Subj : DOS mutex
To   : ALL
From : Jcurtis
Date : Wed Jun 04 2025 10:58:42

I have a shared list. User code adds to it, interrupt code drains it.
The user update sequence must be atomic, so the list can't be corrupted
by interrupt code. Typical method is a cli/sti critical section around
the user update sequence.

But I avoid disabling interrupts when possible, and x86 xchg does the
trick. Indirect addresssing using [bx] was the only way I could make it
work. If anyone has a better trick, please advise.

--- code works with Borland C++ 3.1 ---

int data [] = {3,14,29,33,0,0,0,0,0};   /* list count in element [0] */
int *w = data;                          /* do not disturb when [0] == 0 */

void
main ()
{
    _DX = 0;                            /* list count == 0 means mutex */

    asm mov bx, w;                      /* get list count and set mutex */
    asm xchg dx, [bx];                  /* with CPU interrupts inhibited */

    w[++_DX] = 55;                      /* add a new element to the list */

    asm mov bx, w;                      /* bx got trashed, load it again */
    asm xchg dx, [bx];                  /* updates count and clears mutex */

    _AX = 0;                             /* stopping place for debugger */
}

 * SLMR 2.1a *
---
 þ Synchronet þ Vertrauen þ Home of Synchronet þ [vert/cvs/bbs].synchro.net

.