配置timer2溢出中断，ack ID 1023产生中断，是什么问题？
.align 4
.type IRQ_Handler, %function
IRQ_Handler:
/* Return to the interrupted instruction. */
SUB lr, lr, #4
/* Push the return address and SPSR. */
PUSH {lr}
MRS lr, SPSR
PUSH {lr}
/* Change to supervisor mode to allow reentry. */
CPS #SVC_MODE
/* Push used registers. */
PUSH {r0-r4, r12}
/* Increment nesting count. r3 holds the address of ulPortInterruptNesting
for future use. r1 holds the original ulPortInterruptNesting value for
future use. */
LDR r3, ulPortInterruptNestingConst
LDR r1, [r3]
ADD r4, r1, #1
STR r4, [r3]
/* Read value from the interrupt acknowledge register, which is stored in r0
for future parameter and interrupt clearing use. */
LDR r2, ulICCIARConst
LDR r2, [r2]
LDR r0, [r2]
/* Ensure bit 2 of the stack pointer is clear. r2 holds the bit 2 value for
future use. _RB_ Does this ever actually need to be done provided the start
of the stack is 8-byte aligned? */
MOV r2, sp
AND r2, r2, #4
SUB sp, sp, r2
/* Call the interrupt handler. r4 pushed to maintain alignment. */
PUSH {r0-r4, lr}
LDR r1, vApplicationIRQHandlerConst
BLX r1
POP {r0-r4, lr}
ADD sp, sp, r2
CPSID i
DSB
ISB
/* Write the value read from ICCIAR to ICCEOIR. */
LDR r4, ulICCEOIRConst
LDR r4, [r4]
STR r0, [r4]
/* Restore the old nesting count. */
STR r1, [r3]
/* A context switch is never performed if the nesting count is not 0. */
CMP r1, #0
BNE exit_without_switch
/* Did the interrupt request a context switch? r1 holds the address of
ulPortYieldRequired and r0 the value of ulPortYieldRequired for future
use. */
LDR r1, =ulPortYieldRequired
LDR r0, [r1]
CMP r0, #0
BNE switch_before_exit
exit_without_switch:
/* No context switch. Restore used registers, LR_irq and SPSR before
returning. */
POP {r0-r4, r12}
CPS #IRQ_MODE
POP {LR}/*执行到这一步的时候，lr值就不对，*/
MSR SPSR_cxsf, LR
POP {LR}
MOVS PC, LR
switch_before_exit:
/* A context swtich is to be performed. Clear the context switch pending
flag. */
MOV r0, #0
STR r0, [r1]
/* Restore used registers, LR-irq and SPSR before saving the context
to the task stack. */
POP {r0-r4, r12}
CPS #IRQ_MODE
POP {LR}
MSR SPSR_cxsf, LR
POP {LR}
portSAVE_CONTEXT
/* Call the function that selects the new task to execute.
vTaskSwitchContext() if vTaskSwitchContext() uses LDRD or STRD
instructions, or 8 byte aligned stack allocated data. LR does not need
saving as a new LR will be loaded by portRESTORE_CONTEXT anyway. */
LDR R0, vTaskSwitchContextConst
BLX R0
/* Restore the context of, and branch to, the task selected to execute
next. */
portRESTORE_CONTEXT