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Saving the Context
[RTOS Implementation Building Blocks]

Each real time task has it's own stack memory area so the context can be saved by simply pushing processor registers onto the task stack. Saving the AVR context is one place where assembly code is unavoidable.

portSAVE_CONTEXT() is implemented as a macro, the source code for which is given below:

#define portSAVE_CONTEXT()           \
asm volatile (	                     \
  "push  r0                    \n\t" \ (1)
  "in    r0, __SREG__          \n\t" \ (2)
  "cli                         \n\t" \ (3)
  "push  r0                    \n\t" \ (4)
  "push  r1                    \n\t" \ (5)
  "clr   r1                    \n\t" \ (6)
  "push  r2                    \n\t" \ (7)
  "push  r3                    \n\t" \
  "push  r4                    \n\t" \
  "push  r5                    \n\t" \

    :
    :
    :

  "push  r30                   \n\t" \
  "push  r31                   \n\t" \
  "lds   r26, pxCurrentTCB     \n\t" \ (8)
  "lds   r27, pxCurrentTCB + 1 \n\t" \ (9)
  "in    r0, __SP_L__          \n\t" \ (10)
  "st    x+, r0                \n\t" \ (11)
  "in    r0, __SP_H__          \n\t" \ (12)
  "st    x+, r0                \n\t" \ (13)
);
 

Referring to the source code above:

• Processor register R0 is saved first as it is used when the status register is saved, and must be saved with its original value.
• The status register is moved into R0 (2) so it can be saved onto the stack (4).
• Processor interrupts are disabled (3). If portSAVE_CONTEXT() was only called from within an ISR there would be no need to explicitly disable interrupts as the AVR will have already done so. As the portSAVE_CONTEXT() macro is also used outside of interrupt service routines (when a task suspends itself) interrupts must be explicitly cleared as early as possible.
• The code generated by the compiler from the ISR C source code assumes R1 is set to zero. The original value of R1 is saved (5) before R1 is cleared (6).
• Between (7) and (8) all remaining processor registers are saved in numerical order.
• The stack of the task being suspended now contains a copy of the tasks execution context. The kernel stores the tasks stack pointer so the context can be retrieved and restored when the task is resumed. The X processor register is loaded with the address to which the stack pointer is to be saved (8 and 9).
• The stack pointer is saved, first the low byte (10 and 11), then the high nibble (12 and 13).

Next: RTOS Implementation - Restoring The Context

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