Только 3 ГБ ОЗУ с ядром PAE на 12.04

Я установил 4 ГБ оперативной памяти на Thinkpad X60 под управлением Ubuntu 12.04. Затем я установил ядро ​​с поддержкой PAE. Теперь uname -r выводит

3.2.0-50-generic-pae

Мой ЦП поддерживает PAE:

$ grep pae /proc/cpuinfo
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx constant_tsc arch_perfmon bts aperfmperf pni monitor vmx est tm2 xtpr pdcm dtherm
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx constant_tsc arch_perfmon bts aperfmperf pni monitor vmx est tm2 xtpr pdcm dtherm

BIOS сообщает о 4096 МБ ОЗУ.

Тем не менее free выводит

             total       used       free     shared    buffers     cached
Mem:       3088452    2264192     824260          0     168056     852880
-/+ buffers/cache:    1243256    1845196
Swap:      2085884          0    2085884

Что не так?

Редактировать: Вот раздел e820 dmesg:

[    0.000000]  BIOS-e820: 0000000000000000 - 000000000009f000 (usable)
[    0.000000]  BIOS-e820: 000000000009f000 - 00000000000a0000 (reserved)
[    0.000000]  BIOS-e820: 00000000000d2000 - 00000000000d4000 (reserved)
[    0.000000]  BIOS-e820: 00000000000dc000 - 0000000000100000 (reserved)
[    0.000000]  BIOS-e820: 0000000000100000 - 00000000bf6d0000 (usable)
[    0.000000]  BIOS-e820: 00000000bf6d0000 - 00000000bf6df000 (ACPI data)
[    0.000000]  BIOS-e820: 00000000bf6df000 - 00000000bf700000 (ACPI NVS)
[    0.000000]  BIOS-e820: 00000000bf700000 - 00000000c0000000 (reserved)
[    0.000000]  BIOS-e820: 00000000f0000000 - 00000000f4000000 (reserved)
[    0.000000]  BIOS-e820: 00000000fec00000 - 00000000fec10000 (reserved)
[    0.000000]  BIOS-e820: 00000000fed00000 - 00000000fed00400 (reserved)
[    0.000000]  BIOS-e820: 00000000fed14000 - 00000000fed1a000 (reserved)
[    0.000000]  BIOS-e820: 00000000fed1c000 - 00000000fed90000 (reserved)
[    0.000000]  BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved)
[    0.000000]  BIOS-e820: 00000000ff800000 - 0000000100000000 (reserved)