I have some PCI spec 2.1 motherboards (mid 1990s). I wanted multiple
serial cards sharing a single IRQ.

With 5 or more PCI slots, at least two are hard wired to share an INT
pin. Easy solution for two cards sharing one IRQ. Just find which slots
share the pin, and there you go.

Beyond that, you can move cards around and tweak BIOS setup options.
Maybe you get lucky, maybe not.

Reprogramming the PCI IRQs is a lot of trouble, according to some web
sites. But if you have a PCI 2.1 BIOS, the PCI 2.1 spec document, and
the PCI 2.1 BIOS spec document, it's easy: use the INT 1A interface.

I wrote some code. It's for Borland C++ 3.1, but it's small enough to
easily port.

Run "pd >pi.txt"

Edit the output file and change IRQs (last column) as desired.

Run "pi" to update the IRQs.

Beware of hardwired slot sharing. Both must be updated with the same
values. Experiment and see.

---- build.bat

bcc -Tt pd.c
bcc pi.c
del *.obj

---- pd.c

# pragma inline

# include <stdio.h>
# include <stdlib.h>
# include <dos.h>

int
detect (void)
{ unsigned long tag;
int spec, max;

asm {
.386;

xor edx, edx;
mov ax, 0xb101;
int 0x1a;
jc nope;
or ah, ah;
jnz nope;
mov tag, edx;
mov spec, bx;
mov ch, 0;
mov max, cx;

.8086;
}

if (tag != 0x20494350)
goto nope;

if (spec < 0x210)
goto nope;

return max;

nope:
return -1;
}

unsigned int
read_word (int bdf, int reg)
{ _BX = bdf;
_DI = reg & 0xfe;
_AX = 0xb109;
geninterrupt (0x1a);
return _CX;
}

void
list (int max)
{ unsigned int irq, pin;
int vid, did, type;
register int bdf, bus, dev, fun;

for (bus = 0; bus <= max; bus++) {
for (dev = 0; dev < 32; dev++) {
for (fun = 0; fun < 8; fun++) {
bdf = bus << 8 | dev << 3 | fun;

if ((vid = read_word (bdf, 0)) == -1) {
if (fun > 0)
continue;
break;
}

type = read_word (bdf, 0xe);
if (type & 1) /* omit bridge */
continue;

irq = read_word (bdf, 0x3c);
pin = (irq & 0xff00) >> 8;
if (!pin)
continue;

irq &= 0x00ff;
did = read_word (bdf, 2);

printf ("%04x %04x ", vid, did);
printf ("%2x:%02x:%1x ", bus, dev, fun);
printf ("%1d %2d\n", pin, irq);

if (fun == 0 && !(type & 1 << 7))
break;
}
}
}
}

void
main (void)
{ int max;

if ((max = detect ()) == -1) {
printf ("no PCI BIOS 2.1\n");
exit (1);
}

list (max);

exit (0);
}

---- pi.c

# include <stdio.h>
# include <stdlib.h>
# include <dos.h>

void
write_irq (int bdf, int pin, int irq)
{ asm push ds;
_BX = bdf;
_CH = irq;
_CL = pin + 9;
_DS = _AX = 0xf000;
_AX = 0xb10f;
geninterrupt (0x1a);
asm pop ds;

_BX = bdf;
_CL = irq;
_DI = 0x3c;
_AX = 0xb10b;
geninterrupt (0x1a);
}

void
main (void)
{ static FILE *file;
static int b, d, f, p, i;

file = fopen ("pi.txt", "rt");

while (fscanf (file, "%*x %*x %x:%x:%x %d %d", &b, &d, &f, &p, &i) == 5) {
printf ("%2x:%02x:%1x %1d %2d\n", b, d, f, p, i);
write_irq (b << 8 | d << 3 | f, p, i);
}

exit (0);
}