kopia lustrzana https://github.com/micropython/micropython
docs: Describe issue with hard ISRs and globals.
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@ -170,11 +170,17 @@ is partially updated. When the ISR tries to read the object, a crash results. Be
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on rare, random occasions they can be hard to diagnose. There are ways to circumvent this issue, described in
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:ref:`Critical Sections <Critical>` below.
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It is important to be clear about what constitutes the modification of an object. An alteration to a built-in type
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such as a dictionary is problematic. Altering the contents of an array or bytearray is not. This is because bytes
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or words are written as a single machine code instruction which is not interruptible: in the parlance of real time
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programming the write is atomic. A user defined object might instantiate an integer, array or bytearray. It is valid
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for both the main loop and the ISR to alter the contents of these.
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It is important to be clear about what constitutes the modification of an object. Altering the contents of an array
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or bytearray is safe. This is because bytes or words are written as a single machine code instruction which is not
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interruptible: in the parlance of real time programming the write is atomic. The same is true of updating a
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dictionary item because items are machine words, being integers or pointers to objects. A user defined object might
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instantiate an array or bytearray. It is valid for both the main loop and the ISR to alter the contents of these.
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The hazard arises when the structure of an object is altered, notably in the case of dictionaries. Adding or deleting
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keys can trigger a rehash. If a hard ISR runs while a rehash is in progress and attempts to access an item, a crash
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may occur. Internally globals are implemented as a dictionary. Consequently the main program should create all
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necessary globals before starting a process that generates hard interrupts. Application code should also avoid
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deleting globals.
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MicroPython supports integers of arbitrary precision. Values between 2**30 -1 and -2**30 will be stored in
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a single machine word. Larger values are stored as Python objects. Consequently changes to long integers cannot
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