The role of system mode in arm

Only after paying money and being forced to learn, will you work hard to study it. It's really cheap.

I knew before that arm has 7 basic working modes:
FIQ, IRQ enter by interruption,
UNDEF, ABORT enter by program exception,
SVC enter by power-on and soft interrupt
, user enters by SVC handler
But there is also a system, which uses the same registers as user, but does not have SPSR, and can also execute privileged instructions
Such an alternative, it is not safe for OS to use it as user, and there is no automatic entry method when it is used as an exception or interrupt It is awkward
no matter how you look at it, of course, this is also realized after understanding, system is used to solve the problem of reentrant interrupts in arm

I searched and found that there is only one explanation about reentrant interrupts in arm on the Internet, and it does not consider it completely http://blog.chinaunix.net/u1/58640/showart_513501.html
The default interrupt handling function will push lr to the stack first, this article does not consider this point, and thinks the damage is serious

Such theoretical knowledge is rarely used in practice. Only those who pursue professionalism will delve into it, so that they can meet special requirements and do things that ordinary people cannot do.

First of all, the armcc keyword __irq cannot be used to write reentrant interrupts.
When the interrupt is reentrant, use BL to call the sub-function in the interrupt handling function. At this time, the problem comes. There are two situations.
If the sub-function is not declared in the __irq way and does not have the habit of pushing lr on the stack, then if there is another irq interrupt at this time, lr_irq will be flushed by the new value
, and it will fall into an endless loop.
If the sub-function is a good kid who will actively push lr on the stack, then only when the instruction of pushing lr is executed, the irq interrupt will cause the above problem, but the conditions for triggering the problem become very harsh.

So, at this time, the system mode is needed to save the world.
To write a reentrant interrupt, you must use the following assembly code

IRQHandler
        ;LR_IRQ ,SPSR_IRQ,r12 is pushed onto the stack to prevent the next IRQ interrupt from flushing it
        sub lr,lr,#4
        stmfd sp!,{lr}
        mrs r14,spsr
        STMFD sp!,{r12,r14}  

        ; Read and clear interrupt controller interrupt source
        ; omit related code         
        mov r12,#IntBase
        LDR r12,[r12,#IntSource]

        ;Switch to system mode and enable IRQ
        mrs r14,cpsr
        bic r14,r14,#0x9f
        orr r14,r14,#0x1f
        msr ​​CPSR_c,r14

        ;Save r0-r3,LR_user to the user stack, then call the c subroutine, and pass the interrupt source r0 as a parameter to the c processing function
        BL C_irq_handler;Looking at the name, you know that this c program is declared in the __irq form
                                ; it is a good kid who can push LR on the stack by itself
        LDMFD sp!,{r0-r3,lr}

        ;Switch to IRQ mode and disable IRQ
        mrs r12,cpsr
        bic r12,r12,#0x1f
        orr r12,r12,#0x92
        msr CPSR_c,r12

        ;Restore LR_irq, SPSR_irq and working register r12, then exit IRQ
        LDMFD sp!,{r12,r14}
        msr SPSR_csxf,r14
        LDMFD sp!,{PC}^