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What is FS and GS?
The registers FS and GS are segment registers. They have no processor-defined purpose, but instead are given purpose by the OS’s running them. In Windows 64-bit the GS register is used to point to operating system defined structures. FS and GS are commonly used by OS kernels to access thread-specific memory.
What is Fsgsbase?
FSGSBASE is an Intel® architecture extension that allows applications to directly write to the FS and GS segment registers.
What is GS segment?
The Functional Group Header (designated GS) segment appears at the beginning, and the Functional Group Trailer (designated GE) segment appears at the end. Many Transaction Sets can be included in the Functional Group, but all local transactions must be of the same type.
What are general purpose registers?
General purpose registers are used to store temporary data within the microprocessor. It is of 16 bits and is divided into two 8-bit registers BH and BL to also perform 8-bit instructions. It is used to store the value of the offset. Example: MOV BL, [500] (BL = 500H) CX – This is the counter register.
What does GS stand for in EDI?
Below find the exact description of the Functional Group Header (GS). This is the second segment of all EDI transactions including 837 EDI transaction. The purpose of this segment is to be able to group similar transactions in an interchange envelope.
What is the purpose of the GS and FS registers?
In Windows 64-bit the GS register is used to point to operating system defined structures. FS and GS are commonly used by OS kernels to access thread-specific memory. In windows, the GS register is used to manage thread-specific memory.
How many FS and GS segments are there in Linux?
In 32-bit mode the CPU provides 6 segments, which also support segment limits. The limits can be used to enforce address space protections. In 64-bit mode the CS/SS/DS/ES segments are ignored and the base address is always 0 to provide a full 64bit address space.
How are the FS / GS registers used in Linux AMD64?
The TCB and DTV are explained pretty well in Ulrich Drepper’s TLS document (look for the diagrams in chapter 3). To actually answer your fs:0 question: The x86_64 ABI requires that fs:0 contains the address “pointed to” by fs itself. That is, fs:-4 loads the value stored at fs:0 – 4.
When to use FS and GS in 64bit mode?
In 64-bit mode the CS/SS/DS/ES segments are ignored and the base address is always 0 to provide a full 64bit address space. The FS and GS segments are still functional in 64-bit mode. 24.8.1. Common FS and GS usage ¶ The FS segment is commonly used to address Thread Local Storage (TLS). FS is usually managed by runtime code or a threading library.