tpa           tab           pula           pshb       save old CCR           sei        make atomic, entering critical section           ldab Size           cmpb #FifoSize Full if Size==FifoSize           beq  PutNotFull           clra           bra  PutDonePutNotFull incb           stab Size      Size++           ldx  #Fifo           ldab PutI           abx            staa 0,x       Put data into fifo            incb           cmpb #FifoSize           bne  PutNoWrap skip if no wrapping needed           clrb           Wrap PutNoWrap  clra           sucess           coma           RegA=-1 means OK           stab PutIPutDone    tab        end critical section           pula           tpa        restore CCR to previous value           tba           rts; Program 4.25. Assembly routine to get from a two index with counter FIFO. ***********Get a byte from the FIFO******************* Input  RegX points to place for 8 bit data from Get* Output RegA is -1 if successful, 0 if Fifo was empty when calledGetFifo    tpa           psha       save old CCR           sei        make atomic, entering critical section           clra       assume it will fail           tst  Size           beq  GetDone           ldy  #Fifo           ldab GetI           aby             coma           RegA=-1 means OK           ldab 0,y       Data from FIFO           stab 0,x       Return by reference            ldab GetI           incb           cmpb #FifoSize           bne  GetNoWrap skip if no wrapping needed           clrb           Wrap GetNoWrap  stab GetIGetDone    tab        end critical section           pula           tpa        restore CCR to previous value           tba           rts; Program 4.27. Example of a vectored interrupt. ; MC68HC11A8TimeHan ldaa #$80  ;TOF is bit 7        staa TFLG2 ;clear TOF;*Timer interrupt calculations*        rtiExtHan  ldaa PIOC          ldaa PORTCL ;clear STAF;*External interrupt calculations*        rti        org  $FFDE  ;timer overflow        fdb  TimeHan        org  $FFF2  ;IRQ external        fdb  ExtHan; Program 4.28. Example of a polled interrupt. ; MC68HC11A8ExtHan  ldaa PIOC   ;which one        bita #$80   ;STAF?        bne  STAFHan          ldaa OtherStatus1         bita #$80   ;External?        bne  OtherHan          swi        ;errorSTAFHan ldaa PORTCL ;clear STAF;*STAF interrupt calculations*        rtiOtherHan ldaa OtherData;*Other interrupt calculations*        rti        org  $FFF2  ;IRQ external        fdb  ExtHan; Program 4.29. Interrupting keyboard software. ; MC68HC11A8; PC6-PC0 inputs = keyboard DATA; STRA=STROBE interrupt on rise; 6 STAI 1  Interrupts armed; 5 CWOM 0  Normal outputs; 4 HNDS 0  No handshake; 3 OIN  0; 2 PLS  0  STRB not used; 1 EGA  1  STAF set on rise of READY; 0 INVB 0  STRB not used Init   sei          ;Make this atomic       ldaa #$80    ;PC7 is an output       staa DDRC    ;PC6-0 inputs       ldaa #$42           staa PIOC       ldaa PIOC    ;clears STAF       ldaa PORTCL       clr  PORTC   ;Make PC7=0       jsr  InitFifo       cli          ;Enable IRQ       rtsExtHan ldaa PIOC    ;poll STAF       bmi  KeyHan         swi          ;errorKeyHan ldaa PORTCL  ;clear STAF       jsr  PutFifo       rti       org  $FFF2   ;IRQ external       fdb  ExtHan; Program 4.31. Helper routines for the printer interface. ; MC68HC11A8;*****goes in RAM**************OK    rmb  1   ;0=busy, 1=doneLine  rmb  20  ;ASCII, end with 0Pt    rmb  2   ;pointer to Line;*****goes in ROM**************;Input RegX=>stringFill  ldy  #Line ;RegX=>string      sty  Pt    ;initialize pointerFloop ldaa 0,X   ;copy data      staa 0,Y        inx      iny      tsta       ;end?      bne  Floop      clr  OK      rts;Return RegA=dataGet   ldx  Pt      ldaa 0,X   ;read data       inx      stx  Pt      rts; Program 4.32. Initialization routines for the printer interface. ; MC68HC11A8; PC6-PC0 outputs = printer DATA; STRA=READY interrupt on rise; 6 STAI 1  Interrupts armed; 5 CWOM 0  Normal outputs; 4 HNDS 1  Output handshake; 3 OIN  1; 2 PLS  1  START=STRB pulse; 1 EGA  1  STAF set on rise of READY; 0 INVB 0  STRB not used ;Input RegX=>stringInit   sei          ;Make this atomic       bsr  Fill    ;Init global        ldaa #$FF    ;PC7 is an output       staa DDRC    ;PC6-0 outputs       ldaa #$5E           staa PIOC       bsr  Get     ;start first        staa PORTCL       cli          ;Enable IRQ       rts; Program 4.33. ISR routines for the printer interface. ; MC68HC11A8ExtHan ldaa PIOC    ;poll STAF?       bmi  PrtHan         swi          ;errorPrtHan bsr  Get       tsta       beq  Disarm       staa PORTCL  ;start next       bra  Done Disarm ldaa #$1E    ;STAI=0          staa PIOC       inc  OK      ;line completeDone   rti       org  $FFF2   ;IRQ external       fdb  ExtHan; Program 4.35. Assembly software for the XIRQ interrupt. * Called to initialize the power systemRITUAL ldaa #0       Backup power initially off       staa PORTB    Set the flip flop, make XIRQ=1       ldaa #1       staa PORTB    Flip flop ready to receive rising edge of TooLow         ldaa #$10     Enable XIRQ, Disable IRQ       tap       rts           Back to main, foreground thread*Note that the software can only enable XIRQ and can not disable XIRQ.* In this way, XIRQ is non-maskable.XIRQHAN ldaa #2        staa PORTB   Enable BackUp power, acknowledge XIRQ        ldaa #3        staa PORTB   Will thread another rising edge of TooLow          rti        org  $FFF4        fdb  XIRQHAN  XIRQ interrupt vector; Program 4.37. 6811 assembly structure for interrupt polling using linked lists. start   fdb  llSTAF   place to start pollingSreg    equ  0        Index to Status RegisterAmask   equ  2               and maskCmask   equ  3               compare maskDevHan  equ  4               device handlerNextPt  equ  6               next pointernum     fcb  3        number of devicesllSTAF  fdb  $1002    address of PIOC        fcb  $ff      look at all the bits in PIOC        fcb  $C0      expect exact match with $C0        fdb  STAFhan  device handler        fdb  llCA1    pointer to next device to pollllCA1   fdb  $2011    address of 6821 Port A Control/Status        fcb  $87      look at bits 7,2,1,0        fcb  $85      expect bits 7,2,1,0 to be 1,1,0,1        fdb  CA1han   device handler for CA1        fdb  llCB2    pointer to next device to pollllCB2   fdb  $2013    address of 6821 Port B Control/Status        fcb  $7C      look at bits 6,5,4,3,2        fcb  $4C      expect bits 6,5,4,3,2 to be 1,0,0,1,1        fdb  CB2han   device handler for CB2        fdb  0        no more; Program 4.38.  6811 assembly implementation of interrupt polling using linked lists. IrqHan  ldx  start    Reg X points to linked list place to start        ldab num      number of possible devicesnext    ldy  Sreg,x   Reg Y points to status reg        ldaa ,y       read status        anda Amask,x  clear bits that are indeterminate        cmpa Cmask,x  expected value if this device active        bne  Notyet   skip if this device not requesting        ldy  DevHan,x Reg Y points to device handler        jsr  ,y       call device handler, will return hereNotyet  ldx  NextPt,x Reg X points to next entry        decb          device counter        bne  next     check next device        rti           ; Program 4.45. 6811 assembly implementation of a periodic interrupt using an external clock. TIME   rmb  2      incremented every 1msRITUAL sei         disable interrupts during RITUAL       ldaa #$42   PIOC STAI=1, HNDS=0, EGA=1       staa $1002  Arm interrupt on rise of STRA       ldd  #0       std  TIME   initialize variable       ldaa $1005  initially clear STRA       cli         enable       rts* POLL for zeros and ones  IRQHAN ldaa $1002  11000010 if interrupting       cmpa #$C2       beq  CLKHAN       swiCLKHAN ldaa $1005  Acknowledge       ldx  TIME       inx       stx  TIME       rti; Program 4.46. 6811 assembly implementation of a periodic interrupt using real time interrupt. RITUAL sei          disable interrupts during RITUAL       ldaa  #3     Set RTR1,RTR0 = 11 Interrupt period = 32.768ms       staa  $1026	       ldaa  #$40    Set RTII=1       staa  $1024       cli           Enable IRQ interrupts       rtsRTIHAN ldaa $1025    Polling for zeros and ones expect=X1XX0000       anda #$4F     ignore TOF, PAOVF, and PAIF       cmpa #$40     RTIF should equal 1       beq  OK       swi           ErrorOK     ldaa  #$40    RTIF is cleared by writing to TFLG2       staa  $1025          with bit 6 set* service occurs every 32.768ms or about 30.517Hz       rti 1 This other device could be a 6821 parallel port connected to the 6811 in expanded mode.