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Performing cold reset remotely

It is not possible to perform 100% cold boot without physically disconnecting power supply, some registers cannot be reset otherwise. BKDG differentiates between cold boot, cold reset and warm reset, but even within one type of reset some bits marked with "Cold reset: 0" are cleared and others are not.

Discovered ways to perform reset

Table below gives initial values, they are changed during boot sequence. Bits changing in D18F0x6C (Link Initialization Control) are set and checked by AGESA to differentiate cold boot from warm boot. Some bits (0x1003ff) in PMxC0 (S5/Reset Status) are checked by FCH initialization code, they result in additional reset when set.

Reset type D18F0x6C PMxC0
Cold boot 0x000ff800 0x00000800
Reboot 0x000ffe00 0x40080400
Reset 0x000ff800 0x40010400
Power button 0x000ff800 0x40200402
FullRst 0x000ff800 0x40200400
ACPI reset 0x000ff800 0x40000400
PCI reset 0x000ffe00 0x40020400

Bits 10 and 11 in PMxC0 can't be changed by software, even though they are described as read-write in BKDG. Bits describing reset reasons can be cleared by writing 1 to them, others were not tested.


Done with reboot in shell or after changing options in sortbootorder. It performs warm reset. Sets DoReset in PMxC0.

Reset button

Done by temporary shorting reset pin to the ground. Performs cold reset from AGESA's point of view. Sets UsrReset in PMxC0.

Power button

By shorting power button pin to the ground for more than 4 seconds platform enters S5 state. After second, short press the platform starts up. It is considered to be cold reset, but during FCH initialization platform will perform another reset (after which PMxC0 is set as in reboot, D18F0x6C as in cold boot). This was one of causes for doubled sign of life. During first reboot SleepReset and FourSecondPwrBtn are set in PMxC0.

FullRst through IO port CF9

Bit 3 of IO port CF9 is:

FullRst. Read-write. Reset: 0. 0=Assert reset signals only. 1=Place system in S5 state for 3 to 5 seconds.

Bits 1 (SysRst) and 2 (RstCmd) need to be set as well. CF9 can be accessed through its shadow in ACPI space (PMxC5, at address 0xFED803C5 when MMIO is enabled). Performs cold reset and gives time for most peripheral devices to get into stable state. SleepReset is set in PMxC0.

ACPI emulated reset button

Done by setting bit 6 (ResetAllAcpi) of PMxC4 (Reset Command). It is described as emulating a reset button event, but it does not set UsrReset in PMxC0, while physical button does.

PCI reset

Also triggered through PMxC4. Reset is done after setting bit 0 (Reset), but only if bit 7 (ResetEn) was set. They need to be set with two separate writes. Performs "soft PCI reset" (SoftPciRst in PMxC0), which is a warm reset.

Forcing cold reset from started OS

After flashing new firmware it is highly recommended (sometimes even necessary) to perform a cold boot. It is not always an option for remote devices, so tests were made to find another way of forcing cold boot path in firmware. Note that this cannot guarantee that when any device on board entered an unexpected state it can be brought back to defined state without full power cycle.

As these steps are most likely to be run on production software stack options like using custom kernel/driver or applying kernel parameters at boot time were not considered. Tests were made with Debian 9 and pfSense 2.4.2.

FullRst was chosen as most promising one: it can be done both by IO as well as memory mapped ACPI register. It also puts platform in state closest to cold boot from all tested options by staying in S5 for some time.


By default it is impossible to access physical memory space without changes to the kernel, however access to IO ports is unrestricted for root user. Thus, to perform FullRst type as root:

echo -ne "\xe" | dd of=/dev/port bs=1 count=1 seek=$((0xcf9))

\xe is a bitmap for FullRst, RstCmd and SysRst. After this command platform enters S5 immediately, so save your work, sync all filesystems etc. using e.g. SysRq, of course do not perform last step (reboot or shutdown), as it will be done by FullRst. We found that sometimes 5 seconds is not enough for syncing. Full sequence required for reset is:

for i in s u; do echo $i | sudo tee /proc/sysrq-trigger; sleep 15; done
echo -ne "\xe" | dd of=/dev/port bs=1 count=1 seek=$((0xcf9))

Shell in pfSense does not support backslash in echo command arguments. It also does not allow to use subshells to convert hex to decimal. IO ports cannot be accessed, but MMIO can.

printf "\016" | dd of=/dev/mem bs=1 count=1 seek=4275569605

\016 is the same as \xe, in octal. Seek value is 0xFED803C5 written in decimal. There is no SysRq as in Linux, so manual sync and read-only remount is required, most likely it needs to be forced because of open file descriptors. It needs to be performed for all mounted filesystems, this example for forced reboot shows only root filesystem:

sync && mount -u -f -r /
printf "\016" | dd of=/dev/mem bs=1 count=1 seek=4275569605