diff --git a/libcpu/arm/cortex-m3/cpuport.c b/libcpu/arm/cortex-m3/cpuport.c
index 66e2aa4809..0f09930554 100644
--- a/libcpu/arm/cortex-m3/cpuport.c
+++ b/libcpu/arm/cortex-m3/cpuport.c
@@ -389,12 +389,12 @@ exit
 int __rt_ffs(int value)
 {
     __asm volatile(
-        "CMP     r0, #0x00            \n"
+        "CMP     %1, #0x00            \n"
         "BEQ     1f                   \n"
 
-        "RBIT    r0, r0               \n"
-        "CLZ     r0, r0               \n"
-        "ADDS    r0, r0, #0x01        \n"
+        "RBIT    %1, %1               \n"
+        "CLZ     %0, %1               \n"
+        "ADDS    %0, %0, #0x01        \n"
 
         "1:                           \n"
 
diff --git a/libcpu/arm/cortex-m4/cpuport.c b/libcpu/arm/cortex-m4/cpuport.c
index f060fa122d..717b4ba3df 100644
--- a/libcpu/arm/cortex-m4/cpuport.c
+++ b/libcpu/arm/cortex-m4/cpuport.c
@@ -474,11 +474,11 @@ exit
 int __rt_ffs(int value)
 {
     __asm volatile(
-        "CMP     %0, #0x00            \n"
+        "CMP     %1, #0x00            \n"
         "BEQ     1f                   \n"
 
-        "RBIT    %0, %0               \n"
-        "CLZ     %0, %0               \n"
+        "RBIT    %1, %1               \n"
+        "CLZ     %0, %1               \n"
         "ADDS    %0, %0, #0x01        \n"
 
         "1:                           \n"
diff --git a/libcpu/arm/cortex-m7/cpuport.c b/libcpu/arm/cortex-m7/cpuport.c
index 6b748255c5..274cc19c88 100644
--- a/libcpu/arm/cortex-m7/cpuport.c
+++ b/libcpu/arm/cortex-m7/cpuport.c
@@ -473,12 +473,12 @@ exit
 int __rt_ffs(int value)
 {
     __asm volatile(
-        "CMP     r0, #0x00            \n"
+        "CMP     %1, #0x00            \n"
         "BEQ     1f                   \n"
 
-        "RBIT    r0, r0               \n"
-        "CLZ     r0, r0               \n"
-        "ADDS    r0, r0, #0x01        \n"
+        "RBIT    %1, %1               \n"
+        "CLZ     %0, %1               \n"
+        "ADDS    %0, %0, #0x01        \n"
 
         "1:                           \n"