Task Switching

As mentioned earlier, user code can often unwittingly initiate a context switch by reading from the console (and other miscellaneous things). There are a few situations where this can cause a problem, namely inside interrupt handlers. While the kernel makes an attempt to prevent this, it cannot predict every conceivable problem. The kernel attempts to detect and prevent context switches inside interrupt handlers by checking for the following situations.

• Is the system busy flag non-zero? (The busy flag is located at address $E100FF.)

• Is the “No-Compact” flag set? (Located at $E100CB.)

• Does the stack pointer point to anything in thr range $0100-$01FF?

• Is the interrupt bit in the processor status register set?

If any of these conditions are met, a context switch will not take place. This can cause problems in certain circumstances. The basic rule is to avoid making Kernel calls that might cause a context switch or change in process state from inside an interrupt handler. This includes the following:

• reading from the console
• accessing a pipe
• any of the following kernel traps: _execve(2) (or other calls in the exec family), fork(2), fork2(2), kill(2), pause(2), procreceive(2), sigpause(2), or wait(2).

Calls such as procsend(2), however, may be used from inside an interrupt handler, and in fact are very useful in such situations.