Quote Originally Posted by nealh View Post
Problem is is you disable EIST vcore is unchanged..yes clock speed is lower...but vcore is the same...
Actually C1E *does* decrease the vcore when it switches down to the low-speed multiplier, which is why the cpu draws less power and runs cooler...

To better understand C1E one must first have a basic understanding of how the Windows operating system itself works. Windows is a totally event-driven operating system. What this means is that it's normal state is to do absolutely *nothing*, ie, it just sits there like it's totally dead waiting for some type of stimulus to respond to. This stimulus may be a key being pressed, a mouse click, a timer event or some other type of hardware event that needs attention.

A good way to think of Windows is that it's like a telephone sitting on a desk. A large percentage of the time it just sits there waiting for an incoming call, or for you to pick up the handset and start dialing.

When Windows is in it's "I'm waiting for something to respond to" idle state, it executes a HLT instruction. It's this HLT instruction that activates the C1E mode to drop the cpu multiplier down to slow-speed mode and reduce it's cpu voltage level. Since the cpu is drawing less power it naturally runs a lot cooler. Since C1E works via the common HLT instruction, this makes it totally transparent to the o/s and no special support mechanism is necessary for it to work...

EIST (Enhanced SpeedStep) is a different form of cpu speed/voltage control altogether. Unlike C1E, which is only a two-speed idle/active control, EIST has multiple speeds/voltage levels. However, EIST doesn't work via the built-in HLT instruction. Instead, it must be invoked via the ACPI interface and the o/s must have an EIST support mechanism in place to specify what speed changes to make and when to make them.

These are some techreport excerpts that explain the C1E and EIST (Enhanced SpeedStep) in more detail...

  • C1E enhanced halt state -- Introduced in the Pentium 4 500J-series processors, the C1E halt state replaces the old C1 halt state used on the Pentium 4 and most other x86 CPUs. The C1 halt state is invoked when the operating system's idle process issues a HLT command. (Windows does this constantly when not under a full load.) Entering halt state, which is a lower-power state, will cut a CPU's power consumption and heat production. Intel's new C1E halt state is also invoked by the HLT command, but it turns down the entire CPU's clock frequency (via multiplier control) and voltage in order to work its mojo. This more robust halt state requires significantly less power than the old C1 implementation.
  • EIST (Enhanced SpeedStep) -- SpeedStep also modulates the CPU clock speed and voltage according to load, but it is invoked via another mechanism. The operating system must be aware of SpeedStep, as must the system BIOS, and then the OS can request frequency changes via ACPI. SpeedStep is more granular than C1E halt, because it offers multiple rungs up and down the ladder between the maximum and minimum CPU multiplier and voltage levels.

-Surgeon-