;              c) OSTCBCur              = OSTCBHighRdy;
;              d) SP                    = OSTCBHighRdy->OSTCBStkPtr;
;              e) Restore the new task's context from the new task's stack
;              f) Return to new task's code
;
;           3) Upon entry:
;              OSTCBCur      points to the OS_TCB of the task to suspend
;              OSTCBHighRdy  points to the OS_TCB of the task to resume
;*********************************************************************************************************

        RSEG CODE:CODE:NOROOT(2)
        CODE32

OSIntCtxSw
        LDR     R0, ??OS_TaskSwHook     ; OSTaskSwHook();
        MOV     LR, PC
        BX      R0

        LDR     R4,??OS_PrioCur         ; OSPrioCur = OSPrioHighRdy
        LDR     R5,??OS_PrioHighRdy
        LDRB    R6,[R5]
        STRB    R6,[R4]

        LDR     R4,??OS_TCBCur          ; OSTCBCur  = OSTCBHighRdy;
        LDR     R6,??OS_TCBHighRdy
        LDR     R6,[R6]
        STR     R6,[R4]

        LDR     SP,[R6]                 ; SP = OSTCBHighRdy->OSTCBStkPtr;

                                        ; RESTORE NEW TASK'S CONTEXT
        LDMFD   SP!, {R4}               ;    Pop new task's CPSR
        MSR     SPSR_cxsf, R4

        LDMFD   SP!, {R0-R12,LR,PC}^    ;    Pop new task's context


;*********************************************************************************************************
;                                      IRQ Interrupt Service Routine
;*********************************************************************************************************

        RSEG CODE:CODE:NOROOT(2)
        CODE32

OS_CPU_IRQ_ISR

        STMFD   SP!, {R1-R3}                   ; PUSH WORKING REGISTERS ONTO IRQ STACK

        MOV     R1, SP                         ; Save   IRQ stack pointer

        ADD     SP, SP,#12                     ; Adjust IRQ stack pointer

        SUB     R2, LR,#4                      ; Adjust PC for return address to task

        MRS     R3, SPSR                       ; Copy SPSR (i.e. interrupted task's CPSR) to R3

        MSR     CPSR_c, #(NO_INT | SVC32_MODE) ; Change to SVC mode

                                               ; SAVE TASK'S CONTEXT ONTO TASK'S STACK
        STMFD   SP!, {R2}                      ;    Push task's Return PC
        STMFD   SP!, {LR}                      ;    Push task's LR
        STMFD   SP!, {R4-R12}                  ;    Push task's R12-R4

        LDMFD   R1!, {R4-R6}                   ;    Move task's R1-R3 from IRQ stack to SVC stack
        STMFD   SP!, {R4-R6}
        STMFD   SP!, {R0}                      ;    Push task's R0    onto task's stack
        STMFD   SP!, {R3}                      ;    Push task's CPSR (i.e. IRQ's SPSR)

                                               ; HANDLE NESTING COUNTER
        LDR     R0, ??OS_IntNesting            ; OSIntNesting++;
        LDRB    R1, [R0]
        ADD     R1, R1,#1
        STRB    R1, [R0]

        CMP     R1, #1                         ; if (OSIntNesting == 1) {
        BNE     OS_CPU_IRQ_ISR_1

        LDR     R4, ??OS_TCBCur                ;     OSTCBCur->OSTCBStkPtr = SP
        LDR     R5, [R4]
        STR     SP, [R5]                       ; }

OS_CPU_IRQ_ISR_1
        MSR     CPSR_c, #(NO_INT | IRQ32_MODE) ; Change to IRQ mode (to use the IRQ stack to handle interrupt)

        LDR     R0, ??OS_CPU_IRQ_ISR_Handler   ; OS_CPU_IRQ_ISR_Handler();
        MOV     LR, PC
        BX      R0

        MSR     CPSR_c, #(NO_INT | SVC32_MODE) ; Change to SVC mode

        LDR     R0, ??OS_IntExit               ; OSIntExit();
        MOV     LR, PC
        BX      R0

                                               ; RESTORE NEW TASK'S CONTEXT
        LDMFD   SP!, {R4}                      ;    Pop new task's CPSR
        MSR     SPSR_cxsf, R4

        LDMFD   SP!, {R0-R12,LR,PC}^           ;    Pop new task's context


;*********************************************************************************************************
;                                      FIQ Interrupt Service Routine
;*********************************************************************************************************

        RSEG CODE:CODE:NOROOT(2)
        CODE32

OS_CPU_FIQ_ISR

        STMFD   SP!, {R1-R3}                   ; PUSH WORKING REGISTERS ONTO FIQ STACK

        MOV     R1, SP                         ; Save   FIQ stack pointer

        ADD     SP, SP,#12                     ; Adjust FIQ stack pointer

        SUB     R2, LR,#4                      ; Adjust PC for return address to task

        MRS     R3, SPSR                       ; Copy SPSR (i.e. interrupted task's CPSR) to R3

        MSR     CPSR_c, #(NO_INT | SVC32_MODE) ; Change to SVC mode

                                               ; SAVE TASK'S CONTEXT ONTO TASK'S STACK
        STMFD   SP!, {R2}                      ;    Push task's Return PC
        STMFD   SP!, {LR}                      ;    Push task's LR
        STMFD   SP!, {R4-R12}                  ;    Push task's R12-R4

        LDMFD   R1!, {R4-R6}                   ;    Move task's R1-R3 from FIQ stack to SVC stack
        STMFD   SP!, {R4-R6}
        STMFD   SP!, {R0}                      ;    Push task's R0    onto task's stack
        STMFD   SP!, {R3}                      ;    Push task's CPSR (i.e. FIQ's SPSR)

                                               ; HANDLE NESTING COUNTER
        LDR     R0, ??OS_IntNesting            ; OSIntNesting++;
        LDRB    R1, [R0]
        ADD     R1, R1,#1
        STRB    R1, [R0]

        CMP     R1, #1                         ; if (OSIntNesting == 1) {
        BNE     OS_CPU_FIQ_ISR_1

        LDR     R4, ??OS_TCBCur                ;     OSTCBCur->OSTCBStkPtr = SP
        LDR     R5, [R4]
        STR     SP, [R5]                       ; }

OS_CPU_FIQ_ISR_1
        MSR     CPSR_c, #(NO_INT | FIQ32_MODE) ; Change to FIQ mode (to use the FIQ stack to handle interrupt)

        LDR     R0, ??OS_CPU_FIQ_ISR_Handler   ; OS_CPU_FIQ_ISR_Handler();
        MOV     LR, PC
        BX      R0

        MSR     CPSR_c, #(NO_INT | SVC32_MODE) ; Change to SVC mode

        LDR     R0, ??OS_IntExit               ; OSIntExit();
        MOV     LR, PC
        BX      R0

                                               ; RESTORE NEW TASK'S CONTEXT
        LDMFD   SP!, {R4}                      ;    Pop new task's CPSR
        MSR     SPSR_cxsf, R4

        LDMFD   SP!, {R0-R12,LR,PC}^           ;    Pop new task's context


;*********************************************************************************************************
;                                     POINTERS TO VARIABLES
;*********************************************************************************************************

        DATA

??OS_TaskSwHook:
        DC32    OSTaskSwHook

??OS_CPU_IRQ_ISR_Handler:
        DC32    OS_CPU_IRQ_ISR_Handler

??OS_CPU_FIQ_ISR_Handler:
        DC32    OS_CPU_FIQ_ISR_Handler

??OS_IntExit:
        DC32    OSIntExit

??OS_IntNesting:
        DC32    OSIntNesting

??OS_PrioCur:
        DC32    OSPrioCur

??OS_PrioHighRdy:
        DC32    OSPrioHighRdy

??OS_Running:
        DC32    OSRunning

??OS_TCBCur:
        DC32    OSTCBCur

??OS_TCBHighRdy:
        DC32    OSTCBHighRdy

        END