; Chapter 5 6811 assembly language programs; Jonathan W. Valvano; This software accompanies the book,; Real Time Embedded Systems published by Brooks Cole;; Program 5.1. Assembly code that statically allocates three threads.TCB1 fdb TCB2     fdb IS1      fcb 1      rmb 49    IS1  rmb 1     fcb $40        fdb 0,0,0      fdb ProgATCB2 fdb TCB3     fdb IS2     fcb 2       rmb 49    IS2  rmb 1     fcb $40        fdb 0,0,0      fdb ProgATCB3 fdb TCB1 link     fdb IS3  SP     fcb 4    Id     rmb 49    IS3  rmb 1     fcb $40   CCR     fdb 0,0,0 DXY     fdb ProgB PC; Program 5.2. Assembly code that implements the preemptive thread switcher.Next  equ  0       pointer to next TCBSP    equ  2       Stack pointer for this threadId    equ  4       Used to visualize which thread is current runningRunPt rmb  2       pointer to thread that is currently runningMain  ldaa #$FF      staa DDRC    PortC displays which program is executing      ldx  #TCB1   First thread to run      jmp  Start* Suspend thread which is currently runningOC5Han ldx  RunPt       sts  SP,x    save Stack Pointer in TCB* launch next thread       ldx  Next,xStart  stx  RunPt       ldaa Id,x       staa PORTB   visualizes running thread       lds  SP,x    set SP for this new thread       ldd  TOC5       addd #20000  interrupts every 10 ms       std  TOC5       ldaa #$08    ($20 on the 6812)       staa TFLG1   acknowledge OC5       rti; Program 5.3. Assembly code for the two main programs and shared subroutine.ProgA pshx           tsx      ldd  #5      std  0,xLoopA ldaa #2      staa PORTC      ldd  0,x      jsr  sub      std  0,x      bra  LoopAProgB pshx           tsx      ldd  #5      std  0,xLoopB ldaa #4      staa PORTC      ldd  0,x      jsr  sub      std  0,x      bra  LoopBSub pshx         tsx    std  0,x    ldaa #1    staa PORTC    ldd  0,x    addd #1    pulx    rts; Program 5.8. 6811/6812 assembly code for a spinlock counting semaphore.S      fcb  1     semaphore counter initialized to 1wait   sei        make read-modify-write atomic       ldaa S     current value of semaphore       bhi  OK    available if >0       cli       bra  wait  **interrupts can occur here**OK     deca       staa S     S=S-1       cli       rtssignal inc  S     S=S+1, this is atomic       rts ; Program 5.12. Assembly code to initialize a blocking semaphore.S       rmb  1   semaphore counterBlockPt rmb  2   Pointer to threads blocked on SInit    tpa        psha            Save old value of I        sei             Make atomic        ldaa #1        staa S          Init semaphore value        ldx  #Null        stx  BlockPt    empty list        pula        tap             Restore old value of I        rts; Program 5.13. Assembly code helper function to block a thread, used to implement a blocking semaphore.* To block a thread on semaphore S, execute SWISWIhan ldx  RunPt   running process "to be blocked"       sts  SP,x    save Stack Pointer in its TCB* Unlink "to be blocked" thread from RunPt list       ldy  Next,x  find previous thread       sty  RunPt   next one to runlook   cpx  Next,y  search to find previous       beq  found       ldy  Next,y       bra  lookfound  ldd  RunPt  one after blocked       std  Next,y  link previous to next to run* Put "to be blocked" thread on block list       ldy  BlockPt       sty  Next,x   link "to be blocked"       stx  BlockPt* Launch next thread       ldx  RunPt       lds  SP,x    set SP for this new thread       ldd  TCNT    Next thread gets a full 10ms time slice       addd #20000  interrupt after 10 ms       std  TOC5       ldaa #$08    ($20 on the 6812)       staa TFLG1   clear OC5F       rti