SB),7,$0
+	MOVW	0(FP), R1	// *val
+	MOVW	4(FP), R2	// old
+	MOVW	8(FP), R3	// new
+l:
+	LDREX	(R1), R0
+	CMP		R0, R2
+	BNE		fail
+	STREX	R3, (R1), R0
+	CMP		$0, R0
+	BNE		l
+	MOVW	$1, R0
+	MOVW	R0, 16(SP)
+	RET
+fail:
+	MOVW	$0, R0
+	MOVW	R0, 16(SP)
+	RET