I have been reading about the exception mechanism of Cortex-M (M4 to be precise). The exception request is accepted by the processor if the current priority level of the processor is less than the incoming exception (this is one of the conditions to accept the request). So I am a bit confused about what is meant by 'Current priority level of processor'. I can guess that when the processor is in handler mode, it will be the priority level of that exception. But when the processor is in thread mode, say just running a normal user program after reset, what is the priority level of the processor?
If it is in thread mode, and if none of the interrupt masking registers (e.g. PRIMASK, FAULTMASK, BASEPRI) is set, we call this a base level.
The architecture implies that in this state the priority level is 256.
If you have the ARMv7-M Architecture Reference Manual (issue D), please see section B1.5.4, page B1-639.
Hope this helps.
This means that without any of these registers, the current priority is the lowest one (256) and thus any exception is accepted. The current priority can be raised with the help of PRIMASK, FAULTMASK, BASEPRI.
When PRIMASK is 1 - This raises the execution priority to zero and only the exceptions with negative priority numbers like NMI, Hardfault etc. will be accepted.
When FAULTMASK is 1 - This raises the execution priority -1 and so only the exceptions more negative than Hardfault so only NMI is accepted (and Reset).
When BASEPRI is non-zero (say x )- This raises the execution priority to value = x and only those exceptions with higher priority (lower number than x ) are accepted.
Is this correct?
Yes, that's correct.
Hi I know this thread s old but I would like to ask about it too.
In ARM documentation is written somehing like this:
To access the exception mask registers use the MSR and MRS instructions, or the CPS instruction to change the value of PRIMASK or FAULTMASK
0 = no effect
1 = prevents the activation of all exceptions with configurable priority.
So if I want to use MRS instruction to reset this bit how to do this. Lets say not to use CPS. ARM says written 0 has no effect! Why it is so?
I think you need a newer version of the reference manual. I see the wording you quoted in the SAM3X8 datasheet, but NOT in either the current ARMv6m or ARMv7m Architecture Reference Manuals. (ie https://developer.arm.com/documentation/ddi0403/d/System-Level-Architecture/System-Level-Programmers--Model/Registers/The-special-purpose-mask-registers )
Those are also clear in the MSR instruction definition that it copies the low order bit of the specified register to the PRIMASK.PM bit.