rudi1895 wrote on Tuesday, September 27, 2016:
The example files named portASM.s in the FreeRTOS V9.0.0 and earlier demos for Cortex-A and Cortex-R devices contain the following instruction sequence:
/* Ensure bit 2 of the stack pointer is clear. r2 holds the bit 2 value for
future use. */
MOV r2, sp
AND r2, r2, #4
SUB sp, sp, r2
/* Call the interrupt handler. */
PUSH {r0-r3, lr}
LDR r1, vApplicationIRQHandlerConst
BLX r1
POP {r0-r3, lr}
ADD sp, sp, r2
The first three instructions adjust the IRQ stack to be 8-byte aligned. But the following instruction pushes five registers on the stack. IMHO this push will result in an unaligned stack for the called handler.
The instruction sequence proposed for this purpose by ARM pushes two registers and thus keeps the alignment. There is a link to a document on the stack alignment from ARM on the mentioned page.
As I just came across looking for a working IRQ handler for my bare metal application without a (Free)RTOS, I cannot check my assumption. But there are questions in this forum on unaligned stacks which indicate problems as discussed in the document from ARM.
BTW: Comparing the IRQ handlers from ARM and from FreeRTOS there seems to be room for some optimization in FreeRTOS IRQ handler.
HTH Rudi