Implementing OS_Tick API in RTX5 for Cortex-M3

I can see from the OS Tick API docs that any clock source I use for RTX5 ticking must override the nine weak routines that comprise the API.

My question is then, how do I code up the interrupt handler itself?  I'm trying this on an EFM32GG, using the RTC peripheral to replace the Systick used by RTX5 as default.

Can I implement my ISR in C, or does it have to follow the form of RTX5's own SysTick_Handler (in RTX/Source/GCC/irq_cm3.S):

SysTick_Handler:

        PUSH     {R0,LR}                // Save EXC_RETURN
        BL       osRtxTick_Handler      // Call osRtxTick_Handler
        POP      {R0,LR}                // Restore EXC_RETURN
        MRS      R12,PSP
        B        SVC_Context

If I implement my handler in C and just call osRtxTick_Handler, I'd be missing out on the

MRS R12, PSP

B SVC_Context

which I imagine are crucial for correct operation.

I am also unclear how the second parameter to OS_Tick_Setup is used.  In RTX5, this function is called by svcRtxKernelStart, passing OS_TICK_HANDLER which is a define for SysTick_Handler?  So I have to redefine OS_TICK_HANDLER to be MY isr, i.e. RTC_IRQHandler?

I have been unable to find ANY non-SysTick impl for RTX5 using Cortex-M3, if anyone knows of one, please share.

Stuart