At the top is the user space layer, or application layer, which defines the users of the network stack.

最上面是用户空间层，或称为应用层，其中定义了网络栈的用户。

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On 32-bit systems, the default address-space model is that it USES a single segment for user and stack data with a maximum aggregate size close to 256 MB.

在32位系统上，默认的地址空间模型是对于用户和堆栈数据使用单一数据段，最大总大小接近256 MB。

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Once the kernel schedules the process for execution, it sets the stack to grow down from the top of user space, and the heap to grow up from the end of the executable image.

一旦内核调度进程执行，它就设置栈和堆，前者从用户空间的顶部向下增长，后者从可执行映像的尾部向上增长。

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