[En-Nut-Discussion] Stack size of exeptions in ARM.

[rtems]

Hi,

I have begun porting ethernut to AT91SAM7X MCU from ETHERNUT3 package. With WinARM and openocd, I can debug the software on an AT91SAM7X-EK clone board now.

When working on the assembly start code, I noticed these lines to set the exception stacks:

IRQ_STACK_SIZE = (3*8*4)     @ 3 words per interrupt priority level
FIQ_STACK_SIZE = (3*4)       @ 3 words
ABT_STACK_SIZE = (1*4)       @ 1 word
UND_STACK_SIZE = (1*4)       @ 1 word

…
        .section .init2,"ax",%progbits
        /*
         * Set exception stack pointers and enable interrupts.
         */
        ldr     r0, =__xcp_stack
        msr     CPSR_c, #ARM_MODE_FIQ | 0xC0
        mov     r13, r0
        sub     r0, r0, #FIQ_STACK_SIZE
        msr     CPSR_c, #ARM_MODE_IRQ | 0xC0
        mov     r13, r0
        sub     r0, r0, #IRQ_STACK_SIZE
        msr     CPSR_c, #ARM_MODE_ABORT | 0xC0
        mov     r13, r0
        sub     r0, r0, #ABT_STACK_SIZE
        msr     CPSR_c, #ARM_MODE_UNDEF | 0xC0
        mov     r13, r0
        sub     r0, r0, #UND_STACK_SIZE
        msr     CPSR_c, #ARM_MODE_SVC | 0xC0
        mov     r13, r0
        b       __enter_user
...

I understand ARM uses different stacks in every mode. But why the stack size for every mode is so small?

For example, FIQ_STACK_SIZE is only 3 words, while the following IRQ_ENTRY code already uses more then it.

#define IRQ_ENTRY() \
    asm volatile("sub   lr, lr,#4"          "\n\t"  /* Adjust LR */ \
                 "stmfd sp!,{r0-r12,lr}"    "\n\t"  /* Save registers on IRQ stack. */ \
                 "mrs   r1, spsr"           "\n\t"  /* Save SPSR */ \
                 "stmfd sp!,{r1}"           "\n\t")     /* */

I ever guessed that the exceptions in ethernut can not be nested.  But if so, all them may share the same stack and need not deal with these several xxx_STACK_SIZE.

I’m a newbie to ARM. Any comment is welcome.

Thanks.

HU Daidai
2006-05-29