我有一块华擎 G31M-GS 主板，安装了 4GB 内存，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，各 2G)：
[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 O.E.M.
Product Name: To Be Filled By O.E.M.
Version: To Be Filled By O.E.M.
Serial Number: To Be Filled By O.E.M.
UUID: 00020003-0004-0005-0006-000700080009
Wake-up Type: Power Switch
SKU Number: To Be Filled By O.E.M.
Family: To Be Filled By O.E.M.
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 O.E.M.
Type: Desktop
Lock: Not Present
Version: To Be Filled By O.E.M.
Serial Number: To Be Filled By O.E.M.
Asset Tag: To Be Filled By O.E.M.
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 O.E.M.
Asset Tag: To Be Filled By O.E.M.
Part Number: To Be Filled By O.E.M.
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
同样的事情（多或少几 MB）也会发生在 gentoo 2008.x x86_64 livecd 上。
这是怎么回事？64 位操作系统不是可以解决此类问题吗？
答案1
我个人不太了解这款主板，但这里有一个建议：进入 BIOS。应该有一个内存重映射功能选项（可能在高级菜单中）默认情况下处于禁用状态。启用它并重试。
还要确保您使用的是最新版 BIOS。（不要忘记安装 CentOS 补丁，因为您没有运行最新的内核。:)）
答案2
您的系统中有哪种视频接口？一些板载视频芯片会将系统内存用于自己的用途，从而使操作系统无法使用。它很可能可以在您的 BIOS 中配置。
答案3
可能是主板的芯片组阻碍了你。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/