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By Sauermann J., Thelen M.

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After finishing the critical section, it enters supervisor mode again at T=13 and returns to user mode with interrupts enabled (via S7 at T=14) at T=15. 3. Kernel Implementation 41 As already mentioned, we check for tasks switches at every return to user mode. Instead, it would also be possible to switch tasks immediately, whenever desired. However, it is of no use to switch tasks while in supervisor mode, as the task switch would come into effect only at return to user mode. Switching tasks immediately could lead to several task switches while in supervisor mode, but only one of these task switches would have any effect.

1 shows the part of the kernel that is (along with the functions called from there) executed in supervisor mode. S. S is divided into the start-up code, functions for accessing the hardware, interrupt service routines, the task switch (scheduler), and the semaphore functions that are written in assembler for performance reasons. 1 shows the rest of the kernel, which is executed in user mode. e. every arrow from the middle to the lower part is related to one or several TRAP instructions which cause a change to supervisor mode.

To avoid direct usage of these functions, they are made protected so that only classes derived from RingBuffer can use them. 33 34 35 36 37 38 ... template inline void RingBuffer::GetItem(Type & dest) { dest = data[get++]; if (get >= size) get = 0; count--; } 3. Kernel Implementation 40 41 42 43 44 45 53 template inline void RingBuffer::PutItem(const Type &src) { data[put++] = src; if (put >= size) put = 0; count++; } Finally, it has shown to be useful to provide polled access to both ends of a queue, even if semaphores are used.

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