diff --git a/libcpu/arm/cortex-m3/cpuport.c b/libcpu/arm/cortex-m3/cpuport.c
index 66e2aa48093727858812aca7442968a7fc12b4ac..0f09930554aba1c879d88d2c215bf86288fb5580 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 f060fa122d3cbd0545035bd43d5377777331c1f8..717b4ba3df903b81aa73106741f4e3d51c2bfcbb 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 6b748255c594f376a5472c909c3a0df5e6c72c21..274cc19c8805be1deb8362a66748b71710a8a164 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"