Question

I have a startup code for ARM926ej-s which supports ISA ARMv5TEJ. Startup code looks like below, but i switched some commands/data with comments for clarity. Lines which are still unclear to me are marked with a comment “@???????????????????????????????????????????????????????“.

I wonder, why do we need to substract “#4” from value (location of “arm926ejs_reset“) stored in register “r3“? And then load it into a stack pointer 4 lines later, where we are setting the stacks for the first processor mode which is fast interrupt mode.

__start:
arm926ejs_reset:

@here there is image header data needed by ISROM

arm926ejs_reset_handler:

    @here we disable MMU, I-cache, D-cache, flush them... and prepare the mcpu.

    LDR   r5, =inVirtMem

    @here we enable MMU.

    MOV   pc, r5 

    NOP
    NOP
    NOP

inVirtMem:
    ADR   r3, arm926ejs_reset @load location of "arm926ejs_reset" label in r3
    MOV   r1, #IF_MASK

    SUB   r3, r3, #4 @???????????????????????????????????????????????????????

    ORR   r0, r1, #MODE_FIQ
    MSR   cpsr_cxsf, r0
    MOV   sp, r3 @???????????????????????????????????????????????????????????
    SUB   r3, r3, #FIQ_STACK_SIZE

...

Answer 1

I found similar startup codes as yours on the net which have their code well commented. In this start up code, you can see on Line 186 that r3 is pointing to the MMU base address. MMU spans ahead of its base address. i.e if MMU_BASE_ADDRESS=0x400, MMU entries will be at 0x400, 0x404 0x408 etc. Now in Line 217 r3 is being subtracted by 4 and then assigned to SP on Line 222. If we don’t subtract 4, our first entry of stack ( observe stack is growing in the reverse direction of MMU layout ) will overwrite on the first MMU entry! Hence the subtraction.

Likewise in your example if we don’t subtract 4, your stack will overwrite on the reset handler itself. Your program will go for a toss.

One more example is this start up code where they are going to end of IRAM and subtracting 4. End of IRAM implies just outside the IRAM and hence the subtraction to come inside the IRAM

Observe that stack is growing upwards in this particular example( or towards lower addresses ).( ARM stacks can grow in either ways )