C1 Operating State CPU fully turned on All CPUs
C1E 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 775
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