Experimental chill

Недавно понадобилось снова открыть документацию и посмотреть что значат регистры %fs и %gs: я помню что-то про thread local storage, но не более.

Наткнулся на очень красивый ответ об истории

https://stackoverflow.com/questions/10810203/what-is-the-fs-gs-register-intended-for/10810340

The original intention behind the segment registers was to allow a program to access many different (large) segments of memory that were intended to be independent and part of a persistent virtual store. The idea was taken from the 1966 Multics operating system, that treated files as simply addressable memory segments. No BS "Open file, write record, close file", just "Store this value into that virtual data segment" with dirty page flushing.
...
There was still a need for threads to access thread local store, and each thread needed a a pointer ... somewhere in the immediately accessible thread state (e.g, in the registers) ... to thread local store. Since Windows and Linux both used FS and GS (thanks Nick for the clarification) for this purpose in the 32 bit version, AMD decided to let the 64 bit segment registers (GS and FS) be used essentially only for this purpose (I think you can make them point anywhere in your process space; I don't know if the application code can load them or not). Intel in their panic to not lose market share to AMD on 64 bits, and Andy being retired, decided to just copy AMD's scheme.

Stack Overflow

What is the "FS"/"GS" register intended for?

So I know what the following registers and their uses are supposed to be:
CS = Code Segment (used for IP)
DS = Data Segment (used for MOV)
ES = Destination Segment (used for MOVS, etc.)
SS = Stack

www.group-telegram.com/sg/experimentalchill.com/265

15.9K viewsJan 10, 2024 at 13:29

Недавно понадобилось снова открыть документацию и посмотреть что значат регистры %fs и %gs: я помню что-то про thread local storage, но не более.

Наткнулся на очень красивый ответ об истории

https://stackoverflow.com/questions/10810203/what-is-the-fs-gs-register-intended-for/10810340

The original intention behind the segment registers was to allow a program to access many different (large) segments of memory that were intended to be independent and part of a persistent virtual store. The idea was taken from the 1966 Multics operating system, that treated files as simply addressable memory segments. No BS "Open file, write record, close file", just "Store this value into that virtual data segment" with dirty page flushing.
...
There was still a need for threads to access thread local store, and each thread needed a a pointer ... somewhere in the immediately accessible thread state (e.g, in the registers) ... to thread local store. Since Windows and Linux both used FS and GS (thanks Nick for the clarification) for this purpose in the 32 bit version, AMD decided to let the 64 bit segment registers (GS and FS) be used essentially only for this purpose (I think you can make them point anywhere in your process space; I don't know if the application code can load them or not). Intel in their panic to not lose market share to AMD on 64 bits, and Andy being retired, decided to just copy AMD's scheme.

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