Editing Real-time operating system (section)

===Temporarily masking/disabling interrupts===
General-purpose operating systems usually do not allow user programs to mask (disable) [[interrupt]]s, because the user program could control the CPU for as long as it is made to. Some modern CPUs do not allow [[user mode]] code to disable interrupts as such control is considered a key operating system resource. Many embedded systems and RTOSs, however, allow the application itself to run in [[kernel mode]] for greater [[system call]] efficiency and also to permit the application to have greater control of the operating environment without requiring OS intervention.

On single-processor systems, an application running in kernel mode and masking interrupts is the lowest overhead method to prevent simultaneous access to a shared resource. While interrupts are masked and the current task does not make a blocking OS call, the current task has ''exclusive'' use of the CPU since no other task or interrupt can take control, so the [[critical section]] is protected. When the task exits its critical section, it must unmask interrupts; pending interrupts, if any, will then execute. Temporarily masking interrupts should only be done when the longest path through the critical section is shorter than the desired maximum [[interrupt latency]]. Typically this method of protection is used only when the critical section is just a few instructions and contains no loops. This method is ideal for protecting hardware bit-mapped registers when the bits are controlled by different tasks.