diff --git a/rtl/msdos/prt0.asm b/rtl/msdos/prt0.asm
index 04cb06abb4..f1454f4851 100644
--- a/rtl/msdos/prt0.asm
+++ b/rtl/msdos/prt0.asm
@@ -89,7 +89,7 @@ FPC_MSDOS_CARRY:
         stc
         global FPC_MSDOS
 FPC_MSDOS:
-        mov ax, 21h
+        mov al, 21h  ; not ax, because only the low byte is used
         pop dx
         pop cx
         push ax