diff --git a/doc/sys_arch.txt b/doc/sys_arch.txt
index 95d0add7..b2c45e03 100644
--- a/doc/sys_arch.txt
+++ b/doc/sys_arch.txt
@@ -143,6 +143,13 @@ to be implemented as well:
   more information. This function is only required if your port is supporting
   an operating system.
 
+Note:
+
+Be carefull with using mem_malloc() in sys_arch. When malloc() refers to
+mem_malloc() you can run into a circular function call problem. In mem.c
+mem_init() tries to allcate a semaphore using mem_malloc, which of course
+can't be performed when sys_arch uses mem_malloc.
+
 -------------------------------------------------------------------------------
 Additional files required for the "OS support" emulation layer:
 -------------------------------------------------------------------------------