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What does the word UNPREDICTABLE exactly mean and what is its effect on the system?

Hi,

In many of ARM documentation result of operations are reported as unpredictable in some situations. For example, "If the system includes the Security Extensions, accesses to CNTVOFF from Secure Monitor mode are UNPREDICTABLE." what does the word UNPREDICTABLE exactly mean and what is its effect on the system? would that causes an exception and system switch to a certain mode?

Many thanks.

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  • No you're not guaranteed an interrupt. It depends on what you're talking about and hopefully the reference manual will have a glossary saying what it means in your case. However in general about all you are guaranteed is that you won't be able to do anything that you couldn't do anyway in your current state. For instance you can't read secure data if you're not current executing in monitor mode. And in user mode you shouldn't be able to crash the system or anything like that.

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  • No you're not guaranteed an interrupt. It depends on what you're talking about and hopefully the reference manual will have a glossary saying what it means in your case. However in general about all you are guaranteed is that you won't be able to do anything that you couldn't do anyway in your current state. For instance you can't read secure data if you're not current executing in monitor mode. And in user mode you shouldn't be able to crash the system or anything like that.

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