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？