diff --git a/README.md b/README.md
index 69f3598d..ef1d4851 100644
--- a/README.md
+++ b/README.md
@@ -47,7 +47,7 @@ make boot
 
 You may need to install the following (unless you already have it):
 * GRUB2 bootloader tools - `grub2-common` package (e.g. `apt install grub2-common`)
-* ISO generation tools - `xorriso` package (e.g. `apt install xorriso`)
+* ISO generation tools - `xorriso` and `mtools` package (e.g. `apt install xorriso mtools`)
 
 #### Kernel image build (for example to be used with QEMU)
 
diff --git a/common/usermode.c b/common/usermode.c
index 45915c36..ecc254f1 100644
--- a/common/usermode.c
+++ b/common/usermode.c
@@ -49,8 +49,13 @@ static inline void syscall_restore(void) {
 }
 
 static inline long syscall_return(long return_code) {
-    register long ret asm(STR(_ASM_AX)) = return_code;
-    return ret;
+    /* clang-format off */
+    asm volatile(
+            "mov %[ret], %%" STR(_ASM_AX) "\n"
+            ::[ret] "r"(return_code)
+    );
+    /* clang-format on */
+    return return_code;
 }
 
 static inline void stack_switch(void) {