The CPU core voltage () is the last major power rail to turn on. Because the CPU draws massive amounts of current, its power delivery must be precisely regulated.
The PCH asserts (Status Sleep 5) and SLP_S4# (Status Sleep 4) high (typically 3.3V). desktop motherboard power sequence pdf
The power supply sends a constant 5V through the purple wire of the 24-pin ATX connector to the Super I/O (SIO) chip and the PCH. The CPU core voltage () is the last
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The PSU sends a +5V signal down Pin 8 ( PWR_GOOD ) to signify its outputs are completely stable. The motherboard VRMs generate their own VRM_GD signals.
Typically 1.1V to 1.35V for DDR4/DDR5.