The C Users' Group Library 1994 August/wc-cdrom-cusersgrouplibrary-1994-08.iso/vol_300/330_02/tskasm.asm

Assembly Source File | 1990-10-12 | 27KB | 1,312 lines

;
;    --- Version 2.2 90-10-12 10:37 ---
;
;    CTask - Scheduler and miscellaneous utilities
;
;    Public Domain Software written by
;        Thomas Wagner
;        Ferrari electronic Gmbh
;        Beusselstrasse 27
;        D-1000 Berlin 21
;        Germany
;
;
; NOTE: Version 2.1 enables interrupts shortly after setting the in_sched
;    flag. This greatly reduces interrupt latency, but it has its
;    pitfalls. The main culprit is modification of the current TCB
;    while the scheduler is busy processing it. However, the situations
;    where a "foreign" TCB is modified are rare, so it is possible to
;    handle those cases in the appropriate places. Note that it was
;    never a good idea to make a task waiting or, worse yet, kill the
;    current task, while the scheduler was active. This was not detected
;    in previous releases, but would still cause crashes, even though
;    the interrupt was enabled only in the eligible waiting loop.
;
;    The only legitimate action is to make a task eligible while it
;    is being scheduled out. Since this only modifies the queue head
;    and task status, no adverse effects are possible. But making a task
;    waiting that is already scheduled out (as it was possible in
;    all versions up to 2.1) simply won't work. The wait action requires
;    that the scheduler switch the context away from the running task,
;    which it can't do when it's already active and there is no real
;    "running task". So the scheduler would immediately return, and
;    the wait would never occur, most likely causing some very
;    strange effects. Version 2.1 now explicitly checks for this error,
;    and will terminate the tasker if this should ever happen.
;
;    Other safeguards include not calling the INT 8 timer interrupt
;    chain for early INT8 processing while the scheduler is active.
;    If INT8_EARLY is used, the tick chain is called from within
;    the interrupt handler. Since the interrupt could have hit
;    while the scheduler was active, and the original INT 8
;    may have TSRs attached that call DOS, dangerous conflicts are
;    possible. For the same reason, the scheduler will set the In-DOS
;    flag while it is active, to keep TSR's activated by other
;    interrupts (esp. the keyboard) from calling DOS.
;    Using the same delayed processing as is used in INT 8 in
;    the INT 9 handler was tried, but led to lost keystrokes depending
;    on system load.
;
;
;    Interrupts also are enabled in most other places in the scheduler,
;    except when traversing the queues.
;
;    Having interrupts enabled during the save/restore code also
;    eliminates a deadlock problem with floating point exceptions
;    on the 8087, which simplifies saving and restoring the '87 registers.
;
;
;    Version 2.2 eliminates a race condition problem introduced by
;    the above changes. If a task was made waiting, only the queue head
;    reflected this. Only after the scheduler had entered the task into
;    the appropriate queue would it have been safe to enable interrupts,
;    since an interrupt handler calling (indirectly) tsk_runable would
;    never see the task as waiting if it hit before this time.
;    To keep interrupts enabled, the logic was changed in that the
;    wait action now inserts the task into the appropriate queue
;    directly, without waiting for the scheduler to do it. Naturally,
;    this means a change here so that the scheduler doesn't touch
;    the queue if the task is already enqueued. Only if the task is not
;    in any queue is the queue head used to determine the place to
;    enqueue, which now can occur only when making a running task
;    eligible.
;
;**************************  CAUTION:  ***********************************
;
;    If the DOS flag is set, the scheduler uses an undocumented feature
;    of DOS versions >= 3.10 to save and restore certain variables
;    local to the DOS kernel. This allows for fast, secure task switching
;    with tasks owning different PSP's. To save and restore the complete
;    state of DOS on every task switch would not be feasible with any
;    other documented method.
;
;    NOTE that this method relies on certain inner workings of DOS
;    that may not be compatible with future releases or "compatible"
;    operating systems. It has been tested with all DOS versions
;    from 3.10 through 3.20, 3.30, up to PC/MS-DOS 4.01. It
;    also has been reported to work with DR-DOS 3.41.
;
;*************************************************************************
;
    name    tskasm
;
    include    tsk.mac
    include    tskdeb.h
;
    .tsk_model
;
    public    tsk_scheduler
    public    sched_int
;
    Pubfunc    schedule
    Pubfunc    yield
    Pubfunc    c_schedule
;
    Pubfunc    tsk_callfunc
    Pubfunc    tsk_dis_int
    Pubfunc    tsk_ena_int
    Pubfunc    tsk_cli
    Pubfunc    tsk_sti
    Pubfunc    tsk_dseg
    Pubfunc    tsk_flags
    Pubfunc    tsk_outp
    Pubfunc    tsk_inp
    Pubfunc    tsk_inpw
    Pubfunc    tsk_nop
    Pubfunc    tsk_memcpy
;
    IF    CLOCK_MSEC
    Pubfunc    tsk_timeout
    ENDIF
;
    IF    CHECKING
    CGlbext    tsk_fatal_pmd
    ENDIF
;
    IF    DEBUG AND DEB_TSKDIS
    extrn    tsk_debtask: word
    ENDIF
    IF    DEBUG AND DEB_FLASHERS
    Pubfunc    tsk_inccdis
    extrn    tsk_debflash: word
    ENDIF
    IF    DEBUG
    extrn    tsk_dtposn: dword
    ENDIF
;
    global_ext
;
;
LSTACK_SIZE    =    512    ; local stack size in words
;
INT_FLAG    =    2h    ; Int enable flag in upper byte of flag reg
;
sr_flags    =    8    ; Offset of flag register on task stack
sr_ds        =    2    ; Offset of DS on task stack
;
psp_savoff    =    2eh    ; Offset of PSP stack pointer save dword
;
    IF    FAR_STACKS
ctask_stacks    segment word public 'CTASK_STACKS'
    ELSE
    .tsk_data
    ENDIF
;
    dw    LSTACK_SIZE dup(?)
slocal_stack    label    word
;
    IF    FAR_STACKS
ctask_stacks    ends
    .tsk_data
    ENDIF
;
    .tsk_edata
    .tsk_code
;
tsk_dgroup    dw    @CTASK_DATA
;
    IF    FAR_STACKS
stackseg    dw    SEG ctask_stacks
    ELSE
stackseg    equ    <tsk_dgroup>
    ENDIF
;
;------------------------------------------------------------------------
;
;    enq    Enqueue tcb in queue. For local use only.
;        entry:    es:di = tcb to enqueue
;        exit:    -
;        uses:    ax, cx, si
;    NOTE:    Interrupts must be disabled on entry.
;
enq    macro
;
    push    ds
    lds    cx,es:cqueue.q_next[di]    ; this is NULL if not in queue
    mov    ax,ds
    or    ax,cx
    jnz    enq_end            ; don't touch if enqueued
    lds    si,es:qhead[di]        ; queue head pointer
    mov    ax,ds
    or    ax,si
    jz    enq_end            ; nothing left to do if queue null
;
    mov    cx,es:cqueue.q_el.q_prior[di]
    mov    ax,es:cqueue.q_el.q_ini_prior[di]
    mov    es:cqueue.q_el.q_prior[di],ax
    lds    si,q_last[si]        ; last queue element
;
enq_loop:
    test    q_kind[si],Q_HEAD    ; at head?
    jnz    enq_found        ; then insert
    cmp    q_el.q_prior[si],cx    ; else check priority
    jae    enq_found        ; if above or equal, insert
;
    lds    si,q_prev[si]        ; backup one element
    jmp    enq_loop        ; and try again
;
enq_found:
    mov    word ptr es:q_prev[di],si    ; elem->prev = curr
    mov    word ptr es:q_prev+2[di],ds
    mov    ax,word ptr q_next[si]        ; elem->next = curr->next;
    mov    word ptr es:q_next[di],ax
    mov    cx,word ptr q_next+2[si]
    mov    word ptr es:q_next+2[di],cx
    mov    word ptr q_next[si],di        ; curr->next = elem;
    mov    word ptr q_next+2[si],es
    mov    si,ax
    mov    ds,cx
    mov    word ptr q_prev[si],di        ; elem->next->prev = elem
    mov    word ptr q_prev+2[si],es
;
enq_end:
    pop    ds
;
    endm
;
;    upd_prior: Update priority of tasks in eligible queue.
;               Only activated if tsk_var_prior is nonzero.
;
;    entry:    ds:bx = global variable block
;    exit:    -
;    uses:    ax,di,es
;
;    NOTE:    Contrary to what earlier versions said, this loop
;        must be protected by interrupt disable.
;        Since it steps through a pointer chain, and this
;        pointer chain might be modified by external events
;        (a task becoming eligible to run), there is indeed
;        a danger of race conditions.
;
upd_prior    macro
;
    les    di,eligible_queue.q_first[bx]
;
pinc_loop:
    test    es:q_kind[di],Q_HEAD    ; queue head?
    jnz    updp_end
    inc    es:q_el.q_prior[di]
    jnz    pinc_nxt
    dec    es:q_el.q_prior[di]
pinc_nxt:
    les    di,es:q_next[di]
    jmp    pinc_loop
;
updp_end:
;
    endm
;
;-------------------------------------------------------------------------
;
    IF    DEBUG AND DEB_TSKDIS
;
fill8    macro
    mov    cx,10
    mov    ax,0720h
    rep stosw
    endm
;
@strcpy8    proc    near
    mov    cx,8
    mov    ah,7
strcpyl:
    lodsb
    or    al,al
    jz    strcpyfill
    stosw
    loop    strcpyl
;
strcpyfill:
    add    cx,2
    mov    al,' '
    rep stosw
    ret
;
@strcpy8    endp
;
    ENDIF
;
    IF    DEBUG AND DEB_FLASHERS
;
;    tsk_inccdis - increment counter on display
;
;    Entry:    AX is counter first digit offset
;        DS is CTask dgroup
;
;    Uses:    All registers preserved
;
Localfunc    tsk_inccdis
;
    push    ds
    push    si
    push    cx
    lds    si,tsk_dtposn
    add    si,ax
    mov    cx,DEBFLASH_NDIGS
    add    si,(DEBFLASH_NDIGS - 1) * 2
incdis_loop:
    inc    byte ptr [si]
    cmp    byte ptr [si],'9'
    jbe    incdis_end
    mov    byte ptr [si],'0'
    sub    si,2
    loop    incdis_loop
incdis_end:
    pop    cx
    pop    si
    pop    ds
    ret
;
tsk_inccdis    endp
;
    ENDIF
;
;-------------------------------------------------------------------------
;
;    The scheduler. Note that this routine is entered with the stack
;    set up as for an interrupt handler.
;
tsk_scheduler    proc    far
;
    cli            ; better safe than sorry
;
;    First, check if we're already in the scheduler. This might
;    happen if an interrupt handler schedules, and it would have
;    catastrophic results if the scheduler would be re-entered.
;    Previous versions used a code-segment based variable to store
;    this flag. Starting at version 2.0, the flag is located in the
;    global variable block, to support ROM-based implementations,
;    and to avoid multiple entries into the scheduler when code
;    sharing is not used in secondary invocations.
;
;    Version 2.1 sets the 'pretick' flag when the scheduler is busy.
;    This allows an immediate re-schedule if the schedule request
;    hit after loading the new task.
;
    push    ds
    push    bx
    mov    ds,cs:tsk_dgroup
    IF    SINGLE_DATA
    cmp    tsk_glob_rec.in_sched,0    ; already in the scheduler?
    je    sched_ok    ; continue if not
    mov    tsk_glob_rec.pretick,1    ; mark schedule pending
    ELSE
    lds    bx,tsk_global
    cmp    in_sched[bx],0    ; already in the scheduler?
    je    sched_ok    ; continue if not
    mov    pretick[bx],1    ; mark schedule pending
    ENDIF
    pop    bx
    pop    ds        ; else return immediately
    iret
;
;    Not the second time into the scheduler, set the in_sched flag,
;    and load the current task's TCB.
;    If we're running under DOS, additionally set the In-DOS flag
;    for the reasons outlined above.
;
sched_ok:
    IF    SINGLE_DATA
    inc    tsk_glob_rec.in_sched
    IF    DOS
    push    ds
    lds    bx,tsk_glob_rec.dos_in_use
    inc    byte ptr ds:[bx]
    pop    ds
    ENDIF
    lds    bx,tsk_glob_rec.current_task
    ELSE
    inc    in_sched[bx]
    IF    DOS
    push    ds
    push    bx
    lds    bx,dos_in_use[bx]
    inc    byte ptr [bx]
    pop    bx
    pop    ds
    ENDIF
    lds    bx,current_task[bx]
    ENDIF
    push    ax
    mov    ax,ds
    or    ax,bx
    pop    ax
;
;    Registers are stored in the TCB to keep stack usage minimal.
;    If there is no current task (i.e. it has been killed),
;    we can't store the registers.
;
    IF    CHECKING
    jnz    chk_rsave
    jmp    no_rsave
chk_rsave:
    CHECK_TCBPTR_R    ds,bx,"scheduler: current task"
    ELSE
    jz    no_rsave
    ENDIF
    cmp    state[bx],ST_RUNNING
    jne    store_regs
    mov    state[bx],ST_ELIGIBLE
;
store_regs:
    sti
    mov    t_sp[bx],sp
    mov    t_ss[bx],ss
    mov    t_ax[bx],ax
    mov    t_cx[bx],cx
    mov    t_dx[bx],dx
    mov    t_bp[bx],bp
    mov    t_si[bx],si
    mov    t_di[bx],di
    mov    t_es[bx],es
    mov    bp,sp
    or    byte ptr sr_flags+1[bp],INT_FLAG ; enable interrupts on return
    mov    ax,sr_ds[bp]
    mov    t_ds[bx],ax
;
;    This is the entry when restarting the scheduler.
;    Note that registers don't have to be saved again.
;
sched_restart:
no_rsave:
    cli
    mov    ss,cs:stackseg        ; switch to local stack
    mov    sp,offset slocal_stack
    sti
    cld
    mov    ds,cs:tsk_dgroup    ; establish addressing of our vars
    IF    DEBUG AND DEB_FLASHERS
    cmp    tsk_debflash,0
    je    debdd0
    mov    ax,DEBP_CNTSCHED
    call    tsk_inccdis
debdd0:
    ENDIF
    IF    DEBUG AND DEB_TSKDIS
    cmp    tsk_debtask,0
    je    debdd1
    les    di,tsk_dtposn
    mov    byte ptr es:[di],'*'
debdd1:
    ENDIF
    IF    SINGLE_DATA
    mov    bx,offset tsk_glob_rec
    ELSE
    lds    bx,tsk_global
    ENDIF
;
    cmp    var_prior[bx],0
    je    no_var_pri
;
    cli
    upd_prior            ; update priorities in eligible queue
    sti
;
no_var_pri:
;
    and    preempt[bx],1        ; Turn off temp preempt flag
;
    les    di,current_task[bx]    ; get current tcb again
    mov    ax,es            ; check if NULL (current task killed)
    or    ax,di
    IF    NOT CHECKING
    jz    no_current
    ELSE
    jnz    do_chkstk
    jmp    no_current
;
do_chkstk:
    push    ds
    lds    si,es:stkbot[di]
    mov    cx,8
    xor    ah,ah
chkstk:
    lodsb
    cmp    ah,al
    jne    stkoflo
    inc    ah
    loop    chkstk
    jmp    short chkstend
;
stkmsg    db    "Stack Overflow",0
;
stkoflo:
    callp    tsk_fatal_pmd,<<cs,#stkmsg>>
;
chkstend:
    pop    ds
    ENDIF
;
;    If there is a scheduler-entry routine in the current task's
;    TCB, call it. (This is used by the DOS handler module)
;    NOTE: Interrupts are still enabled.
;
    IF    DOS
    mov    ax,word ptr es:sched_ent_func[di]
    or    ax,word ptr es:sched_ent_func[di]+2
    jz    no_schentcall
;
    push    es
    push    di
    push    ds
    push    bx
    call    es:sched_ent_func[di]
    pop    bx
    pop    ds
    pop    di
    pop    es
;
no_schentcall:
    ENDIF
;
    cli
    enq                ; Enqueue current task
    sti
;
no_current:
    lea    si,eligible_queue[bx]
;
;    Now we enter an enabled loop if there is no task in the
;    eligible queue.
;
wait_elig:
    IF    DEBUG AND DEB_TSKDIS
    push    ds
    mov    ds,cs:tsk_dgroup
    cmp    tsk_debtask,0
    je    debdd2
    les    di,tsk_dtposn
    add    di,DEBP_ELIGTSK
    pop    ds
    push    ds
    push    si
    lds    si,q_first[si]
    test    q_kind[si],Q_HEAD
    jnz    deb101
    push    si
    lea    si,tname.nname[si]
    call    @strcpy8
    pop    si
    lds    si,q_next[si]
    test    q_kind[si],Q_HEAD
    jnz    deb102
    lea    si,tname.nname[si]
    call    @strcpy8
    jmp    short deb105
;
deb101:
    fill8
deb102:
    fill8
deb105:
    pop    si
debdd2:
    pop    ds
    ENDIF
    jmp    $+2            ; allow ints
    cli
    les    di,q_first[si]
    test    es:q_kind[di],Q_HEAD
    jz    not_empty        ; jump if not
;
    sti                       ; enable interrupts
    IF    DEBUG AND DEB_FLASHERS
    cmp    tsk_debflash,0
    je    debdidle
    mov    ax,DEBP_CNTIDLE
    call    tsk_inccdis
debdidle:
    ENDIF
    jmp    $+2
    jmp    wait_elig
;
;    Eligible queue not empty, activate first eligible task.
;    First, take it off the queue.
;    The preemption tick flag is cleared here. If it should be
;    set by an interrupt handler trying to schedule after this point,
;    we immediately re-schedule when hitting the end.
;
not_empty:
    CHECK_TCBPTR_R    es,di,"scheduler: eligible task"
    mov    pretick[bx],0        ; No preemption tick
    push    di
    push    es
    mov    bp,sp            ; address new pointer thru BP
;
    mov    ax,word ptr es:cqueue.q_next[di] ; next in eligible queue
    mov    cx,word ptr es:cqueue.q_next+2[di]
    mov    word ptr es:cqueue.q_next[di],0    ; mark not in queue
    mov    word ptr es:cqueue.q_next+2[di],0
    mov    di,ax
    mov    es,cx
    mov    word ptr q_first[si],di
    mov    word ptr q_first+2[si],es    ; eligible_queue.first = next
    mov    word ptr es:q_prev[di],si
    mov    word ptr es:q_prev+2[di],ds    ; next->prev = eligible_queue
;
;    Again, interrupts can be enabled here.
;
    sti
;
;    Now check if the new task is the same as the old one.
;    If that's the case, we skip the save/restore function calls,
;    and the DOS variable copy.
;
    les    di,current_task[bx]    ; the previously running task
    mov    ax,es
    cmp    ax,[bp]            ; same as the new one ?
    jne    chk_sav            ; jump if not same segment
    cmp    di,2[bp]
    jne    chk_sav            ; jump if not same offset
    jmp    sched_complete        ; don't save/restore if same task
;
chk_sav:
    or    ax,di
    jz    set_new_task        ; don't save if no previous
;
;    First, call the save function if one is defined in the old TCB.
;
    mov    ax,word ptr es:save_func[di]
    or    ax,word ptr es:save_func+2[di]
    jz    no_savfcall        ; jump if no save function
;
    push    ds            ; save our registers
    push    bx
    push    es
    push    di
;
    push    es            ; push TCB address
    push    es            ; push segment again
    pop    ds            ; and put into DS
    push    di
;
    call    es:save_func[di]    ; call function
        add     sp,4
;
        pop     di            ; restore registers
        pop     es
        pop     bx
        pop     ds
;
;    Save the DOS variables, and the DOS-related interrupt vectors
;    in the old TCB.
;
no_savfcall:
    IF    DOS
    mov    es:t_new[di],0        ; task is no longer new
;
    push    ds
    push    di
    mov    ax,ds
    mov    ds,es:base_psp[di]    ; get base PSP address
    mov    si,psp_savoff        ; offset to save (caller's SS:SP)
    add    di,psp_sssp        ; destination
    movsw                ; save two words
    movsw
    mov    ds,ax
    mov    cx,l_swap[bx]        ; swap area addr & size
    jcxz    no_swap_sav
    lds    si,dos_vars[bx]
    rep movsb
;
no_swap_sav:
    xor    cx,cx            ; copy int21-24 interrupt vectors
    mov    ds,cx
    mov    si,21h*4
    mov    cx,8
    rep movsw
;
    pop    di
    pop    ds
    ENDIF
;
;    if EMS support is installed, call the page map save function.
;
    IF    EMS
    mov    ax,word ptr ems_save[bx]
    or    ax,word ptr ems_save+2[bx]
    jz    no_emssav
    call    ems_save[bx]
;
no_emssav:
    ENDIF
;
;    If the NDP is active, save the NDP context.
;
    IF    NDP
    test    es:flags[di],F_USES_NDP
    jz    no_ndpsave
    wait
    fsave    es:ndpsave[di]
;
no_ndpsave:
    ENDIF
;
;    Save complete. The new TCB becomes the current task.
;
set_new_task:
    IF    DEBUG AND DEB_TSKDIS
    push    ds
    mov    ds,cs:tsk_dgroup
    cmp    tsk_debtask,0
    je    deb205
    les    di,tsk_dtposn
    pop    ax            ; saved DS
    pop    ds            ; the new task's TCB
    pop    si
    push    si
    push    ds
    push    ax            ; saved DS
    mov    byte ptr es:[di],'+'
    add    di,DEBP_CURRTSK
    lea    si,tname.nname[si]
    call    @strcpy8
    pop    ds
    push    ds
    lds    si,current_task[bx]    ; the previous task
    lea    si,tname.nname[si]
    call    @strcpy8
    pop    ds
    push    ds
    lds    si,eligible_queue.q_first[bx]
    test    q_kind[si],Q_HEAD
    jnz    deb201
    push    si
    lea    si,tname.nname[si]
    call    @strcpy8
    pop    si
    lds    si,q_next[si]
    test    q_kind[si],Q_HEAD
    jnz    deb202
    lea    si,tname.nname[si]
    call    @strcpy8
    jmp    short deb205
;
deb201:
    fill8
deb202:
    fill8
deb205:
    pop    ds
    ENDIF
    pop    es
    pop    di
    cli
    mov    word ptr current_task[bx],di     ; set tcb into current
    mov    word ptr current_task+2[bx],es
    sti
;
;    if EMS support is installed, call the page map restore function.
;
    IF    EMS
    mov    ax,word ptr ems_rest[bx]
    or    ax,word ptr ems_rest+2[bx]
    jz    no_emsrest
    call    ems_rest[bx]
no_emsrest:
    ENDIF
;
;    Now check for a restore function in the new TCB.
;
    mov    ax,word ptr es:rest_func[di]
    or    ax,word ptr es:rest_func+2[di]
    jz    no_resfcall            ; jump if no restore function
;
    push    bx                ; save our regs
    push    ds
    push    di
    push    es
;
    push    es                ; push TCB addr
    push    es                ; push segment again
    pop    ds                ; and put into DS
    push    di
;
    call    es:rest_func[di]    ; call restore function
        add     sp,4
;
        pop     es
        pop     di
        pop     ds
        pop     bx
;
no_resfcall:
;
;    If NDP is enabled, restore 80x87 context.
;
    IF    NDP
    test    es:flags[di],F_USES_NDP
    jz    no_ndprest
    cmp    es:t_new[di],0        ; is this TCB a fresh one?
    jne    no_ndprest        ; then NDP state isn't valid
;
    wait
    frstor    es:ndpsave[di]
;
no_ndprest:
    ENDIF
;
;    Restore DOS variables and int vectors from new TCB.
;
    IF    DOS
    cmp    es:t_new[di],0        ; is this TCB a fresh one?
    jne    sched_complete        ; then the DOS info isn't valid.
;
    push    di
    push    es
    push    ds
    mov    dx,ds
    mov    ax,es
    mov    ds,ax
    mov    si,di
    add    si,base_psp
    lodsw
    mov    es,ax
    mov    di,psp_savoff        ; offset to restore (caller's SS:SP)
    cli
    movsw                ; restore two words
    movsw
    sti
;
    mov    ax,ds
    mov    ds,dx
    mov    cx,l_swap[bx]
    jcxz    no_swap_rest
    les    di,dos_vars[bx]
    mov    ds,ax
    rep movsb
;
no_swap_rest:
    mov    ds,ax
    mov    di,21h*4        ; restore int21-24
    xor    cx,cx
    mov    es,cx
    mov    cx,8
    cli
    rep movsw
    sti
;
    pop    ds
    pop    es
    pop    di
    ENDIF
;
;    And that's it. Wrap it up by resetting the priority,
;    restoring the registers, resetting the in_sched flag,
;    and returning to the task.
;    Note: to allow keeping interrupts enabled as long as possible,
;    the in_sched flag is cleared after reloading the registers.
;
sched_complete:
    mov    ax,es:q_el.q_ini_prior[di]    ; reset current tasks priority
    mov    es:q_el.q_prior[di],ax
    mov    es:state[di],ST_RUNNING        ; set task state
;
    push    es
    push    di
    IF    DEBUG AND DEB_TSKDIS
    mov    ds,cs:tsk_dgroup
    cmp    tsk_debtask,0
    je    debddx
    les    di,tsk_dtposn
    mov    byte ptr es:[di],' '
debddx:
    ENDIF
    pop    bx
    pop    ds
    mov    es,t_es[bx]            ; restore all registers
    mov    di,t_di[bx]
    mov    si,t_si[bx]
    mov    bp,t_bp[bx]
    mov    dx,t_dx[bx]
    mov    cx,t_cx[bx]
    mov    ax,t_ax[bx]
    mov    ss,t_ss[bx]
    mov    sp,t_sp[bx]
;
;    All done except for resetting the in_sched flag.
;    Now we check the preemption tick flag, and restart the schedule
;    if it is set. To avoid needless reschedules, the priority of the
;    current task is compared to the priority of the first task in
;    the eligible queue. If there is no task eligible, or if its
;    priority is less or equal to the current task, the current task
;    is allowed to continue.
;
    mov    ds,cs:tsk_dgroup
    cli
    IF    SINGLE_DATA
    cmp    tsk_glob_rec.pretick,0
    je    sched_end
    mov    bx,offset tsk_glob_rec
    ELSE
    lds    bx,tsk_global
    cmp    pretick[bx],0
    je    sched_end
    ENDIF
;
    push    es
    push    di
    les    di,current_task[bx]    ; get current tcb again
    lds    bx,eligible_queue.q_first[bx]
    test    q_kind[bx],Q_HEAD
    jnz    no_restart
    mov    bx,cqueue.q_el.q_prior[bx]    ; prior of eligible task
    cmp    bx,es:cqueue.q_el.q_prior[di]    ; prior of current task
    jbe    no_restart
;
;    Note: registers don't have to be popped, since the stack
;    is reset anyway at the sched_restart entry.
;
do_rest:
    jmp    sched_restart
;
;
no_restart:
    pop    di
    pop    es
    mov    ds,cs:tsk_dgroup
    IF    NOT SINGLE_DATA
    lds    bx,tsk_global
    ENDIF
;
sched_end:
    IF    SINGLE_DATA
    mov    tsk_glob_rec.in_sched,0
    IF    DOS
    lds    bx,tsk_glob_rec.dos_in_use
    dec    byte ptr [bx]
    ENDIF
    ELSE
    mov    in_sched[bx],0
    IF    DOS
    lds    bx,dos_in_use[bx]
    dec    byte ptr [bx]
    ENDIF
    ENDIF
;
    pop    bx
    pop    ds
    iret
;
tsk_scheduler    endp
;
;
;--------------------------------------------------------------------------
;
;
;    sched_int
;
;    Is the scheduler entry for interrupt handlers.
;    It checks if preemption is allowed, returning if not.
;    The stack is assumed to be set up as on interrupt entry.
;
sched_int    proc    far
;
    push    ds
    push    bx
    mov    ds,cs:tsk_dgroup
    IF    SINGLE_DATA
    mov    bx,offset tsk_glob_rec
    ELSE
    lds    bx,tsk_global
    ENDIF
    cmp    preempt[bx],0        ; preempt flags 0?
    jne    no_sched1        ; no scheduling if set
    lds    bx,current_task[bx]    ; current running task
    test    flags[bx],F_CRIT    ; preemption allowed for this task?
    jnz    no_sched        ; no scheduling if flag set
    pop    bx            ; else go schedule
    pop    ds
    jmp    tsk_scheduler
;
no_sched:
    mov    ds,cs:tsk_dgroup
    IF    SINGLE_DATA
    mov    bx,offset tsk_glob_rec
    ELSE
    lds    bx,tsk_global
    ENDIF
no_sched1:
    mov    pretick[bx],1        ; Mark preemption pending
    pop    bx
    pop    ds
    iret
;
sched_int    endp
;
;
;    void far schedule (void)
;
;    Entry for calling the scheduler. Rearranges the stack to
;    contain flags.
;    NOTE: Uses ax,bx.
;
Globalfunc    schedule
;
    IF    NEAR_CODE
    pop    ax
    pushf
    push    cs
    push    ax
    ELSE
    pop    ax
    pop    bx
    pushf
    push    bx
    push    ax
    ENDIF
    cli
    jmp    tsk_scheduler
;
schedule    endp
;
;
;    void far yield (void)
;
;    Entry for calling the scheduler with priority temporarily
;       set to zero. Rearranges the stack to contain flags.
;    NOTE: Uses ax,bx, es.
;
Globalfunc    yield
;
    IF    NEAR_CODE
    pop    ax
    pushf
    push    cs
    push    ax
    ELSE
    pop    ax
    pop    bx
    pushf
    push    bx
    push    ax
    ENDIF
;
    IF    SINGLE_DATA
    IFDEF    LOAD_DS
    mov    es,cs:tsk_dgroup
    les    bx,es:tsk_glob_rec.current_task
    ELSE
    les    bx,tsk_glob_rec.current_task
    ENDIF
    ELSE
    IFDEF    LOAD_DS
    mov    es,cs:tsk_dgroup
    les    bx,es:tsk_global
    ELSE
    les    bx,tsk_global
    ENDIF
        les     bx,es:current_task[bx]
    ENDIF
        cli
        mov     es:q_el.q_prior[bx],0
    jmp    tsk_scheduler
;
yield    endp
;
;
;    void far c_schedule (void)
;
;    Entry for conditionally calling the scheduler. Rearranges
;    the stack to contain flags, then jumps to _sched_int.
;    NOTE: Uses ax,bx.
;
Globalfunc    c_schedule
;
    IF    NEAR_CODE
    pop    ax
    pushf
    push    cs
    push    ax
    ELSE
    pop    ax
    pop    bx
    pushf
    push    bx
    push    ax
    ENDIF
    cli
    jmp    sched_int
;
c_schedule    endp
;
;--------------------------------------------------------------------------
;
;    void tsk_callfunc (farptr funcad, farptr param)
;
;    Calls the given function, placing the segment address of
;    the parameter into the DS register.
;
Localfunc    tsk_callfunc,<uses ds, funcad: far ptr, param: far ptr>
;
    lds    ax,param
    push    ds
    push    ax
    call    funcad
    add    sp,4
    ret
tsk_callfunc    endp
;
;
;    word tsk_dseg (void)
;
;    Returns current contents of DS register.
;
Globalfunc    tsk_dseg
    mov    ax,ds
    ret
tsk_dseg    endp
;
;
;    word tsk_flags (void)
;
;    Returns current contents of Flag register.
;
Globalfunc    tsk_flags
    pushf
    pop    ax
    ret
tsk_flags    endp
;
;
;    int tsk_dis_int (void)
;
;    Returns current state of the interrupt flag (1 if ints were
;    enabled), then disables interrupts.
;
Globalfunc    tsk_dis_int
;
    pushf
    pop    ax
    mov    al,ah
    shr    al,1
    and    ax,1
    cli
    ret
;
tsk_dis_int    endp
;
;
;    void far tsk_ena_int (int state)
;
;    Enables interrupts if 'state' is nonzero.
;
Globalfunc    tsk_ena_int,<istate: word>
;
    cmp    istate,0
    je    teiend
    sti
teiend:
    ret
;
tsk_ena_int    endp
;
;
;    tsk_cli/tsk_sti: disable/enable int
;    NOTE: These routines are normally replaced by intrinsics.
;
Globalfunc    tsk_cli
    cli
    ret
tsk_cli    endp
;
;
Globalfunc tsk_sti
    sti
    ret
tsk_sti    endp
;
;
;    tsk_inp/tsk_outp: input/output from/to port
;    NOTE: These routines are normally replaced by intrinsics,
;          except for Turbo C tsk_inpw.
;
Globalfunc tsk_inp,<port: word>
;
    mov    dx,port
    in    al,dx
    xor    ah,ah
    ret
;
tsk_inp    endp
;
;
Globalfunc tsk_inpw,<port: word>
;
    mov    dx,port
    in    ax,dx
    ret
;
tsk_inpw    endp
;
;
Globalfunc tsk_outp,<port: word, val: word>
;
    mov    dx,port
    mov    al,byte ptr(val)
    out    dx,al
    ret
;
tsk_outp    endp
;
;
;    void tsk_nop (void)
;
;    Do nothing. Used for very short delays.
;
Globalfunc    tsk_nop
;
    jmp    short tnop1
tnop1:
    jmp    short tnop2
tnop2:
    ret
;
tsk_nop    endp
;
;
;    void tsk_memcpy (farptr dest, farptr src, word nbytes)
;
Globalfunc    tsk_memcpy,<uses ds si di, dest: far ptr, src: far ptr, len: word>
;
    mov    cx,len
    lds    si,src
    les    di,dest
    xor    bx,bx
    shr    cx,1
    rcl    bx,1
    jcxz    cpy_byte
    rep movsw
cpy_byte:
    or    bx,bx
    jz    cpy_end
    movsb
cpy_end:
    ret
;
tsk_memcpy    endp
;
;
    IF    CLOCK_MSEC
;
;    dword Localfunc tsk_timeout (dword tout)
;
;    Translates milliseconds to clock ticks using integer arithmetic.
;    Routine provided by Chris Blum.
;
Localfunc    tsk_timeout,<tout:dword>
;
    mov    dh,byte ptr tout
    mov    dl,byte ptr tout+3
    mov    ax,word ptr tout+1
    or    dx,dx
    jnz    tsk_timmax
    or    ax,ax
    jz    tsk_timok
tsk_timmax:
    push    ds
    mov    ds,cs:tsk_dgroup
    IF    SINGLE_DATA
    mov    bx,offset tsk_glob_rec
    ELSE
    lds    bx,tsk_global
    ENDIF
    mov    cx,tick_factor[bx]
    pop    ds
    xor    dh,dh
    cmp    cx,dx
    jbe    tsk_timhi
    div    cx
    mov    bx,ax
    xor    al,al
    mov    ah,byte ptr tout
    div    cx
    inc    cx
    shr    cx,1
    cmp    dx,cx
    mov    dx,bx
    jb    tsk_timmin
    add    ax,1
    adc    dx,0
    jnc    tsk_timok
tsk_timhi:
    mov    ax,0ffffh
    mov    dx,ax
    jmp short tsk_timok
tsk_timmin:
    or    ax,ax
    jnz    tsk_timok
    or    dx,dx
    jnz    tsk_timok
    inc    ax
tsk_timok:
    ret
;
tsk_timeout    endp
;
    ENDIF
;
    .tsk_ecode
    end