Turning off a PC using the Intel 82801 I/O Controller Hub

Shutting down a PC is normally performed by transitioning the machine into an ACPI S5 state which is achieved by writing some magic value into an ACPI PM control register.  Implementing this is very straight forward once one has navigated a couple of rather information dense documents.

Armed with a copy of an Intel ICH manual one see that section 8.8.3.3 the PM1_CNT (Power Management 1 Control) register is the direct implementation of the ACPI PM1a_CNT_BLK register as described in section 4.7.3.2.1 of version 4.0 of the ACPI specification.


Bits 10:12 control the sleep state type (SLP_TYP), and setting these to 111 selects soft power off. To signal this, one has to set bit 13 to transition into the required sleep state.

On an Intel system, finding the PM1_CNT register is simple, use:

cat /proc/ioports | grep PM1a_CNT_BLK

..on my Dell 1525 this results in:

1004-1005 : ACPI PM1a_CNT_BLK

..so a soft shutdown can be invoked running the following code with root privileges:

#include <unistd.h>

#include <sys/io.h>

#include <stdint.h>

#include <stdio.h>

/* PM1 Sleep types */

#define SLP_ON       0

#define SLP_ST_PCLK  1

#define SLP_S3       5

#define SLP_S5       6
#define SLP_SOFT_OFF 7

int main(int argc, char **argv)
{
        uint16_t val;

        if (ioperm(0x1004, 2, 1) < 0) {
                printf("Cannot access port 0x1004\n");
                exit(0);
        }
        val = inw(0x1004);
        val &= ~(7 << 10); /* Clear SLP_TYPE */
        val |= (SLP_SOFT_OFF << 10); /* Soft power off */
        val |= (1 << 13);  /* Trigger SLP_EN */
        outw(val, 0x1004);
}

..this is akin to the Vulcan death grip, so make sure that one has sync'd and unmounted file systems first! It's quite impressive to see how quickly can shutdown a machine using this method.

Of course, more rugged implementation would be to determine the PM1a_CNT_BLK register from the Fixed ACPI Description Table (FADT), but happily for us Linux reports this register in /proc/ioports, so it's simple to do from user space.