Hi again,

I've been wanting to get into microcontrollers for while now.

Initially I wanted to start with AVR/arduino. Compared to PIC they have larger memory, their assembler language is far saner than that of PIC's and the AVR's toolchain is "nicer". There is also the matter of larger userbase and the convenience arduino boards themselves.

However the is one major thing bugging me about AVR chips and that is the lack of a read-modify-write instruction for critical sections of code. AVR only supports this on the larger chips like the AVR32, but with PIC you get it from the PIC12 and upwards IIRC.

I just don't feel comfortable not having a RMW instruction. Sure you can do it in software, but it would not be 100% reliable.

Are my concerns about RMW instructions justified? Or is that too much of an edge case?

Thanks,

Richard

 

Having RMW instructions is critical for software reliability in the case of fault tolerant process synchronization. You can fake it with mutual exclusion but the possibility of lock corruption due to a process hanging or crashing makes it a poor substitute.

http://en.wikipedia.org/wiki/Consensus_%28computer_science%29#Applications_of_consensus_protocols

But if all you're writing is single threaded app with a few ISR driven data streams I wouldn't lose any sleep over it.

 

The danger with a lack of RMW instructions is if an interrupt changes the I/o at the same time a none RMW write to a port occurs. The I/o will get screwed up.
The only way around is to disable interrupts while the software RMW takes place.

 

As far as I know, all RMW instructions must complete execution even if an interrupt occurs while the instruction is in mid-execution.

AFSIK, the Atmel AVR does not suffer from any such deficiency.
Perhaps you can enlighten me on this.