What is CPU HLT ?
In A7V BIOS Version 1003 the CPU was considerably
cooler when idling (doing nothing) than in all newer versions. This
was due to the fact that this version allowed the CPU to be stopped
via the HLT function. If it had nothing to do, it would be stopped
and consume less power, generating less heat.
You may read more on this behaviour in the article
What happens when HLT is active ?
The CPU only draws very little current while it is
in HLT mode. The value depends on CPU type, frequency and voltage,
but may be somewhere around 10A. As soon as the CPU gets some work
again, it leaves HLT mode and begins drawing the normal amount of
current, somewhere around 30-40A.
Why was HLT disabled ?
When leaving HLT mode the CPU draws the higher current
instantly. The onboard voltage regulator has to keep
the CPU's core voltage (Vcore) steady. When the load shifts (meaning
the current drawn is variated) it has to re-regulate Vcore to get
it back into the limits allowed for the CPU. Now, if there is such
a load shift as from 10A to 40A almost instantly, the voltage regulator
struggles to keep the voltage, but for a very short time can't do
so. This is directly after the load shift. Depending on the CPU
and many other factors, the resulting wrong voltage (too low or
too high), if only present for a few fractions of a second, can
lead to a system crash. This is what you see as a lockup, reboot
or simply a blue screen.
Although this link is in german, it supports what
is written here:ASUS
So, if HLT is disabled, the CPU draws more current
while idle. The load shift when it gets back to work is significantly
smaller, let's say from 30A to 40A. Thus, disabling HLT makes a
mainboard a lot more stable, especially when it's equipped with
a mediocre voltage regulator. Many AMD mainboards use very mediocre
voltage regulators. The A7V for has only a 2 stage regulator as
opposed to potentially better 3 stage regulators on other mainboards.
I can't comment on the A7V regulator's quality, though. Being 2
stage does not automatically mean it's bad...
Should I reenable HLT ?
This depends on what you want.
Leave HLT disabled for a more stable System.
Enable HLT for a longer CPU Life. Remember, for electronic
components 7 to 10 °C more temperature (constantly) mean half
life time !
If you want to reenable HLT, use a special BIOS by
use the register hack described in Reenabling
Idle mode, or use a (working!) software cooling programm.
What to do ?
I prefer a stable System any day, so I recommend leaving
HLT disabled. This means simply use a BIOS newer than 1003, if possible
the most recent one (1008 as of today). Don't use the register hack
and don't use any software cooling (including the one in ASUS probe)