Path: utzoo!attcan!uunet!steinmetz!ge-dab!peora!rtmvax!johnc
From: johnc@rtmvax.UUCP (John Connin)
Newsgroups: comp.os.minix
Subject: critical sections
Keywords: Minix, interrupts, lock, restore
Message-ID: <1804@rtmvax.UUCP>
Date: 19 Jul 88 02:34:02 GMT
Organization: Robert Talley (PRIVATE), Orlando, FL
Lines: 73


I thought the following "critial section" code from Thomas Wagner's
excellent public domain "Ctask" package, might be of interest.  In
addition to the clean notation, note that the 'lock' and 'restore'
routines only operate on the psw interrupt bit -- leaving all other
flags unaltered.  Though restoring the whole psw as done currently
in Minix and Xinu seems to work, only changing the interrupt flag
seems to me like a better way of doing business.


#define CRITICAL  int crit_save
#define C_ENTER   crit_save = lock()
#define C_LEAVE   restore(crit_save)

foo()
{
    CRTIICAL;

    C_ENTER;
    while( .... ) {
        ....
        if ( .... ) {
            C_LEAVE:
            return;
        ....
    }
    C_LEAVE;
}

;--------------------------------------------------------------------
;
;   int lock(void)
;
;   Returns current state of the interrupt flag (1 if ints were
;   enabled), then disables interrupts.
;
_lock   proc
;
        pushf                   ;
        cli                     ;
        pop     ax              ; extract and return the flag
        mov     cl,9            ;  interrupt bit.
        shr     ax,cl           ;
        and     ax,1            ;
        ret                     ;
;                               ;
_lock   endp
;

;--------------------------------------------------------------------
;
;   void  restore(int state)
;
;   Enables interrupts if 'state' is nonzero.
;
_restore proc
;
        push    bp              ;
        mov     bp,sp           ;
        mov     ax,4[bp]        ; ax - state
        pop     bp              ;
        or      ax,ax           ; if (state)  sti
        jz      _rstend         ;
        sti                     ;
_rstend:                        ;
        ret                     ;
;
_restore endp
;

BTW: Ctask is a public domain software package which enables multi-tasking
within MSDOS programs.  Ctask is available on BIX and is authored by
Thomas Wagner, Patschkauer Weg 31, D-1000 Berlin 33, West Germany.