21.8.5 Protect Mode

The Protect mode is used to read the Interrupt Vector register without performing the associated automatic operations. This is necessary when working with a debug system. When a debugger, working either with a Debug Monitor or the ARM processor's ICE, stops the applications and updates the opened windows, it might read the AIC User Interface and thus the IVR. This has adverse consequences:

  • If an enabled interrupt with a higher priority than the current one is pending, it is stacked.
  • If there is no enabled pending interrupt, the spurious vector is returned.

In either case, an End of Interrupt command is necessary to acknowledge and restore the context of the AIC. This operation is generally not performed by the debug system, as the debug system would become strongly intrusive and cause the application to enter an undesired state.

This is avoided by using the Protect mode. Writing PROT in the Debug Control register (AIC_DCR) at 0x1 enables the Protect mode.

When the Protect mode is enabled, the AIC performs interrupt stacking only when a write access is performed on AIC_IVR. Therefore, the Interrupt Service Routines must write (arbitrary data) to AIC_IVR just after reading it. The new context of the AIC, including the value of AIC_ISR, is updated with the current interrupt only when AIC_IVR is written.

An AIC_IVR read on its own (e.g., by a debugger) modifies neither the AIC context nor AIC_ISR. Extra AIC_IVR reads perform the same operations. However, it is recommended to not stop the processor between the read and the write of AIC_IVR of the interrupt service routine to make sure the debugger does not modify the AIC context.

To summarize, in normal operating mode, the read of AIC_IVR performs the following operations within the AIC:

  1. Calculates active interrupt (higher than current or spurious).
  2. Determines and returns the vector of the active interrupt.
  3. Memorizes the interrupt.
  4. Pushes the current priority level onto the internal stack.
  5. Acknowledges the interrupt.

However, while the Protect mode is activated, only operations 1 to 3 are performed when AIC_IVR is read. Operations 4 and 5 are only performed by the AIC when AIC_IVR is written.

Software that has been written and debugged using the Protect mode runs correctly in normal mode without modification. However, in normal mode, the AIC_IVR write has no effect and can be removed to optimize the code.