chap5.asm

Motorola 6812芯片开发的接口程序。

; Chapter 5 6812 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  fcb $40   
     fdb 0,0,0 
     fdb ProgA
TCB2 fdb TCB3
     fdb IS2
     fcb 2  
     rmb 49    
IS2  fcb $40   
     fdb 0,0,0 
     fdb ProgA
TCB3 fdb TCB1 link
     fdb IS3  SP
     fcb 4    Id
     rmb 49    
IS3  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 TCB
SP    equ  2       Stack pointer for this thread
Id    equ  4       Used to visualize which thread is current running
RunPt rmb  2       pointer to thread that is currently running
Main  ldaa #$FF
      staa DDRH    PortH displays which thread is executing
      staa DDRJ    PortJ displays which program is executing
      ldx  #TCB1   First thread to run
      jmp  Start
* Suspend thread which is currently running
OC5Han ldx  RunPt
       sts  SP,x    save Stack Pointer in TCB
* launch next thread
       ldx  Next,x
Start  stx  RunPt
       ldaa Id,x
       staa PORTH   visualizes running thread
       lds  SP,x    set SP for this new thread
       ldd  TC5
       addd #20000  interrupts every 10 ms
       std  TC5
       ldaa #$20    ($08 on the 6811)
       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,x
LoopA ldaa #2
      staa PORTJ
      ldd  0,x
      jsr  sub
      std  0,x
      bra  LoopA
ProgB pshx     
      tsx
      ldd  #5
      std  0,x
LoopB ldaa #4
      staa PORTJ
      ldd  0,x
      jsr  sub
      std  0,x
      bra  LoopB
Sub pshx     
    tsx
    std  0,x
    ldaa #1
    staa PORTJ
    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 1
wait   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
       rts
signal inc  S     S=S+1, this is atomic
       rts 

; Program 5.9. 6812 assembly code for a spinlock binary semaphore.
S      fcb  1     semaphore flag initialized to 1
bWait  clra       new value
loop   minm S     in either case S is now 0, carry set if S used to 1
       bcc  loop  loop until S=1
       rts
bSignal ldaa #1
       staa S     S=1
       rts 

; Program 5.12. Assembly code to initialize a blocking semaphore.
S       rmb  1   semaphore counter
BlockPt rmb  2   Pointer to threads blocked on S
Init    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 SWI
SWIhan 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 run
look   cpx  Next,y  search to find previous
       beq  found
       ldy  Next,y
       bra  look
found  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  TC5
       ldaa #$20    ($08 on the 6811)
       staa TFLG1   clear C5F
       rti