Writing STM32 OS step by step [I] Preface

　　I have always wanted to write an OS similar to uCOS. I have finally made some progress in my spare time while preparing for the postgraduate entrance examination. Back to the point, I think the most difficult part of OS is the context switching problem, which is related to the architecture of MCU, so for different MCUs, this part needs to be ported. Once this problem is solved, the whole OS is half successful, of course, for a simple OS.

　　Well, to write an OS, you first need a development board and an emulator. My development board is STM32F4DISCOVERY, which comes with ST-LINK V2 emulator. The onboard MCU is STM32F407VGT6, which supports FPU, 32-bit ARM Cortex-M4F core, 1024KB FLASH, 192 KB RAM, which is very powerful. For other STM32 series, this OS can be used almost without modification. The development environment is IAR for ARM 6.5. If it is MDK, it is also similar, and the assembly part needs to be modified.

　　After studying the Port part of UCOS-II's Cortex-M4, I found it very good, so I used it directly with very few modifications. First, let's take a look at several important functions in this part. Open the os_cpu_a.asm file and locate the following place. I changed the comments to Chinese. After the OS is initialized, execute OSStart, which finally calls the OSStartHighRdy function. Note that before this, the stacks of the thread mode and exception mode are both MSP, and after this, the stack of the thread mode is PSP, and the stack of the exception mode is still MSP.

OSStartHighRdy
    LDR R0, =NVIC_SYSPRI14 ; Set the exception interrupt priority of PendSV LDR R1, =NVIC_PENDSV_PRI
    STRB R1, [R0]

    MOVS R0, #0 ; Initialize PSP=0 MSR PSP, R0

    LDR R0, =OS_CPU_ExceptStkBase ; Initialize exception stack MSP address LDR R1, [R0]
    MSR MSP, R1    

    LDR R0, =OSRunning; Set OSRunning = TRUE
    MOVS R1, #1
    STRB R1, [R0]

    LDR R0, =NVIC_INT_CTRL ; Trigger PendSV exception (cause context switch) LDR R1, =NVIC_PENDSVSET STR R1, [R0]

    CPSIE I ; Enable interrupt, then enter PendSV exception OSStartHang
    B OSStartHang ; Normally, it should not run to this

　　The most core function should be OS_CPU_PendSVHandler, which handles the PendSV interrupt and completes the context switch.

OS_CPU_PendSVHandler
    CPSID I ; Disable interrupt
    MRS R0, PSP ; Get PSP
    CBZ R0, OS_CPU_PendSVHandler_nosave; If PSP is 0, jump to OS_CPU_PendSVHandler_nosave, that is, do not save the previous context and directly enter the next context.
                                           ; Why ask? Because it is the first call, there is no context.
                                           ; Save the previous text
    SUBS R0, R0, #0x20; Because the register is 32 bits, 4 bytes aligned, there are 8 registers pushed automatically, so the offset is 8*0x04=0x20
    STM R0, {R4-R11}; In addition to the registers that are automatically pushed onto the stack, R4-R11 must be pushed onto the stack manually
    LDR R1, =OSTCBCur ; Save the SP pointer above OSTCBCur->OSTCBStkPtr = SP; LDR R1, [R1] STR R0, [R1]                                            

                                                                
OS_CPU_PendSVHandler_nosave ; Switch to the following context
    PUSH {R14}; LR is pushed onto the stack, and the C function is called next
    LDR R0, =OSTaskSwHook ; Call OSTaskSwHook(); BLX R0 POP {R14}

    LDR R0, =OSPrioCur; Set OSPrioCur = OSPrioHighRdy; LDR R1, =OSPrioHighRdy
    LDRB R2, [R1]
    STRB R2, [R0]

    LDR R0, =OSTCBCur; Set OSTCBCur = OSTCBHighRdy; LDR R1, =OSTCBHighRdy
    LDR R2, [R1] STR R2, [R0]

    LDR R0, [R2]; The value in R0 is the SP of the new task; SP = OSTCBHighRdy->OSTCBStkPtr;
    LDM R0, {R4-R11} ; Manually pop R4-R11 ADDS R0, R0, #0x20
   
        
    MSR PSP, R0; PSP = New mission SP
    ORR LR, LR, #0x04 ; Ensure that PSP CPSIE I is used after the exception return
    BX LR; Exit abnormally, pop up xPSR, PC, LR, R0-R3 from PSP, and enter new task running

　　Then we built our own OS on this basis. First, we completed the mutual calling of the two tasks, then added SysTick task scheduling, and finally added semaphores, mailboxes and other functions.