??? subsp,sp,#4??????? ??? ;decrement sp(to store jump address) ; 先假設sp=0x1014, sp-4=0x1010

??? stmfd?? sp!,{r0}???? ;PUSH the work register to stack(lr does't push because it return to original address)

??????????????????????????? ;sp=sp-4=0x100C,r0->sp[0x100C],

??? ldr? r0,= HandleIRQ? ;load the address of HandleIRQ to r0, HandleIRQ所指向的地址內裝的是IsrIRQ的入口地址

??????????????????? ?????? ;所以下面的語句實際上是加載IsrIRQ的入口地址到r0

??? ldr? r0,[r0]????? ;load the contents(service routine start address? 即IsrIRQ的入口地址) of HandleIRQ to r0

??? str? r0,[sp,#4]????? ;store the contents(ISR) of HandleIRQ to stack, r0也就是IsrIRQ的入口地址to SP

??????????????????????? ;r0->sp[0x100C]+4(=0x1010),but sp=0x100C不變

??? ldmfd?? sp!,{r0,pc}???? ;POP the work register and pc(jump to ISR)? IsrIRQ的入口地址to PC