Hi all, This is my second message on the issue to clarify the question. From the documentation: **************** Standart C51 functions must not be called simultaneously from several tasks or interrupt procedures. These functions store their parameters and local data in static memory segments; for this reason, this data is overwritten by a repetitive call. In order to solve this problem, C51 provides reentrant functions. ..... Since RTX51 Tiny does not contaion any management for the reentrant stack, the round robin task switching must be disabled when reentrant functions are used. RTX51 Tiny can manage reentrant functions in the compact model. **************** In the light of these, can I use reentrant functions with task switching in the compact model? If I cannot use task switching with reentrant function, where is the multi-tasking property of RTX51? If round-robin task switching disabled, is a task switching performed when an os_wait() function is executed? regards,