;********************************************************************************************************
;                                              uC/OS-III
;                                        The Real-Time Kernel
;
;                    Copyright 2009-2021 Silicon Laboratories Inc. www.silabs.com
;
;                                 SPDX-License-Identifier: APACHE-2.0
;
;               This software is subject to an open source license and is distributed by
;                Silicon Laboratories Inc. pursuant to the terms of the Apache License,
;                    Version 2.0 available at www.apache.org/licenses/LICENSE-2.0.
;
;********************************************************************************************************

;********************************************************************************************************
;
;                                             ARMv7-M Port
;
; File      : os_cpu_a.asm
; Version   : V3.08.01
;********************************************************************************************************
; For       : ARMv7-M Cortex-M
; Mode      : Thumb-2 ISA
; Toolchain : IAR EWARM
;********************************************************************************************************
; Note(s)   : (1) This port supports the ARM Cortex-M3, Cortex-M4 and Cortex-M7 architectures.
;             (2) It has been tested with the following Hardware Floating Point Unit.
;                 (a) Single-precision: FPv4-SP-D16-M and FPv5-SP-D16-M
;                 (b) Double-precision: FPv5-D16-M
;********************************************************************************************************
;

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

                                                                ; External references.
    EXTERN  OSPrioCur
    EXTERN  OSPrioHighRdy
    EXTERN  OSTCBCurPtr
    EXTERN  OSTCBHighRdyPtr
    EXTERN  OSIntExit
    EXTERN  OSTaskSwHook
    EXTERN  OS_CPU_ExceptStkBase
    EXTERN  OS_KA_BASEPRI_Boundary


    PUBLIC  OSStartHighRdy                                      ; Functions declared in this file
    PUBLIC  OSCtxSw
    PUBLIC  OSIntCtxSw
    PUBLIC  OS_CPU_PendSVHandler


;********************************************************************************************************
;                                               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
;********************************************************************************************************

    RSEG CODE:CODE:NOROOT(2)
    THUMB



;********************************************************************************************************
;                                         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) During task execution, PSP is used as the stack pointer.
;              When an exception occurs, the core will switch to MSP until the exception return.
;
;           3) 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) Get current high priority, OSPrioCur = OSPrioHighRdy;
;              e) Get current ready thread TCB, OSTCBCurPtr = OSTCBHighRdyPtr;
;              f) Get new process SP from TCB, SP = OSTCBHighRdyPtr->StkPtr;
;              g) Restore R0-R11 and R14 from new process stack;
;              h) Enable interrupts (tasks will run with interrupts enabled).
;********************************************************************************************************

OSStartHighRdy
    CPSID   I                                                   ; Prevent interruption during context switch
    MOV32   R0, NVIC_SYSPRI14                                   ; Set the PendSV exception priority
    MOV32   R1, NVIC_PENDSV_PRI
    STRB    R1, [R0]

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

    MOV32   R0, OS_CPU_ExceptStkBase                            ; Initialize the MSP to the OS_CPU_ExceptStkBase
    LDR     R1, [R0]
    MSR     MSP, R1

    BL      OSTaskSwHook                                        ; Call OSTaskSwHook() for FPU Push & Pop

    MOV32   R0, OSPrioCur                                       ; OSPrioCur   = OSPrioHighRdy;
    MOV32   R1, OSPrioHighRdy
    LDRB    R2, [R1]
    STRB    R2, [R0]

    MOV32   R0, OSTCBCurPtr                                     ; OSTCBCurPtr = OSTCBHighRdyPtr;
    MOV32   R1, OSTCBHighRdyPtr
    LDR     R2, [R1]
    STR     R2, [R0]

    LDR     R0, [R2]                                            ; R0 is new process SP; SP = OSTCBHighRdyPtr->StkPtr;
    MSR     PSP, R0                                             ; Load PSP with new process SP

    MRS     R0, CONTROL
    ORR     R0, R0, #2
    BIC     R0, R0, #4                                          ; Clear FPCA bit to indicate FPU is not in use
    MSR     CONTROL, R0
    ISB                                                         ; Sync instruction stream

    LDMFD    SP!, {R4-R11, LR}                                  ; Restore r4-11, lr from new process stack
    LDMFD    SP!, {R0-R3}                                       ; Restore r0, r3
    LDMFD    SP!, {R12, LR}                                     ; Load R12 and LR
    LDMFD    SP!, {R1, R2}                                      ; Load PC and discard xPSR
    CPSIE    I
    BX       R1


;********************************************************************************************************
;                       PERFORM A CONTEXT SWITCH (From task level) - OSCtxSw()
;                   PERFORM A CONTEXT SWITCH (From interrupt level) - OSIntCtxSw()
;
; Note(s) : 1) OSCtxSw() is called when OS wants to perform a task context switch.  This function
;              triggers the PendSV exception which is where the real work is done.
;
;           2) OSIntCtxSw() is called by OSIntExit() when it determines a context switch is needed as
;              the result of an interrupt.  This function simply triggers a PendSV exception which will
;              be handled when there are no more interrupts active and interrupts are enabled.
;********************************************************************************************************

OSCtxSw
OSIntCtxSw
    LDR     R0, =NVIC_INT_CTRL                                  ; Trigger the PendSV exception (causes context switch)
    LDR     R1, =NVIC_PENDSVSET
    STR     R1, [R0]
    BX      LR


;********************************************************************************************************
;                                       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-M.  This is because the Cortex-M auto-saves half of the
;              processor context on any exception, and restores same on return from exception.  So only
;              saving of R4-R11 & R14 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
;              b) Save remaining regs r4-r11 & r14 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 and R14 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).
;
;           5) Increasing priority using a write to BASEPRI does not take effect immediately.
;              (a) IMPLICATION  This erratum means that the instruction after an MSR to boost BASEPRI
;                  might incorrectly be preempted by an insufficient high priority exception.
;
;              (b) WORKAROUND  The MSR to boost BASEPRI can be replaced by the following code sequence:
;
;                  CPSID i
;                  MSR to BASEPRI
;                  DSB
;                  ISB
;                  CPSIE i
;********************************************************************************************************

OS_CPU_PendSVHandler
    CPSID   I                                                   ; Cortex-M7 errata notice. See Note #5
    MOV32   R2, OS_KA_BASEPRI_Boundary                          ; Set BASEPRI priority level required for exception preemption
    LDR     R1, [R2]
    MSR     BASEPRI, R1
    DSB
    ISB
    CPSIE   I

    MRS     R0, PSP                                             ; PSP is process stack pointer
#ifdef __ARMVFP__
                                                                ; Push high vfp registers if the task is using the FPU context
    TST       R14, #0x10
    IT        EQ
    VSTMDBEQ  R0!, {S16-S31}
#endif

    STMFD   R0!, {R4-R11, R14}                                  ; Save remaining regs r4-11, R14 on process stack

    MOV32   R5, OSTCBCurPtr                                     ; OSTCBCurPtr->StkPtr = SP;
    LDR     R1, [R5]
    STR     R0, [R1]                                            ; R0 is SP of process being switched out

                                                                ; At this point, entire context of process has been saved
    MOV     R4, LR                                              ; Save LR exc_return value
    BL      OSTaskSwHook                                        ; Call OSTaskSwHook() for FPU Push & Pop

    MOV32   R0, OSPrioCur                                       ; OSPrioCur   = OSPrioHighRdy;
    MOV32   R1, OSPrioHighRdy
    LDRB    R2, [R1]
    STRB    R2, [R0]

    MOV32   R1, OSTCBHighRdyPtr                                 ; OSTCBCurPtr = OSTCBHighRdyPtr;
    LDR     R2, [R1]
    STR     R2, [R5]

    ORR     LR,  R4, #0x04                                      ; Ensure exception return uses process stack
    LDR     R0,  [R2]                                           ; R0 is new process SP; SP = OSTCBHighRdyPtr->StkPtr;
    LDMFD   R0!, {R4-R11, R14}                                  ; Restore r4-11, R14 from new process stack

#ifdef __ARMVFP__
                                                                ; Pop the high vfp registers if the next task is using the FPU context
    TST       R14, #0x10
    IT        EQ
    VLDMIAEQ  R0!, {S16-S31}
#endif

    MSR     PSP, R0                                             ; Load PSP with new process SP

    MOV32   R2, #0                                              ; Restore BASEPRI priority level to 0
    CPSID   I                                                   ; Cortex-M7 errata notice. See Note #5
    MSR     BASEPRI, R2
    DSB
    ISB
    CPSIE   I
    BX      LR                                                  ; Exception return will restore remaining context

    END
