;;  mos4  all functions are in S2
;;     Happy Sonkran Day ! 
;;     13 Apr 2016
;;     semaphore       14 Apr 2016

;;  two processes share a variable

;; memory map

;; 0..900     code
;; 1000       int vec
;; 1500       global var
;; 1700       system stack
;; 2000       process queue
;; 2100       PCB
;; 2300       process stack
;; 3000       semaphore area

;;  trap 19   print3 show which process
;;  trap 20   sem = newsem()
;;  trap 21   wait(sem)
;;  trap 22   signal(sem)

.symbol
	stop		0
	di		15
	ei		16
	print1		17
	print2		18
	print3		19
	newsem		20
	wait		21
	signal		22
	cnt		1600
	sem		1601

	;; global registers
	sp		28     ;; system stack pointer
	retval		27
	link		15     ;; must be r1..r15

	;; OS variables	
	qend		1510
	qindex		1511
	nump		1500   ;; number of process
	currentp	1501   ;; pointer to current proc
	pcbptr		1502   ;; PCB block pointer
	stackptr	1503   ;; process stack pointer
	;; OS constant
	Q		2000   ;; Q at &2000

.code	0
	trap di
	;;  ... initialise
	mv r1 #tswitch
	st r1 1000	 ;; set int vec
	mv sp #1700      ;; sp = &1700
	st r0 nump
	st r0 currentp
	mv r1 #Q	 ;; Q at &2000
	st r1 qend       ;; pointer to Qend
	st r0 @0 r1	 ;; Q[end] = 0
	st r0 qindex	 ;; qindex = 0
	mv r1 #2100
	st r1 pcbptr	 ;; pcbptr = &2100
	mv r1 #2316	 ;; offset +16, for first resr
	st r1 stackptr	 ;; stackptr = &2316
	st r0 cnt	 ;; cnt = 0
	
	trap newsem
	st retval sem	 ;; sem = newsem()

	mv r1 #process1  ;; create process1
	jal link createp
	mv r1 #process1  ;; create process2
	jal link createp

	;; boot make zeroth process not in Q
	;; then switch to process in Q
	jal link newp
	st sp @1 retval
	st retval currentp  ;; zeroth p
	trap ei
	int 0             ;; start by switch p
	trap stop

;; creat new PCB, enqueue it
;;   input: &program in r1
;;   return:  process

:createp		
	push sp link	   ;; save ret
	jal link newp	   ;; newp()
	st r1 @0 retval    ;; p.PC = &program
	mv r1 retval	   ;; keep process
	jal link newStack  ;; newStack()
	st retval @1 r1    ;; p.SP = newStack
	jal link enqueue   ;; enqueue()
	mv retval r1	   ;; return process
	pop sp link	   ;; restore ret
	ret link

;; newp()
;;  a = pcbptr
;;  pcbptr += 16
;;  return a

:newp	
	ld r5 pcbptr
	mv retval r5	
	add r5 r5 #16
	st r5 pcbptr	
	ret link

:newStack
	ld r5 stackptr
	mv retval r5	
	add r5 r5 #64
	st r5 stackptr
	ret link

;; enqueue(p)
;;   Q[end] = p
;;   Q[end+1] = 0
;;   end++
;;   nump++

:enqueue		;; input in r1
	ld r6 qend
	st r1 @0 r6	;; Q[end] = p
	add r6 r6 #1
	st r0 @0 r6	;; Q[end+1] = 0
	st r6 qend	;; end++
	ld r6 nump
	add r6 r6 #1
	st r6 nump	;; nump++
	ret link

;;  terminate()
;;     Q[qindex] = 1
;;     nump--

:terminate		;; critical section
	trap di
	ld r6 qindex
	mv r7 #Q
	mv r5 #1
	st r5 +r6 r7	;; Q[qindex] = 1
	ld r5 nump
	sub r5 r5 #1
	st r5 nump	;; nump--
	trap ei
	ret link

;;   include compact Q (delete "1") 
;; nextp()
;;   if( nump == 0 ) return 0
;;   if( Q[qindex] == 1 )
;;      compact Q from qindex to qend
;;      qend--
;;   else
;;      qindex++
;;   if( Q[qindex] == 0 ) qindex = 0
;;   return Q[qindex]

:nextp
	ld r5 nump
	jt r5 skip
	mv retval r0
	ret link
:skip   ld r5 qindex
	mv r6 #Q
	ld r7 +r5 r6    ;; Q[qindex]
	eq r8 r7 #1	;; Q[qindex] == 1
	jf r8 skip2
	;; compact from qindex to qend
	add r8 r5 r6	;; r8 &Q[qindex]
	ld r9 qend
	sub r9 r9 #1
	st r9 qend	;; qend--
:loop	le r10 r8 r9	;; ptr <= qend?
	jf r10 skip2
	ld r10 @1 r8	;; 
	st r10 @0 r8	;; *prt = *(ptr+1)
	add r8 r8 #1
	jmp loop
:skip2  add r5 r5 #1
	ld r7 +r5 r6    
	jt r7 exit2     ;; Q[qindex] == 0 ?
	mv r5 r0	;; qindex = 0
	ld r7 +r5 r6
:exit2  mv retval r7	;; ret Q[qindex]
	st r5 qindex
	ret link

:process1		;; count 1..n
:loop1	ld r1 sem
	trap wait	;; wait(sem)
	ld r5 cnt
	add r5 r5 #1
	st r5 cnt
	trap signal	;; signal(sem)
	mv r1 r5
	trap print3
	gt r6 r5 #20
	jf r6 loop1
        jal link terminate
	int 0		;; force task switch
	trap stop

:tswitch		;; use r20,21
	trap di
 	ld r21 currentp
	savt r20	;; get RetAds
	st r20 @0 r21	;; p.PC = RetAds
	savr sp		;; save current context + link
	st sp @1 r21	;; p.SP = sp
	jal link nextp
	mv r21 retval
	jf r21 exit	;; no process in the queue

	st r21 currentp ;; update currentp
	ld sp @1 r21    ;; get p.SP
	resr sp		;; restore context
	ld r20 @0 r21	;; get p.PC
	rest r20	;; RetAds = p.PC
	trap ei	
	reti
:exit	trap stop

.end