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联想主板上有 8GB 内存，其中 4GB 内存空闲。内存设备映射地址与内存阵列映射地址

tag-icon • tag-icon lenovo ram motherboard dmidecode
联想主板上有 8GB 内存，其中 4GB 内存空闲。内存设备映射地址与内存阵列映射地址

我有一块联想主板，上面有 2 个 4GB DIMM。从 Ubuntu Grub 启动菜单中选择并运行内存测试时，我看到了 8GB。

但是当我启动 ubuntu 16.04 时，free只显示 4GB。

当我运行 dmidecode 时，我看到了两个 4GB DIMM：

BIOS Information
    Vendor: LENOVO
    Version: I7KT32AUS
    Release Date: 12/26/2013
    Address: 0xF0000
    Runtime Size: 64 kB
    ROM Size: 4096 kB
...
Handle 0x0017, DMI type 16, 23 bytes
Physical Memory Array
    Location: System Board Or Motherboard
    Use: System Memory
    Error Correction Type: None
    Maximum Capacity: 8 GB
    Error Information Handle: Not Provided
    Number Of Devices: 4

Handle 0x0018, DMI type 19, 31 bytes
Memory Array Mapped Address
    Starting Address: 0x00000000000
    Ending Address: 0x000FFFFFFFF
    Range Size: 4 GB
    Physical Array Handle: 0x0017
    Partition Width: 255

Handle 0x0019, DMI type 17, 34 bytes
Memory Device
    Array Handle: 0x0017
    Error Information Handle: Not Provided
    Total Width: 64 bits
    Data Width: 64 bits
    Size: No Module Installed
    Form Factor: SODIMM
    Set: None
    Locator: DIMM 0
    Bank Locator: CHANNEL B
    Type: DDR3
    Type Detail: None
    Speed: Unknown
    Manufacturer: A1_Manufacturer2
    Serial Number: A1_SerialNum2
    Asset Tag: A1_AssetTagNum2
    Part Number: A1_PartNum2
    Rank: Unknown
    Configured Clock Speed: Unknown

Handle 0x001A, DMI type 17, 34 bytes
Memory Device
    Array Handle: 0x0017
Error Information Handle: Not Provided
Total Width: 64 bits
Data Width: 64 bits
Size: 4096 MB
Form Factor: DIMM
Set: None
Locator: DIMM 1
Bank Locator: CHANNEL B
Type: DDR3
Type Detail: Synchronous Unbuffered (Unregistered)
Speed: 1600 MHz
Manufacturer: Samsung
Serial Number: 93FB5498
Asset Tag: A1_AssetTagNum3
Part Number: M378B5173QH0-CK0
Rank: 1
Configured Clock Speed: 1600 MHz

看起来奇怪并且可能解释差异的是，我有一个 4 GB 内存阵列映射地址和一个 4 GB 设备映射地址。

Handle 0x0018, DMI type 19, 31 bytes
    Memory Array Mapped Address
    Starting Address: 0x00000000000
    Ending Address: 0x000FFFFFFFF
    Range Size: 4 GB
    Physical Array Handle: 0x0017
    Partition Width: 255

....

Memory Device Mapped Address
    Starting Address: 0x00000000000
    Ending Address: 0x000FFFFFFFF
    Range Size: 4 GB
    Physical Device Handle: 0x001A
    Memory Array Mapped Address Handle: 0x0018
    Partition Row Position: Unknown
    Interleave Position: Unknown
    Interleaved Data Depth: Unknown

我一直无法弄清楚什么是存储设备，什么是存储阵列，以及什么可以设置它。

更普遍地说，为什么只有大约 4GB 可用和可见free？

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