systick.c

__attribute__((naked)) void SysTick_Handler(void)
{
	/// STEP 1 - SAVE THE CURRENT TASK CONTEXT

	/// At this point the processor has already pushed PSR, PC, LR, R12, R3, R2, R1 and R0
	/// onto the stack. We need to push the rest(i.e R4, R5, R6, R7, R8, R9, R10 & R11) to save the
	/// context of the current task.
	/// Disable interrupts
    __asm("CPSID   I");
    /// Push registers R4 to R7
    __asm("PUSH    {R4-R7}");
    /// Push registers R8-R11
    __asm("MOV     R4, R8");
    __asm("MOV     R5, R9");
    __asm("MOV     R6, R10");
    __asm("MOV     R7, R11");
    __asm("PUSH    {R4-R7}");
    /// Load R0 with the address of pCurntTcb
    __asm("LDR     R0, =pCurntTcb");
    /// Load R1 with the content of pCurntTcb(i.e post this, R1 will contain the address of current TCB).
    __asm("LDR     R1, [R0]");
    /// Move the SP value to R4
    __asm("MOV     R4, SP");
    /// Store the value of the stack pointer(copied in R4) to the current tasks "stackPt" element in its TCB.
    /// This marks an end to saving the context of the current task.
    __asm("STR     R4, [R1]");


    /// STEP 2: LOAD THE NEW TASK CONTEXT FROM ITS STACK TO THE CPU REGISTERS, UPDATE pCurntTcb.

    /// Load the address of the next task TCB onto the R1.
    __asm("LDR     R1, [R1,#4]");
    /// Load the contents of the next tasks stack pointer to pCurntTcb, equivalent to pointing pCurntTcb to
    /// the newer tasks TCB. Remember R1 contains the address of pCurntTcb.
    __asm("STR     R1, [R0]");
    /// Load the newer tasks TCB to the SP using R4.
    __asm("LDR     R4, [R1]");
    __asm("MOV     SP, R4");
    /// Pop registers R8-R11
    __asm("POP     {R4-R7}");
    __asm("MOV     R8, R4");
    __asm("MOV     R9, R5");
    __asm("MOV     R10, R6");
    __asm("MOV     R11, R7");
    /// Pop registers R4-R7
    __asm("POP     {R4-R7}");
    __asm("CPSIE   I ");
    __asm("BX      LR");
}