Arm system call exception assembly

A7 Processor exception swi and M3 Processor exception svc

A special interrupt ：SVCall

sketch ： An interrupt triggered by a program , Default on

origin ：SVC（ System service call , Also called system call ） It is mostly used in software development on the operating system .SVC Call requests used to generate system functions . for example , The operating system does not allow the user program to directly access the hardware , But by providing some system service functions , User program use SVC Make a call request to the system service function , Call them in this way to access the hardware indirectly . therefore , When a user program wants to control specific hardware , It will create a SVC abnormal , And then the operating system provides SVC Exception service routine executed , It then calls the relevant operating system functions , The latter completes the service requested by the user program .

purpose ： Can be set by , Enable a piece of code to be interrupted by certain interrupts , Instead of being interrupted by other interruptions , For example, it can be used to ensure simulation IIC The time sequence of is not interrupted, resulting in communication failure

Be careful ：

• 　SVC   Exceptions must be responded to immediately （ If the priority is not higher than the current one , Or other reasons that make it impossible to immediately   Respond to , Will lead to HardFault）
•     The concept of master-slave priority is the same as that of ordinary interrupts （ And have the same status , Even the special interrupt is not special ）// I wish ： By default , except HardFault and NMI, The priority of other interrupts is 0,0（ Additional reminders ,group Setting must precede priority Set up ）,！！BUT！！, The setting of interrupt priority needs to be called ：NVIC_SetPriority(SVCall_IRQn,NVIC_EncodePriority(NVIC_GetPriorityGrouping(), 1, 1));

  stay C Use in SVCall

　　SVC The service function uses the stack to pass parameters , so C Language version of the SVC The service function requires an assembly operation , Used to extract parameters from the stack into registers

__asm void SVC_Handler(void) // The function name is in Keil Chung Tung USART2_IRQHandler wait 
{

//   Assembly operations , Used to propose the starting position of the stack frame , And on the R0 in , Then jump to the actual SVC In the service routine  
IMPORT svc_handler 
TST LR, #4 
ITE EQ 
MRSEQ R0, MSP 
MRSNE R0, PSP 
B svc_handler 
}

// “ real ” Service function of , Accept a pointer parameter （pwdSF）： Stack the starting address of the stack . 
// pwdSF[0] = R0 , pwdSF[1] = R1 
// pwdSF[2] = R2 , pwdSF[3] = R3 
// pwdSF[4] = R12, pwdSF[5] = LR 
// pwdSF[6] =  The return address （ On stack PC） 
// pwdSF[7] = xPSR 
unsigned long svc_handler(unsigned int* pwdSF) 
{ 
unsigned int svc_number; 
unsigned int svc_r0; 
unsigned int svc_r1; 
unsigned int svc_r2; 
unsigned int svc_r3; 
int retVal; // Used to store the return value  
svc_number = ((char *) pwdSF[6])[-2]; //  I didn't think! ,C The array of can be used so much ！ 
svc_r0 = ((unsigned long) pwdSF[0]); 
svc_r1 = ((unsigned long) pwdSF[1]); 
svc_r2 = ((unsigned long) pwdSF[2]); 
svc_r3 = ((unsigned long) pwdSF[3]); 
printf (“SVC number = %xn”, svc_number); 
printf (“SVC parameter 0 = %x\n”, svc_r0); 
printf (“SVC parameter 1 = %x\n”, svc_r1); 
printf (“SVC parameter 2 = %x\n”, svc_r2); 
printf (“SVC parameter 3 = %x\n”, svc_r3); 
// Do some work , And store the return value in retVal in  
pwdSF[0]=retVal; 
return 0; 
}

// Be careful , This function does not return 0！ Further , Gray text is only used to make the compiler happy , Specific reference Cortex-M3 Authoritative guide P169

How to trigger an interrupt ？

step1.   Declare functions （__svc The corresponding function will be automatically generated ）//__svc when keil A macro in

unsigned long __svc(0x03) CallSvc3(unsigned long svc_r0, unsigned long 
svc_r1, unsigned long svc_r2, unsigned long svc_r3);

step2.  Call function

unsigned long svcRet; // The return value of the system service  
svcRet=CallSvc3(p0, p1, p2, p3); //  call 3 System service No , And deliver 4 Parameters , In turn ：p1,p2,p3,p4, Then receive the return value to svcRet in （ Don't forget , The origin of this return value is unusual ）

Re:STM32 SVCall

flyangchina 2016-08-19 17:25

Pretty good , stay svc_handler There should be code to call the corresponding function in .
You can add a SVC_Table Array to hold function pointers , Such as this ：

extern int __SVC_0(int, int);
extern int __SVC_1(int);
void * SVCTable[256] = {
	__SVC_0,
	__SVC_1
};

Then write like this svc_handler writes ：

void svc_handler(unsigned int* pwdSF) {

	//  Use r0~r3 Call the corresponding function for the parameter ,
        //  And store the return value in the caller's stack r0 It's about  
	pwdSF[0] = ((int (*)(int, int, int, int))SVCTable[((char *) pwdSF[6])[-2]])
		(
			pwdSF[0], 
			pwdSF[1], 
			pwdSF[2], 
			pwdSF[3]
		);
}

Add a little more __SVC_0 The definition of , In a random file ,__SVC_1 Forget it ：

int __SVC_0      (int i1, int i2) {
  return (i1 + i2);
}

Use as follows ：

int __svc(0) add (int i1, int i2);

...

void test() {
    add(1,1);
}

int main() { ... }

In this way, there is a whole use svc Realize the simple architecture of system call

Re:STM32 SVCall

Serval 2015-02-10 16:10

About “svc_number = ((char *) pwdSF[6])[-2]; // I didn't think! ,C The array of can be used so much ！ ” Usage of , Looked at half a day , Post it to see if you understand it ：

First, the ultimate goal of this line of code is to get svc #num Medium Num. stay Thumb Code (STM32 Most of them should be Cortex . ) in , An instruction is usually 16 position ,svc #num The instructions of are usually translated as 0xDFxx --> among xx Namely #num. therefore ,Keil The manual has the following explanation ：
0 to 224–1 (a 24-bit value) in an ARM instruction.
0-255 (an 8-bit value) in a Thumb instruction.

The code can be translated into something easy to understand ：
*((char *)pwdSF[6] - 2)

give an example ：
Suppose the system is configured as a small end , Lower the low byte .
Instructions for ：svc #1 ---> Binary for 0xDF01, The address is 0x286.
So actually , Pushed into the stack PC, Deposit is 0x288. because PC What is pressed in is the next instruction .
0x286: DF01 ;svc #1
0x288: ......
Stack: 88 02 00 00
That strange line of code is to get this 01. Several steps ：
1. Convert to (char*)pwdSF[6], Parallel subtraction 2
This step is to make the pointer point to 88 02 The location of .
(char*)pwdSF[6] - 2
2. Pointer value
*((char *)pwdSF[6] - 2)
3. Pointer access is changed to the following table access form
The pointer value can be written in the form of subscript , for example ：*(p + i) Equate to p[i], I suggest that the pointer should be written in the second way
((char *) pwdSF[6])[-2]