BIOS/Global C-state Control
C-States[1]
C-States are just a power saving feature and can be disabled in Bios. No negative effects (apart from a slight increase in power usage) and will not effect the lifespan of your CPU. BIOS C-states option configure the system to adjust the core frequency in small increments when changing C-States. Enabling this option will monitor the workload and modulate the frequency of the core to maintain a high C0 residency. This has power and latency benefits when the core is not 100% utilized.
They are numbered starting at C0, which is the normal CPU operating mode, i.e., the CPU is 100% turned on. The higher the C number is, the deeper is the CPU sleep mode, i.e., more circuits and signals are turned off and more time the CPU will take to go back to C0 mode.
| Mode | Name | What it does | CPUs |
| C0 | Operating State | CPU fully turned on | All CPUs |
| C1 | Halt | Stops CPU main internal clocks via software; bus interface unit and APIC are kept running at full speed. | 486DX4 and above |
| C1E | Enhanced Halt | Stops CPU main internal clocks via software and reduces CPU voltage; bus interface unit and APIC are kept running at full speed. | All socket LGA775 CPUs |
| C1E | — | Stops all CPU internal clocks. | Turion 64, 65-nm Athlon X2, and Phenom CPUs |
| C2 | Stop Grant | Stops CPU main internal clocks via hardware; bus interface unit and APIC are kept running at full speed. | 486DX4 and above |
| C2 | Stop Clock | Stops CPU internal and external clocks via hardware | Only 486DX4, Pentium, Pentium MMX, K5, K6, K6-2, K6-III |
| C2E | Extended Stop Grant | Stops CPU main internal clocks via hardware and reduces CPU voltage; bus interface unit and APIC are kept running at full speed. | Core 2 Duo and above (Intel only) |
| C3 | Sleep | Stops all CPU internal clocks | Pentium II, Athlon, and above, but not on Core 2 Duo E4000 and E6000 |
| C3 | Deep Sleep | Stops all CPU internal and external clocks | Pentium II and above, but not on Core 2 Duo E4000 and E6000; Turion 64 |
| C3 | AltVID | Stops all CPU internal clocks and reduces CPU voltage | AMD Turion 64 |
| C4 | Deeper Sleep | Reduces CPU voltage | Pentium M and above, but not on Core 2 Duo E4000 and E6000 series; AMD Turion 64 |
| C4E/C5 | Enhanced Deeper Sleep | Reduces CPU voltage even more and turns off the memory cache | Core Solo, Core Duo, and 45-nm mobile Core 2 Duo only |
| C6 | Deep Power Down | Reduces the CPU internal voltage to any value, including 0 V | 45-nm mobile Core 2 Duo only |
Depending on the system and BIOS,
Examples
- Supermicro - CPU Configuratio > Global C-state Control
- HPE - System Configuration > BIOS/Platform Configuration (RBSU) > Power and Performance Options > C-State Efficiency Mode.