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Вот чего написаано:
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The time slicing offers some advantages, but it is often unsuitable for process control and difficult to use on 8051 based applications that are characterized by a great number of different tasks; often with a short lifetime. RTX51 thus provides time-slicing capabilities that have been modified for optimized use of the resources available on the 8051 microcontroller.
Specifically, for RTX51, the concept of task is very nearly identical to that of an interrupt service routine. Each task is characterized by a configurable priority level between 0-3. As for interrupts, the priority level feature permits context overlay on the stack, as only the initiated highest priority task takes control of the CPU, i.e. runs. A task is given a time slice only if it is in the waiting state, or if a higher priority task takes control of the CPU.
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Фактически это означает, что существует максимум 4(3) "взаимовытесняющих" задачи, да и те с жёсткими приоритетами, т.е. "вытесняемость" их условная. В остальных случаях надо вручную тормозить задачу (например, с помощью OS_DEL_XX), чтобы она дала дышать остальным (одинакового или меньшего приоритета).
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