Restoring the Context
[RTOS Implementation Building Blocks]

The RTOS macro portRESTORE_CONTEXT() is the reverse of portSAVE_CONTEXT(). The context of the task being resumed was previously stored in the tasks stack. The real time kernel retrieves the stack pointer for the task then POP's the context back into the correct processor registers.

#define portRESTORE_CONTEXT()        \
asm volatile (	
  "lds  r26, pxCurrentTCB      \n\t" \ (1)
  "lds  r27, pxCurrentTCB + 1  \n\t" \ (2)
  "ld   r28, x+                \n\t" \ 
  "out  __SP_L__, r28          \n\t" \ (3)
  "ld   r29, x+                \n\t" \ 
  "out  __SP_H__, r29          \n\t" \ (4)
  "pop  r31                    \n\t" \
  "pop  r30                    \n\t" \

    :
    :
    :

  "pop  r1                     \n\t" \
  "pop  r0                     \n\t" \ (5)
  "out  __SREG__, r0           \n\t" \ (6)
  "pop  r0                     \n\t" \ (7)
);
 

Referring to the code above:

• The FreeRTOS pxCurrentTCB variable holds the address from where the tasks stack pointer can be retrieved. This is loaded into the X register (1 and 2).
• The stack pointer for the task being resumed is loaded into the AVR stack pointer, first the low byte (3), then the high nibble (4).
• The processor registers are then popped from the stack in reverse numerical order, down to R1.
• The status register stored on the stack between registers R1 and R0, so is restored (6) before R0 (7).

Next: RTOS Implementation - Putting It All Together