I have just tried booting the Ubuntu 10.10 64 bit live USB on this machine and to my amazement everything works fine. I even installed the system, after which I checked with uname -a and the result is

Linux T205-04 2.6.35-22-generic #33-Ubuntu SMP Sun Sep 19 20:32:27 UTC 2010 x86_64 GNU/Linux

This is quite confusing to me. To my knowledge Pentium 4 is 32 bit only. How was that possible?

Below is the result of cat /proc/cpuinfo (there are 2 CPUs, but the information is the same)

processor       : 1
vendor_id       : GenuineIntel
cpu family      : 15
model           : 4
model name      : Intel(R) Pentium(R) 4 CPU 3.00GHz
stepping        : 3
cpu MHz         : 2800.000
cache size      : 2048 KB
physical id     : 0
siblings        : 2
core id         : 0
cpu cores       : 1
apicid          : 1
initial apicid  : 1
fpu             : yes
fpu_exception   : yes
cpuid level     : 5
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov
 pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx lm
 constant_tsc pebs bts pni dtes64 monitor ds_cpl est cid cx16 xtpr
bogomips        : 6000.41
clflush size    : 64
cache_alignment : 128
address sizes   : 36 bits physical, 48 bits virtual
power management:

4 Answers 4


From Wikipedia: “In 2004, the initial 32-bit x86 instruction set of the Pentium 4 microprocessors was extended by the 64-bit x86-64 set.”

Your /proc/cpuinfo output shows flags: … lm …. The flag lm stands for “long mode“ which means 64-Bit extension. Thus, you have indeed a 64-bit processor.

  • 1
    48 bits virtual also shows that it has 48-bit addressing. (there are no 64-bit address bus x86 yet, but they all do 64-bit data). The 36 bits physical could also be shown if it has pae but not 64-bit. Jul 24, 2016 at 10:40

Apparently there were some 64 bit pentium 4 chips made. Check your processor with cat /proc/cpuinfo

  • I checked, but don't see any clue whether it is 32 or 64, which part of /proc/cpuinfo says so?
    – phunehehe
    Oct 21, 2010 at 3:04
  • Post the whole thing
    – Falmarri
    Oct 21, 2010 at 3:05
  • the information is the same, really. the only exception was the processor : 1 line
    – phunehehe
    Oct 21, 2010 at 10:35
  • 4
    From the /proc/cpuinfo you posted, it seems that Falmarri was right and that you were able to install it because you have a 64-bit P4. Note the "lm" flag in the output.
    – Steven D
    Oct 21, 2010 at 14:35

If you got P4 630 775 socket, like me, you can easly run 64-bit OSes.

  • socket isn't relevant here, only the microarchitecture is important. Prescott CPUs using socket 478 still support 64-bit mode
    – phuclv
    Sep 24, 2022 at 1:31

I expect you have a DUAL CORE Pentium 4 and this enables you to run 64bit code. The earlier single processor P4s ran at a lower speed and only worked 32bit.

  • Not entirely true. My earlier single processor (but hyper threaded) P4 is only 32-bit, but runs at a higher speed... Dec 12, 2017 at 15:43
  • the number of cores has nothing to do with the bit width
    – phuclv
    Sep 24, 2022 at 1:27

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