您可能已经注意到您的Windows 操作系统^{(Windows OS)}可能会显示可用内存可能小于安装的内存 ( RAM )。例如，您的32 位 Windows 操作系统^{(Windows OS 32-bit)}可能会在安装 4 GB 内存时报告只有 3.5 GB 可用系统内存。或者，如果安装了 8 GB 内存， Windows 操作系统 64 位^{(Windows OS 64-bit)}计算机可能仅显示 7.1 GB 可用系统内存。

已安装 RAM 与可用 RAM

指示性可用内存^{(Usable Memory)}是总物理内存^{( total physical memory)}减去“硬件保留^{(hardware reserved)}”内存的计算量。

在Windows 7中查看已安装内存和可用内存：

• 单击开始^{(Click Start)}，右键单击计算机^(Computer)，然后单击属性。

• 查看^(View)System下的Installed memory ( RAM ) 值。例如，如果它显示 4.00 GB（3.5 GB 可用），这意味着您在 4 GB 已安装内存中有 3.5 GB 可用内存。

Windows 显示的RAM少于安装的 RAM

可用系统内存的减少取决于以下配置：

1. 计算机中安装的设备以及这些设备保留的内存
2. 主板处理内存的能力
3. 系统BIOS^{(System BIOS)}版本和设置
4. 安装的Windows 操作系统版本^{(Windows OS)}
5. 其他系统设置。

阅读^(Read)：Find out Computer RAM, Graphics Card/Video memory of your Windows PC。

例如，如果计算机安装了 4 GB 的RAM ，则在^(RAM)Windows的“^(Windows)系统信息^{(System Information)}”对话框中报告的系统内存会更少。例如，系统信息^{(System Information)}对话框可能会报告安装了 4 GB 内存 (4,096 MB) 的计算机上的系统内存为 3,120 MB。如果计算机安装了许多设备，则可用内存可能会减少到 3 GB 或更少。

但是，在 32 位版本（例如Windows 7）中可用的最大内存通常为 3.12 GB。此行为是某些硬件和软件因素的预期结果。要让Windows在安装了 4 GB 内存的计算机上使用全部 4 GB 内存，该计算机必须满足以下要求：

• 芯片组必须支持至少 8 GB 的地址空间。
• CPU必须支持 x64 指令集。AMD64 CPU和Intel EM64T CPU支持此指令集。

BIOS必须支持内存重新映射功能。内存重新映射功能允许将先前被外围组件互连^{(Peripheral Component Interconnect)}( PCI ) 配置空间覆盖的系统内存段重新映射到 4 GB 地址线之上。必须在计算机的BIOS配置实用程序中启用此功能。查看您的计算机产品文档以获取说明如何启用此功能的说明。

许多面向消费者的计算机可能不支持内存重新映射功能。此功能的文档或BIOS^(BIOS)配置实用程序中没有使用标准术语。因此，您可能必须阅读各种BIOS配置设置的说明，以确定是否有任何设置启用内存重新映射功能。

有几种其他情况可能会导致可用RAM低于预期。

Windows 操作系统支持的 RAM

每个版本的 Windows 10 支持多少 RAM？^{(How much RAM does each edition of Windows 10 support?)} Windows 10 有不同的版本——家庭版、教育^(Education)版、专业版^(Pro)和企业版^(Enterprise)，因此​​支持的RAM也各不相同。

Windows Server最多可支持 24Tb。

如何增加PC 上的可用RAM

您可以尝试以下建议来增加计算机上的可用RAM：

• 在 MSCONFIG 中打开Boot Advanced Options^{(Boot Advanced Options in MSCONFIG)}并取消选中Maximum Memory复选框。
• 更新系统 BIOS^{(Update the system BIOS)}并查看
• 检查 BIOS 设置
  • 启用内存重新映射功能
  • 在BIOS^(BIOS)设置中更改AGP视频光圈大小
• 检查 RAM 是否存在物理缺陷
  • 检查您是否有坏的内存模块
  • 确保^(Make)内存排列正确
  • 检查^(Check)是否使用了内存卡。

另请阅读：^{(Also read:)}

1. Windows 中的物理内存分配和内存状态
2. Windows 的物理内存限制
3. 64 位 Windows 的最大内存 (RAM) 限制。

Windows shows less RAM than installed. Why?

You may haνe noticed that your Windows OS may show that the usable memory may be less than the installed memory (RAM). For instance, yоur Windows OS 32-bit, may report that there is only 3.5 GB of usable system memory when 4 GB of memory has been installed. Or, a Windows OS 64-bit computer may show only 7.1 GB of usable system memory when 8 GB of memory may be installed.

Installed RAM vs Usable RAM

The indicative Usable Memory is a calculated amount of the total physical memory minus “hardware reserved” memory.

To view the installed memory and the usable memory in Windows 7:

• Click Start, right-click Computer, and then click Properties.

• View the Installed memory (RAM) value under System. For example, if it displays 4.00 GB (3.5 GB usable), this means that you have 3.5 GB of usable memory out of 4 GB of installed memory.

Windows shows less RAM than installed

The reduction in available system memory depends on the configuration of the following:

1. The devices that are installed in the computer and the memory that is reserved by those devices
2. The ability of the motherboard to handle memory
3. The System BIOS version and settings
4. The version of Windows OS that is installed
5. Other system settings.

Read: Find out Computer RAM, Graphics Card/Video memory of your Windows PC.

If, say, a computer has 4 GB of RAM installed, the system memory that is reported in the System Information dialog box in Windows is less. For example, the System Information dialog box may report 3,120 MB of system memory on a computer that has 4 GB of memory installed (4,096 MB). If a computer has many installed devices, the available memory may be reduced to 3 GB or less.

However, the maximum memory available in 32-bit versions of, say, Windows 7, is typically 3.12 GB. This behavior is the expected result of certain hardware and software factors. For Windows to use all 4 GB of memory on a computer that has 4 GB of memory installed, the computer must meet the following requirements:

• The chipset must support at least 8 GB of address space.
• The CPU must support the x64 instruction set. The AMD64 CPU and the Intel EM64T CPU support this instruction set.

The BIOS must support the memory remapping feature. The memory remapping feature allows for the segment of system memory that was previously overwritten by the Peripheral Component Interconnect (PCI) configuration space to be remapped above the 4 GB address line. This feature must be enabled in the BIOS configuration utility on the computer. View your computer product documentation for instructions that explain how to enable this feature.

Many consumer-oriented computers may not support the memory remapping feature. No standard terminology is used in documentation or in BIOS configuration utilities for this feature. Therefore, you may have to read the descriptions of the various BIOS configuration settings that are available to determine whether any of the settings enable the memory remapping feature.

There are several additional situations that could cause the usable RAM to be less than expected.

RAM supported by your Windows OS

How much RAM does each edition of Windows 10 support? Windows 10 comes in different flavors—Home, Education, Pro, and Enterprise, and so the RAM supported varies.

Windows Server may support up to 24Tb.

How to increase the usable RAM on a PC

You may try the following suggestions to increase the usable RAM on your computer:

• Open the Boot Advanced Options in MSCONFIG and uncheck the Maximum Memory checkbox.
• Update the system BIOS and see
• Check BIOS settings
  • Enable the memory remapping feature
  • Change the AGP video aperture size in the BIOS settings
• Check the RAM for physical defects
  • Check whether you have bad memory modules
  • Make sure that the memory arrangement is correct
  • Check whether memory standoff cards are used.

Also read:

1. Physical memory allocation and memory status in Windows
2. Physical Memory Limits for Windows
3. Maximum memory (RAM) limit for 64-bit Windows .

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