Assembly language hcs12 question! The explanation included of the intire program

Question

Assembly language hcs12 question! The explanation included of the intire program the better really.

Ok, so I can follow this code well enough until the interupts are enabled with the CLI command. It then

hits the RTS command and then goes back to hit the infinite main loop. I would like to know how the

program ever gets to the "IRQ_ISR:" part. It seems to me that the code is set up to not hit this. But I know

this part is needed because is tell what is to be done when a flag is triggered.

;******************************************************************************
;-----------------------------Include Libraries--------------------------------
; Include derivative-specific definitions
INCLUDE 'derivative.inc'

; export symbols
XDEF Entry, _Startup, main
; we use export 'Entry' as symbol. This allows us to
; reference 'Entry' either in the linker .prm file
; or from C/C++ later on

XREF __SEG_END_SSTACK
; symbol defined by the linker for the end of the stack

   INCLUDE 'mc9s12dg256.inc'

;******************************************************************************
;-----------------------------code section-------------------------------------
MyCode: SECTION
main:
_Startup:
Entry:
LDS #__SEG_END_SSTACK ;initialize the stack pointer
  

;******************************************************************************
;-----------------------------Main Code Loop-----------------------------------

BSR Init_Ports   ;Branch to subroutine below

Main Loop:
BRA Main_Loop

Init_Ports:           ;Initialize ports and interrupt hardware
sei                ;Disable Interrupts

ldaa DDRH           ;Setup I/O IE. switches
anda #$FE ;And's the binary value 11111110 with PORT H
;This sets PORT H bit 0 to be zero or input
staa DDRH            ;store new anded value back in DDRH
  
ldaa DDRB           ;Load accumulator A with DDRB, LED's
oraa #$FF           ;OR with 1111 1111, makes all leds outputs
staa DDRB           ;Store into DDRB, so save that configuration

ldaa DDRJ           ;Set up DDRJ, load accumulator A: LED enabler
oraa #$02           ;OR with 0000 0010
staa DDRJ           ;Store into DDRJ

ldaa PTJ           ;Set up PortJ by load
anda #$FE            ;And with 1111 1110
staa PTJ           ;Store into PortJ
;*****************************************************************************

ldaa PPSH           ;Setup Interrupts, PORT POLARITY SELCT REISTER(ppsh, ppsj,ppsp)
anda #$FE           ;And with 1111 1110
staa PPSH            ;Set polarity

ldaa PIEH           ;Load PIEH, PORT INTERRUPT ENABLE REGISTER(pidh,piej,piep)
oraa #$0F           ;OR with 0000 1111
staa PIEH            ;Enable interupt on bit 0

ldaa #$FF           ;Load accumulator A with 1111 1111
staa PIFH            ;Clear all interrupt flags on portH, port interupt flag register
cli                ;Enable Interrupts cli
  
  
  
rts               ;Then, return to subroutine

IRQ_ISR:           ;Interrupt Routine

ldaa PIFH        ;LOAD FLAGS
cmpa #$01           ;JUMP ACCORDING TO WHICH FLAG
BEQ Switch1           ;If equal, go to Switch1 loop
cmpa #$02           ;Compare with 0000 0010
BEQ Switch2           ;If equal, then go to Switch2
bra none           ;Branch always to none loop
  
Switch1:           ;Switch 1 for incrementing

inc PORTB             ;Increment PORTB
bra none

Switch2:           ;Switch 2 for clearing

ldaa PORTB
anda #$00           ;Clear the bits, 0000 0000
staa PORTB
bra none           ;Branch always to none loop below

none:               ;none button

ldaa #$0F           ;CLEAR FLAGS, 0000 1111
staa PIFH
rti                ;Return from Interrupt

;Enter Vector Table  
org $FFCC            ;port h interrupt vector see page 145

dc.w IRQ_ISR

Explanation / Answer

When it implements cli command, the program enters interrupt subroutine that is written after the RTS. After completing the interrupt service routine i.e IRQ_ISR, the program counter comes back to the next command after cli and then RTS is implemented.

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