Understanding /proc/cpuinfo

Understanding /proc/cpuinfo
Example:

$ uname -r
2.6.22-3-486

How many physical processors are there?
$ grep 'physical id' /proc/cpuinfo | sort | uniq | wc -l
2

How many virtual processors are there?
$ grep ^processor /proc/cpuinfo | wc -l
4

Are the processors dual-core (or multi-core)?
$ grep 'cpu cores' /proc/cpuinfo
cpu cores : 2
cpu cores : 2
cpu cores : 2
cpu cores : 2

"2" indicates the two physical processors are dual-core, resulting in 4 virtual processors.

If "1" was returned, the two physical processors are single-core. If the processors are single-core, and the number of virtual processors is greater than the number of physical processors, the CPUs are using hyper-threading. Hyper-threading is supported if ht is present in the CPU flags and you are using an SMP kernel.

Are the processors 64-bit?
A 64-bit processor will have lm ("long mode") in the flags section of cpuinfo. A 32-bit processor will not.

e.g.,

flags : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt lm 3dnowext 3dnow pni cx16 lahf_lm cmp_legacy svm cr8legacy ts fid vid ttp tm stc

What do the CPU flags mean?
The CPU flags are briefly described in the kernel header file cpufeature.h.