diff --git a/vm/cpu-x86.32.S b/vm/cpu-x86.32.S
index cc55532768..2e85be0f81 100644
--- a/vm/cpu-x86.32.S
+++ b/vm/cpu-x86.32.S
@@ -7,7 +7,8 @@
 #define DS_REG %esi
 #define RETURN_REG %eax
 
-#define NV_TEMP_REG %ebx
+#define NV0 %ebx
+#define NV1 %ebp
 
 #define ARITH_TEMP_1 %ebp
 #define ARITH_TEMP_2 %ebx
@@ -82,7 +83,7 @@ DEF(void,set_x87_env,(const void*)):
 	ret
 
 DEF(F_FASTCALL void,throw_impl,(CELL quot, F_STACK_FRAME *rewind_to, void *vm)):
-	mov ARG2,NV_TEMP_REG  /* remember vm ptr in case quot_xt = lazy_jit_compile */		
+	mov ARG2,NV0  /* remember vm ptr in case quot_xt = lazy_jit_compile */		
 	/* clear x87 stack, but preserve rounding mode and exception flags */
 	sub $2,STACK_REG
 	fnstcw (STACK_REG)
@@ -90,7 +91,7 @@ DEF(F_FASTCALL void,throw_impl,(CELL quot, F_STACK_FRAME *rewind_to, void *vm)):
 	fldcw (STACK_REG)
 	/* rewind_to */
 	mov ARG1,STACK_REG
-	mov NV_TEMP_REG,ARG1
+	mov NV0,ARG1
 	jmp *QUOT_XT_OFFSET(ARG0)
 
 DEF(F_FASTCALL void,lazy_jit_compile,(CELL quot, void *vm)):
diff --git a/vm/cpu-x86.64.S b/vm/cpu-x86.64.S
index 90d274e711..5e307f0500 100644
--- a/vm/cpu-x86.64.S
+++ b/vm/cpu-x86.64.S
@@ -7,7 +7,8 @@
 #define CELL_SIZE 8
 #define STACK_PADDING 56
 
-#define NV_TEMP_REG %rbp
+#define NV0 %rbp
+#define NV1 %r12
 
 #define ARITH_TEMP_1 %r8
 #define ARITH_TEMP_2 %r9
diff --git a/vm/cpu-x86.S b/vm/cpu-x86.S
index e9116f8f65..c497a0aad2 100644
--- a/vm/cpu-x86.S
+++ b/vm/cpu-x86.S
@@ -41,7 +41,8 @@ multiply_overflow:
 
 DEF(F_FASTCALL void,c_to_factor,(CELL quot, void *vm)):
 	PUSH_NONVOLATILE
-	mov ARG0,NV_TEMP_REG
+	mov ARG0,NV0
+	mov ARG1,NV1
 	
 	/* Create register shadow area for Win64 */
 	sub $32,STACK_REG
@@ -51,7 +52,8 @@ DEF(F_FASTCALL void,c_to_factor,(CELL quot, void *vm)):
 	call MANGLE(save_callstack_bottom)
 	
 	/* Call quot-xt */
-	mov NV_TEMP_REG,ARG0
+	mov NV0,ARG0
+	mov NV1,ARG1
 	call *QUOT_XT_OFFSET(ARG0)
 
 	/* Tear down register shadow area */