Hello,

            Kindly refer code at path     UBOOT / u-boot / cpu / pxa / start.S

 

Code:

            .macro      irq_save_user_regs

      sub   sp, sp, #S_FRAME_SIZE

      stmia sp, {r0 - r12}                /* Calling r0-r12      */

      add   r8, sp, #S_PC

      stmdb r8, {sp, lr}^                 /* Calling SP, LR      */

      str   lr, [r8, #0]                  /* Save calling PC           */

      mrs   r6, spsr

      str   r6, [r8, #4]                  /* Save CPSR                 */

      str   r0, [r8, #8]                  /* Save OLD_R0               */

      mov   r0, sp

      .endm

 

Why there is need to save R0? If we refer ARM architecture (section what all happens when interrupt comes) there is no need to save R0.

 

Moreover, there is no need of writing such big piece of code it can be replaced with below given four liner code:

 

      Sub   lr, lr, #4

            Stmfd    sp! , {r0-r12, lr}

            MRS     r1, spsr

            Stmfd    sp! , {r1}

 

Similarly we can write small code for restore of stack

             .macro     irq_restore_user_regs

                         Ldmfd  sp! , {r1}

                        MSR         cpsr_c ,  R1
                        ldmfd    sp!,  {r0-r12, pc}^

             .endm

 

Can anybody clarify?

 

Regards

Amit Kumar