背后的Windows x64的44位虚拟内存地址限制 [英] Behind Windows x64's 44-bit virtual memory address limit

问题描述

http://www.alex-ionescu.com/?p=50。

我读了一柱上。

作者解释了单向链表例如为什么的Windows x64只支持44位虚拟内存地址。

Author explains the why Windows x64 supports only 44-bit virtual memory address with singly linked list example.

结构{// 8字节标题         ULONGLONG深度：16;         ULONGLONG顺序：9;         ULONGLONG NextEntry：39; } Header8;

struct { // 8-byte header ULONGLONG Depth:16; ULONGLONG Sequence:9; ULONGLONG NextEntry:39; } Header8;

第一牺牲做出是为了减少对的序列号空间向9位，而不是16位，减少了最大序列号的名单可以实现。这仍然只能留下39位的指针 - 一个平庸的改进32位。通过迫使结构是16字节，分配时对齐，4更多的位可以争取，由于底位现在可以始终被假定为0

The first sacrifice to make was to reduce the space for the sequence number to 9 bits instead of 16 bits, reducing the maximum sequence number the list could achieve. This still only left 39 bits for the pointer — a mediocre improvement over 32 bits. By forcing the structure to be 16-byte aligned when allocated, 4 more bits could be won, since the bottom bits could now always be assumed to be 0.

呵呵，我无法理解。

什么通过迫使结构是16字节，分配时对齐，4更多的位可以争取，由于底位现在可以总是被假设为0。手段？

What "By forcing the structure to be 16-byte aligned when allocated, 4 more bits could be won, since the bottom bits could now always be assumed to be 0." means?

推荐答案

对于2 ^ N字节对齐的指针，它的地址总是能被2整除^ N - 这意味着较低N位始终为零。您可以在其中存储附加信息：

For a 2^N-byte aligned pointer, its address is always divisible by 2^N - which means that lower N bits are always zero. You can store additional information in them:

encode ptr payload = ptr | payload
decode_ptr data = data & ~mask
decode_payload data = data & mask

其中，掩码为（1＆LT;＆LT; N） - 1 - 即一些具有低氮位设置

where mask is (1 << N) - 1 - i.e. a number with low N bits set.

这招经常被用来保存在低级别的code空间（有效载荷可以被GC标志，类型标记等。）

This trick is often used to save space in low-level code (payload can be GC flags, type tag, etc.)

在影响你不保存一个指针，而是一个号码，在指针可以提取。当然，应注意，以不取消引用号码作为不进行解码的指针。

In effect you're not storing a pointer, but a number from which a pointer can be extracted. Of course, care should be taken to not dereference the number as a pointer without decoding.

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