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I'm running my new computer which has 8GB of RAM installed, the memory modules show in the BIOS screen and dmidecode, but the total is only 4GB

Only 4GB is visible in /proc/meminfo

uname -a
Linux localhost 3.0.6-gentoo #2 SMP PREEMPT Sat Nov 19 10:45:22 GMT-- x86_64 AMD   
Phenom(tm) II X4 955 Processor AuthenticAMD GNU/Linux

The result of /proc/meminfo is as follows: (thanks Andrey)

MemTotal:        4021348 kB
MemFree:         1440280 kB
Buffers:           23696 kB
Cached:          1710828 kB
SwapCached:         4956 kB
Active:          1389904 kB
Inactive:         841364 kB
Active(anon):    1337812 kB
Inactive(anon):   714060 kB
Active(file):      52092 kB
Inactive(file):   127304 kB
Unevictable:          32 kB
Mlocked:              32 kB
SwapTotal:       8388604 kB
SwapFree:        8047900 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:        492732 kB
Mapped:            47528 kB
Shmem:           1555120 kB
Slab:             267724 kB
SReclaimable:     177464 kB
SUnreclaim:        90260 kB
KernelStack:        1176 kB
PageTables:        12148 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:    10399276 kB
Committed_AS:    3293896 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      317008 kB
VmallocChunk:   34359398908 kB
AnonHugePages:    120832 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:       23552 kB
DirectMap2M:     3088384 kB
DirectMap1G:     1048576 kB

I have tried using mem=8G as a kernel boot parameter, I read a post about setting HIGHMEM64G to yes, before realising that only applies to 32bit kernels.

Trying dmidecode -t memory

SMBIOS 2.7 present.

Handle 0x0026, DMI type 16, 23 bytes
Physical Memory Array
    Location: System Board Or Motherboard
    Use: System Memory
    Error Correction Type: Multi-bit ECC
    Maximum Capacity: 32 GB
    Error Information Handle: Not Provided
    Number Of Devices: 4

Handle 0x0028, DMI type 17, 34 bytes
Memory Device
    Array Handle: 0x0026
    Error Information Handle: Not Provided
    Total Width: 64 bits
    Data Width: 64 bits
    Size: 4096 MB
    Form Factor: DIMM
    Set: None
    Locator: DIMM0
    Bank Locator: BANK0
    Type: <OUT OF SPEC>
    Type Detail: Synchronous
    Speed: 1333 MHz
    Manufacturer: Manufacturer0
    Serial Number: SerNum0
    Asset Tag: AssetTagNum0
    Part Number: Array1_PartNumber0
    Rank: Unknown

Handle 0x002A, DMI type 17, 34 bytes
Memory Device
    Array Handle: 0x0026
    Error Information Handle: Not Provided
    Total Width: Unknown
    Data Width: 64 bits
    Size: No Module Installed
    Form Factor: DIMM
    Set: None
    Locator: DIMM1
    Bank Locator: BANK1
    Type: Unknown
    Type Detail: Synchronous
    Speed: Unknown
    Manufacturer: Manufacturer1
    Serial Number: SerNum1
    Asset Tag: AssetTagNum1
    Part Number: Array1_PartNumber1
    Rank: Unknown

Handle 0x002C, DMI type 17, 34 bytes
Memory Device
    Array Handle: 0x0026
    Error Information Handle: Not Provided
    Total Width: 64 bits
    Data Width: 64 bits
    Size: 4096 MB
    Form Factor: DIMM
    Set: None
    Locator: DIMM2
    Bank Locator: BANK2
    Type: <OUT OF SPEC>
    Type Detail: Synchronous
    Speed: 1333 MHz
    Manufacturer: Manufacturer2
    Serial Number: SerNum2
    Asset Tag: AssetTagNum2
    Part Number: Array1_PartNumber2
    Rank: Unknown

Handle 0x002E, DMI type 17, 34 bytes
Memory Device
    Array Handle: 0x0026
    Error Information Handle: Not Provided
    Total Width: Unknown
    Data Width: 64 bits
    Size: No Module Installed
    Form Factor: DIMM
    Set: None
    Locator: DIMM3
    Bank Locator: BANK3
    Type: Unknown
    Type Detail: Synchronous
    Speed: Unknown
    Manufacturer: Manufacturer3
    Serial Number: SerNum3
    Asset Tag: AssetTagNum3
    Part Number: Array1_PartNumber3
    Rank: Unknown

My motherboard is a ASUS M5 A99X EVO there are mentions of similar issues on other boards solved by fireware upgrades. I am on the latest firmware, it seems to have made the issue worse.

share|improve this question
    
Yes, try running dmidecode with "-t memory" argument. This will list all the modules visible to the system. –  Andrey Nov 19 '11 at 15:04
    
Does your BIOS report the correct amount? –  Matt Simmons Nov 19 '11 at 15:08
    
Now I have flashed the BIOS to the latests version. No. (it was showing 8GB on the previous version) The total is only 4GB, but I can see two 4096 modules on the details page. See my recent edit. –  Chris Huang-Leaver Nov 19 '11 at 16:52

2 Answers 2

You may have activated some memory redundancy feature like memory mirroring - this reduces the amount of RAM available to the O/S. Some BIOSes also offer a kind of compatibility mode (often labelled "OS install mode) where the total amount of visible memory is capped to work around installer bugs - check for this.

share|improve this answer

Go into your BIOS and find a setting called 'memory remapping'. Turn it ON. Otherwise, your BIOS will set up memory the way a 32-bit operating system expects and cut off access to memory above 4GB.

share|improve this answer
    
I can turn it off the amount of memory goes DOWN :-( The setting is called 'Remapping the memory hole!' on my ASUS board. –  Chris Huang-Leaver Nov 20 '11 at 12:33
1  
Nah, remapping will only allow you to remap the address space reserved for memory-mapped I/O communications typically located between 3 and 4 GB of the system to higher memory addresses above your installed physical RAM boundary, so the gigabyte of RAM is not wasted. It does nothing for the general compatibility with 64-bit systems. See also en.wikipedia.org/wiki/… –  the-wabbit Nov 21 '11 at 21:37
    
On most motherboards, if memory remapping is disabled, they don't map memory above 4GB anywhere. –  David Schwartz Nov 21 '11 at 23:17

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