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PIC32 CP0 operations

Posted by Eric Lofstad on February 15, 2010
The code for the PIC32 in portmacro.h does a read-modify-write on CP0 to control stuff like interrupts, which does not seem safe. For example this code enables interrupts.
ulStatus = _CP0_GET_STATUS()
ulStatus &= ~portALL_IPL_BITS;
_CP0_SET_STATUS( ulStatus );
why do that rather than this

RE: PIC32 CP0 operations

Posted by Richard on February 16, 2010
The code generated by your suggestion is:

9D000020 40026000 mfc0 v0,Status << read
9D000024 00401821 addu v1,v0,zero
9D000028 3C02FFFF lui v0,0xffff
9D00002C 344203FF ori v0,v0,0x3ff
9D000030 00621024 and v0,v1,v0
9D000034 40826000 mtc0 v0,Status << write

so it is still performing a read/modify/write sequence so I'm not sure what is to be gained.

That said, the original code is probably less than ideal, however I don't think it is problematic because each task has its own Status value, and its own stack onto which ulStatus is stored.


RE: PIC32 CP0 operations

Posted by Eric Lofstad on February 16, 2010
Thanks for the reply, I should have looked at the assembly first.
I'm still left wondering how atomic the operation is, but the is independent of FreeRTOS.


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