MSR     SPSR_cxsf, R4
        
        LDMFD   SP!, {R0-R12,LR,PC}^    ;    Pop new task's context

;*********************************************************************************************************
;                   PERFORM A CONTEXT SWITCH (From interrupt level) - OSIntCtxSw()
;
; Note(s) : 1) OSIntCtxSw() is called in SYS mode with BOTH FIQ and IRQ interrupts DISABLED
;
;           2) The pseudo-code for OSCtxSw() is:
;              a) OSTaskSwHook();
;              b) OSPrioCur             = OSPrioHighRdy;
;              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
;*********************************************************************************************************

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

        LDR     R4,=OSPrioCur           ; OSPrioCur = OSPrioHighRdy
        LDR     R5,=OSPrioHighRdy
        LDRB    R6,[R5]
        STRB    R6,[R4]
        
        LDR     R4,=OSTCBCur            ; OSTCBCur  = OSTCBHighRdy;
        LDR     R6,=OSTCBHighRdy
        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
;*********************************************************************************************************

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, =OSIntNesting              ; 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, =OSTCBCur                  ;     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            

EIC_base_addr         EQU    0xFFFFF800; EIC base address.
CICR_off_addr         EQU    0x04      ; Current Interrupt Channel Register.
IVR_off_addr          EQU    0x18      ; Interrupt Vector Register.
IPR_off_addr          EQU    0x40      ; Interrupt Pending Register.

        LDR     R0, =(EIC_base_addr + IVR_off_addr)
        MOV     LR, PC
        BX      R0                             ; Branch to the IRQ handler.
                                               ; Clear pending bit in EIC (using the proper IPRx)
        LDR     R0, =EIC_base_addr
        LDR     R2, [R0, #CICR_off_addr]       ; Get the IRQ channel number.
        MOV     R3, #1
        MOV     R3, R3, LSL r2
        STR     R3, [R0, #IPR_off_addr]        ; Clear the corresponding IPR bit.
        
        MSR     CPSR_c, #(NO_INT | SVC32_MODE) ; Change to SVC mode
        
        LDR     R0, =OSIntExit                 ; 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
;*********************************************************************************************************

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, =OSIntNesting              ; 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, =OSTCBCur                  ;     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, =FIQ_Handler               ; FIQ_Handler();
        MOV     LR, PC
        BX      R0            
        
        MSR     CPSR_c, #(NO_INT | SVC32_MODE) ; Change to SVC mode
        
        LDR     R0, =OSIntExit                 ; 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

        END