Canonical Address

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stay 64 Bit operating system , The virtual address width is 64bits, But most processors don't support 64 Bit wide virtual address space

Current Intel/AMD The processor only uses 48bits To represent the virtual address space

All virtual addresses must be Canonical Address

So what kind of address is Canonical Address Well ？

Intel This is what the developer manual says

here bits 63 It refers to the highest position

0xfffff801c9c11000 The binary form of is

11111111 11111111 11111000 00000001 11001001 11000001 00010000 00000000

So the front one 16 individual 1 Namely bits 63-48

Actually, I made a Symbol expansion operation

Because only 48bit, that bits47 It's the most significant bit ,48-63 It's no use , If the most significant bit is regarded as a sign bit , that 48-63 Is symbolic expansion

because 48-63 Is full of 1, And the best significant bit , That is to say bits47 It's also 1

This is Canonical Address

0xffff080000000000 In binary form ：

11111111 11111111 00001000 00000000 00000000 00000000 00000000 00000000

here 48-63 and 47 atypism , Does not conform to the rules of symbolic expansion , So it's not Canonical Address