Introduction to Cortex-M3 register set, assembly language and C language interface

learn uCOS The task switching of involves assembly code . In order to understand assembly code , I learned about it online Cortex-M3 Register group 、C Knowledge of interface with assembly , Share it here .

Let's first introduce Cortex-M3 Register group ：

Cortex-M3 Have 16 General registers R0-R15.

R0-R12 All are 32 Bit general register , For data manipulation .

R13 It's a stack pointer . stay CM3 There are two stack pointers in the processor core , So it supports two stacks . When referencing R13（SP） when , You are referring to the one currently in use , The other must be accessed with special instructions . The two stack pointers are ：

1. Main stack pointer （MSP）： This is the default stack pointer , It consists of OS kernel 、 Exception service program and application code requiring privileged access .
2. Process stack pointer （PSP）： For general application code .

R14 Is the connection register （LR）. Store the returned address when the function is called .

R15 Program counter （PC）. Point to the address of the current program . If you change its value , Will change the execution order of the program （ Many advanced operations are here ）.

Next, the compilation and C The interface of ：

Give Way C When a program and an assembler interact with each other , We must know how parameters are passed , And how the value is returned , In this way, we can coordinate the work between the main function and the subroutine . These interaction mechanisms are ARM There are clear provisions in , By document 《ARM Architecture Procedure Call Standard(AAPCS,Ref5)》( I haven't seen it ) give . Although I didn't read the official documents , I still read it on Baidu C Mixed programming with assembly , And make the following summary ：

1、 When a function call occurs , The entry parameters pass through R0-R3 Register transfer , among R0 Pass on the first ,R1 Pass on section 2 individual ……, When more than 4 When the parameters , Other parameters are passed through the stack . The return value of the function passes through R0 Register returns . Before the function is called ,R0-R3 The value in will be automatically stacked .

2、R4-R11 For ordinary general purpose registers , When a function call occurs , The data will not be automatically stacked , If the called function needs to use these registers , The called function needs to store the data in these registers on the stack first, and then use these registers . Before the called function returns , You need to take the data out of the stack and reply R4-R11 Value , And then it returns to the main function .

3、R12（IP） You can record calls to subroutines .

R13-R15 The role of is introduced in the previous section , Stop talking .

Last , I use C And compiled a program of running water lamp , This is a demonstration C Language calls assembly functions , among LED The switching between on and off is realized by assembly code . Some codes are attached below ：  Code download address ：https://download.csdn.net/download/qdchenxr/10887924

/******************led.h*******************/
#ifndef __LED_H
#define __LED_H  
#include "stm32f10x.h"

void LED_Init(void);//GPIO initialization 
void LED_Change(unsigned char index);// Assembler function in C Declaration in language header file 
                            
#endif

/******************main.c*******************/
#include "delay.h"
#include "led.h"
 int main(void)
 {  
    unsigned char index=1;
    delay_init();      
    LED_Init();       
    while(1)
    {
        LED_Change(index);// Call the assembly function , Pass a parameter 
        index=!index;
        delay_ms(300);
    }
 }

/******************led.s*******************/
; Global function 
    EXPORT LED_Change           ; The functions defined in this file 
; Constant 
GPIOB_BASE EQU 0x40010C00       ;GPIOB The base address 
GPIOB_BRR  EQU GPIOB_BASE+0x14  ;GPIOB_BRR The address of the register 
GPIOB_BSRR EQU GPIOB_BASE+0x10  ;GPIOB_BSRR The address of the register    
GPIOE_BASE EQU 0x40011800       ;GPIOE The base address 
GPIOE_BRR  EQU GPIOE_BASE+0x14  ;GPIOE_BRR The address of the register 
GPIOE_BSRR EQU GPIOE_BASE+0x10  ;GPIOE_BSRR The address of the register    
LED_LIGHT  EQU 0x0020
; Code generation instructions 
    PRESERVE8
    THUMB
        
    AREA CODE, CODE, READONLY       
;LED Switching function 
LED_Change
    CBZ R0, LED1_Light  ; A parameter consists of R0 Pass on , Judge R0, If the value is 0 Jump to LED1_Light
    
LED2_Light
; Lighten up LED2 
    LDR R1, =GPIOE_BRR  ;R1=GPIOE_BRR;//R1 Zhongcun GPIOE_BRR The address of the register 
    LDR R2, =LED_LIGHT  ;R2=0x0020;
    STR R2, [R1]         ;*R1=R2;
; Extinguish LED1
    LDR R1, =GPIOB_BSRR
    LDR R2, =LED_LIGHT
    STR R2, [R1]
    BX LR               ; The function returns 
    
LED1_Light
; Lighten up LED1 
    LDR R1, =GPIOB_BRR
    LDR R2, =LED_LIGHT
    STR R2, [R1]
; Extinguish LED2
    LDR R1, =GPIOE_BSRR
    LDR R2, =LED_LIGHT
    STR R2, [R1]
    BX LR               ; The function returns 

    NOP 
    END                 ; End of assembly file 

  The effect is as follows ：（ Turn into GIF The back is reversed , I don't know why I'm coming ）