os_cpu_a.asm

UCOS-III

;
;********************************************************************************************************
;                                                uC/OS-III
;                                          The Real-Time Kernel
;
;
;                           (c) Copyright 2009-2010; Micrium, Inc.; Weston, FL
;                    All rights reserved.  Protected by international copyright laws.
;
;                                           ARM Cortex-M3 Port
;
; File      : OS_CPU_A.ASM
; Version   : V3.01.2
; By        : JJL
;             BAN
;
; For       : ARMv7M Cortex-M3
; Mode      : Thumb2
; Toolchain : IAR EWARM
;********************************************************************************************************
;

;********************************************************************************************************
;                                          PUBLIC FUNCTIONS
;********************************************************************************************************

    EXTERN  OSRunning                                           ; External references
    EXTERN  OSPrioCur
    EXTERN  OSPrioHighRdy
    EXTERN  OSTCBCurPtr
    EXTERN  OSTCBHighRdyPtr
    EXTERN  OSIntExit
    EXTERN  OSTaskSwHook
    EXTERN  OS_CPU_ExceptStkBase


    EXPORT  OSStartHighRdy                                      ; Functions declared in this file
    EXPORT  OS_CPU_PendSVHandler

;PAGE
;********************************************************************************************************
;                                               EQUATES
;********************************************************************************************************

NVIC_INT_CTRL   EQU     0xE000ED04                              ; Interrupt control state register.
NVIC_SYSPRI14   EQU     0xE000ED22                              ; System priority register (priority 14).
NVIC_PENDSV_PRI EQU           0xFF                              ; PendSV priority value (lowest).
NVIC_PENDSVSET  EQU     0x10000000                              ; Value to trigger PendSV exception.


;********************************************************************************************************
;                                     CODE GENERATION DIRECTIVES
;********************************************************************************************************

    AREA |.text|, CODE, READONLY, ALIGN=2
    THUMB
    REQUIRE8
    PRESERVE8


;PAGE
;********************************************************************************************************
;                                         START MULTITASKING
;                                      void OSStartHighRdy(void)
;
; Note(s) : 1) This function triggers a PendSV exception (essentially, causes a context switch) to cause
;              the first task to start.
;
;           2) OSStartHighRdy() MUST:
;              a) Setup PendSV exception priority to lowest;
;              b) Set initial PSP to 0, to tell context switcher this is first run;
;              c) Set the main stack to OS_CPU_ExceptStkBase
;              d) Trigger PendSV exception;
;              e) Enable interrupts (tasks will run with interrupts enabled).
;********************************************************************************************************

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

    MOVS    R0, #0                                              ; Set the PSP to 0 for initial context switch call
    MSR     PSP, R0

    LDR     R0, =OS_CPU_ExceptStkBase                           ; Initialize the MSP to the OS_CPU_ExceptStkBase
    LDR     R1, [R0]
    MSR     MSP, R1    

    LDR     R0, =NVIC_INT_CTRL                                  ; Trigger the PendSV exception (causes context switch)
    LDR     R1, =NVIC_PENDSVSET
    STR     R1, [R0]
    
    CPSIE   I                                                   ; Enable interrupts at processor level

OSStartHang
    B       OSStartHang                                         ; Should never get here


;PAGE
;********************************************************************************************************
;                                       HANDLE PendSV EXCEPTION
;                                   void OS_CPU_PendSVHandler(void)
;
; Note(s) : 1) PendSV is used to cause a context switch.  This is a recommended method for performing
;              context switches with Cortex-M3.  This is because the Cortex-M3 auto-saves half of the
;              processor context on any exception, and restores same on return from exception.  So only
;              saving of R4-R11 is required and fixing up the stack pointers.  Using the PendSV exception
;              this way means that context saving and restoring is identical whether it is initiated from
;              a thread or occurs due to an interrupt or exception.
;
;           2) Pseudo-code is:
;              a) Get the process SP, if 0 then skip (goto d) the saving part (first context switch);
;              b) Save remaining regs r4-r11 on process stack;
;              c) Save the process SP in its TCB, OSTCBCurPtr->OSTCBStkPtr = SP;
;              d) Call OSTaskSwHook();
;              e) Get current high priority, OSPrioCur = OSPrioHighRdy;
;              f) Get current ready thread TCB, OSTCBCurPtr = OSTCBHighRdyPtr;
;              g) Get new process SP from TCB, SP = OSTCBHighRdyPtr->OSTCBStkPtr;
;              h) Restore R4-R11 from new process stack;
;              i) Perform exception return which will restore remaining context.
;
;           3) On entry into PendSV handler:
;              a) The following have been saved on the process stack (by processor):
;                 xPSR, PC, LR, R12, R0-R3
;              b) Processor mode is switched to Handler mode (from Thread mode)
;              c) Stack is Main stack (switched from Process stack)
;              d) OSTCBCurPtr      points to the OS_TCB of the task to suspend
;                 OSTCBHighRdyPtr  points to the OS_TCB of the task to resume
;
;           4) Since PendSV is set to lowest priority in the system (by OSStartHighRdy() above), we
;              know that it will only be run when no other exception or interrupt is active, and
;              therefore safe to assume that context being switched out was using the process stack (PSP).
;********************************************************************************************************

OS_CPU_PendSVHandler
    CPSID   I                                                   ; Prevent interruption during context switch
    MRS     R0, PSP                                             ; PSP is process stack pointer
    CBZ     R0, OS_CPU_PendSVHandler_nosave                     ; Skip register save the first time

    SUBS    R0, R0, #0x20                                       ; Save remaining regs r4-11 on process stack
    STM     R0, {R4-R11}

    LDR     R1, =OSTCBCurPtr                                    ; OSTCBCurPtr->OSTCBStkPtr = SP;
    LDR     R1, [R1]
    STR     R0, [R1]                                            ; R0 is SP of process being switched out

                                                                ; At this point, entire context of process has been saved
OS_CPU_PendSVHandler_nosave
    PUSH    {R14}                                               ; Save LR exc_return value
    LDR     R0, =OSTaskSwHook                                   ; OSTaskSwHook();
    BLX     R0
    POP     {R14}

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

    LDR     R0, =OSTCBCurPtr                                    ; OSTCBCurPtr = OSTCBHighRdyPtr;
    LDR     R1, =OSTCBHighRdyPtr
    LDR     R2, [R1]
    STR     R2, [R0]

    LDR     R0, [R2]                                            ; R0 is new process SP; SP = OSTCBHighRdyPtr->StkPtr;
    LDM     R0, {R4-R11}                                        ; Restore r4-11 from new process stack
    ADDS    R0, R0, #0x20
    MSR     PSP, R0                                             ; Load PSP with new process SP
    ORR     LR, LR, #0x04                                       ; Ensure exception return uses process stack
    CPSIE   I
    BX      LR                                                  ; Exception return will restore remaining context

    END