我有装有4GB内存的华擎G31M-GS主板，但是当我在CentOS 5.4（x86_64）下查询内存大小时，
[root@localhost ~]# uname -a Linux localhost.localdomain 2.6.18-164.el5 #1 SMP Thu Sep 3 03:28:30 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux 这是我得到的：
[root@localhost ~]# free -m total used free shared buffers cached Mem: 3261 565 2696 0 25 288 -/+ buffers/cache: 250 3010 Swap: 2047 0 2047 [root@localhost ~]# grep -i memory /var/log/dmesg Memory for crash kernel (0x0 to 0x0) notwithin permissible range Memory: 3336256k/3398336k available (2547k kernel code, 61628k reserved, 1289k data, 208k init) Freeing initrd memory: 3280k freed Total HugeTLB memory allocated, 0 Non-volatile memory driver v1.2 agpgart: Detected 7164K stolen memory. Freeing unused kernel memory: 208k freed [root@localhost ~]# cat /proc/meminfo MemTotal: 3339748 kB MemFree: 2760432 kB Buffers: 26692 kB Cached: 295436 kB SwapCached: 0 kB Active: 102232 kB Inactive: 284276 kB HighTotal: 0 kB HighFree: 0 kB LowTotal: 3339748 kB LowFree: 2760432 kB SwapTotal: 2097144 kB SwapFree: 2097144 kB Dirty: 0 kB Writeback: 0 kB AnonPages: 64336 kB Mapped: 20720 kB Slab: 31628 kB PageTables: 7724 kB NFS_Unstable: 0 kB Bounce: 0 kB CommitLimit: 3767016 kB Committed_AS: 257916 kB VmallocTotal: 34359738367 kB VmallocUsed: 4000 kB VmallocChunk: 34359733883 kB HugePages_Total: 0 HugePages_Free: 0 HugePages_Rsvd: 0 Hugepagesize: 2048 kB
但dmidecode报告4G（DIMM0，DIMM1,2G各）：
[root@localhost ~]# dmidecode # dmidecode 2.9 SMBIOS 2.4 present. 22 structures occupying 1127 bytes. Table at 0x000FC200. Handle 0x0000, DMI type 0, 24 bytes BIOS Information Vendor: American Megatrends Inc. Version: P1.60 Release Date: 07/08/2009 Address: 0xF0000 Runtime Size: 64 kB ROM Size: 512 kB Characteristics: PCI is supported BIOS is upgradeable BIOS shadowing is allowed Boot from CD is supported Selectable boot is supported BIOS ROM is socketed EDD is supported 5.25"/1.2 MB floppy services are supported (int 13h) 3.5"/720 KB floppy services are supported (int 13h) 3.5"/2.88 MB floppy services are supported (int 13h) Print screen service is supported (int 5h) 8042 keyboard services are supported (int 9h) Serial services are supported (int 14h) Printer services are supported (int 17h) CGA/mono video services are supported (int 10h) ACPI is supported USB legacy is supported LS-120 boot is supported ATAPI Zip drive boot is supported BIOS boot specification is supported Function key-initiated network boot is supported Targeted content distribution is supported BIOS Revision: 8.14 Handle 0x0001, DMI type 1, 27 bytes System Information Manufacturer: To Be Filled By OEM Product Name: To Be Filled By OEM Version: To Be Filled By OEM Serial Number: To Be Filled By OEM UUID: 00020003-0004-0005-0006-000700080009 Wake-up Type: Power Switch SKU Number: To Be Filled By OEM Family: To Be Filled By OEM Handle 0x0002, DMI type 2, 15 bytes Base Board Information Manufacturer: ASRock Product Name: G31M-GS Version: Serial Number: Asset Tag: Features: Board is a hosting board Board is replaceable Location In Chassis: Chassis Handle: 0x0003 Type: Motherboard Contained Object Handles: 0 Handle 0x0003, DMI type 3, 21 bytes Chassis Information Manufacturer: To Be Filled By OEM Type: Desktop Lock: Not Present Version: To Be Filled By OEM Serial Number: To Be Filled By OEM Asset Tag: To Be Filled By OEM Boot-up State: Safe Power Supply State: Safe Thermal State: Safe Security Status: None OEM Information: 0x00000000 Height: Unspecified Number Of Power Cords: 1 Contained Elements: 0 Handle 0x0004, DMI type 4, 35 bytes Processor Information Socket Designation: CPUSocket Type: Central Processor Family: Pentium 4 Manufacturer: Intel ID: 7A 06 01 00 FF FB EB BF Signature: Type 0, Family 6, Model 23, Stepping 10 Flags: FPU (Floating-point unit on-chip) VME (Virtual mode extension) DE (Debugging extension) PSE (Page size extension) TSC (Time stamp counter) MSR (Model specific registers) PAE (Physical address extension) MCE (Machine check exception) CX8 (CMPXCHG8 instruction supported) APIC (On-chip APIC hardware supported) SEP (Fast system call) MTRR (Memory type range registers) PGE (Page global enable) MCA (Machine check architecture) CMOV (Conditional move instruction supported) PAT (Page attribute table) PSE-36 (36-bit page size extension) CLFSH (CLFLUSH instruction supported) DS (Debug store) ACPI (ACPI supported) MMX (MMX technology supported) FXSR (Fast floating-point save and restore) SSE (Streaming SIMD extensions) SSE2 (Streaming SIMD extensions 2) SS (Self-snoop) HTT (Hyper-threading technology) TM (Thermal monitor supported) PBE (Pending break enabled) Version: Pentium(R) Dual-Core CPU E5400 @ 2.70GHz Voltage: 1.3 V External Clock: 200 MHz Max Speed: 2700 MHz Current Speed: 2700 MHz Status: Populated, Enabled Upgrade: Other L1 Cache Handle: 0x0005 L2 Cache Handle: 0x0006 L3 Cache Handle: Not Provided Serial Number: To Be Filled By OEM Asset Tag: To Be Filled By OEM Part Number: To Be Filled By OEM Handle 0x0005, DMI type 7, 19 bytes Cache Information Socket Designation: L1-Cache Configuration: Enabled, Not Socketed, Level 1 Operational Mode: Write Back Location: Internal Installed Size: 64 KB Maximum Size: 64 KB Supported SRAM Types: Other Installed SRAM Type: Other Speed: Unknown Error Correction Type: Parity System Type: Data Associativity: 8-way Set-associative Handle 0x0006, DMI type 7, 19 bytes Cache Information Socket Designation: L2-Cache Configuration: Enabled, Not Socketed, Level 2 Operational Mode: Write Back Location: Internal Installed Size: 2048 KB Maximum Size: 2048 KB Supported SRAM Types: Other Installed SRAM Type: Other Speed: Unknown Error Correction Type: Single-bit ECC System Type: Unified Associativity: 8-way Set-associative Handle 0x0007, DMI type 5, 20 bytes Memory Controller Information Error Detecting Method: 64-bit ECC Error Correcting Capabilities: None Supported Interleave: One-way Interleave Current Interleave: One-way Interleave Maximum Memory Module Size: 4096 MB Maximum Total Memory Size: 8192 MB Supported Speeds: Other Supported Memory Types: DIMM SDRAM Memory Module Voltage: 3.3 V Associated Memory Slots: 2 0x0008 0x0009 Enabled Error Correcting Capabilities: None Handle 0x0008, DMI type 6, 12 bytes Memory Module Information Socket Designation: DIMM0 Bank Connections: 0 1 Current Speed: Unknown Type: DIMM SDRAM Installed Size: 2048 MB (Double-bank Connection) Enabled Size: 2048 MB (Double-bank Connection) Error Status: OK Handle 0x0009, DMI type 6, 12 bytes Memory Module Information Socket Designation: DIMM1 Bank Connections: 4 5 Current Speed: Unknown Type: DIMM SDRAM Installed Size: 2048 MB (Double-bank Connection) Enabled Size: 2048 MB (Double-bank Connection) Error Status: OK Handle 0x000A, DMI type 9, 13 bytes System Slot Information Designation: PCI1 Type: 32-bit PCI Current Usage: Available Length: Short ID: 1 Characteristics: 3.3 V is provided Opening is shared PME signal is supported Handle 0x000B, DMI type 9, 13 bytes System Slot Information Designation: PCI2 Type: 32-bit PCI Current Usage: Available Length: Short ID: 2 Characteristics: 3.3 V is provided Opening is shared PME signal is supported Handle 0x000C, DMI type 126, 13 bytes Inactive Handle 0x000D, DMI type 9, 13 bytes System Slot Information Designation: PCIE2 Type: x1 PCI Express Current Usage: Available Length: Short ID: 18 Characteristics: 3.3 V is provided Opening is shared PME signal is supported Handle 0x000E, DMI type 16, 15 bytes Physical Memory Array Location: System Board Or Motherboard Use: System Memory Error Correction Type: None Maximum Capacity: 4 GB Error Information Handle: Not Provided Number Of Devices: 2 Handle 0x000F, DMI type 19, 15 bytes Memory Array Mapped Address Starting Address: 0x00000000000 Ending Address: 0x000FFFFFFFF Range Size: 4 GB Physical Array Handle: 0x000E Partition Width: 0 Handle 0x0010, DMI type 17, 27 bytes Memory Device Array Handle: 0x000E Error Information Handle: Not Provided Total Width: 64 bits Data Width: 64 bits Size: 2048 MB Form Factor: DIMM Set: None Locator: DIMM0 Bank Locator: BANK0 Type: SDRAM Type Detail: Synchronous Speed: Unknown Manufacturer: Manufacturer0 Serial Number: SerNum0 Asset Tag: AssetTagNum0 Part Number: PartNum0 Handle 0x0011, DMI type 20, 19 bytes Memory Device Mapped Address Starting Address: 0x00000000000 Ending Address: 0x0007FFFFFFF Range Size: 2 GB Physical Device Handle: 0x0010 Memory Array Mapped Address Handle: 0x000F Partition Row Position: 1 Interleaved Data Depth: 1 Handle 0x0012, DMI type 17, 27 bytes Memory Device Array Handle: 0x000E Error Information Handle: Not Provided Total Width: 64 bits Data Width: 64 bits Size: 2048 MB Form Factor: DIMM Set: None Locator: DIMM1 Bank Locator: BANK1 Type: SDRAM Type Detail: Synchronous Speed: Unknown Manufacturer: Manufacturer1 Serial Number: SerNum1 Asset Tag: AssetTagNum1 Part Number: PartNum1 Handle 0x0013, DMI type 20, 19 bytes Memory Device Mapped Address Starting Address: 0x00080000000 Ending Address: 0x000FFFFFFFF Range Size: 2 GB Physical Device Handle: 0x0012 Memory Array Mapped Address Handle: 0x000F Partition Row Position: 1 Interleaved Data Depth: 1 Handle 0x0014, DMI type 32, 20 bytes System Boot Information Status: No errors detected Handle 0x0015, DMI type 127, 4 bytes End Of Table
与Gentoo 2008.x x86_64 livecd相同的事情（给或几MB）发生。
这里发生了什么事？ 这不是一个64位的操作系统可以解决这种问题吗？
我个人不知道这个主板，但是这里有个build议：进入BIOS。 应该有一个内存重新映射function选项（可能在高级菜单中），默认情况下是禁用的。 启用它，然后重试。
另外请确保您使用的是最新的BIOS版本。 （不要忘了安装CentOS补丁，因为你没有运行最新的内核:)）
你的系统中有什么样的video接口？ 某些机载video芯片使用系统内存供自己使用，使其无法在操作系统上使用。 这是最有可能configuration在您的BIOS。
可能是你的主板的芯片组阻止你回来。 例如，请参阅此超级用户主题： https ： //superuser.com/questions/35731/how-to-enable-4gb-in-my-windows-7-64-bit/