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 *
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Re: DOS mutex
By: Jcurtis to ALL on Wed Jun 04 2025 10:58 am

If anyone has a better trick

Looks like something was lost along the way:

int w = data;

should be

int *w = data;

The pointer indirection is what makes the [bx] two-instruction sequence necessary. If "w" was a regular variable, one asm instruction is enough.

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