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What is a FS register?
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 segment registers x86?
16 bits can represent any integer between 0 and 64k. This means that if we set A to be a fixed value and allow B to change we can address a 64k area of memory. This 64k area is called a segment. The x86 line of computers have 6 segment registers (CS, DS, ES, FS, GS, SS). They are totally independent of one another.
What is FS 0x28?
It is a segment register. In protected mode, fs is a selector into the GDT. There are hidden “base” and “limit” registers associated with it, that you cannot see. So fs:0x28 is really [hidden_fs_base + 0x28] .
What is the use of SS register?
The stack segment register (SS) is usually used to store information about the memory segment that stores the call stack of currently executed program. SP points to current stack top. By default, the stack grows downward in memory, so newer values are placed at lower memory addresses.
Is segment a register?
Segment Registers Code Segment − It contains all the instructions to be executed. A 16-bit Code Segment register or CS register stores the starting address of the code segment. Data Segment − It contains data, constants and work areas.
Are segment registers still used?
Every process access its own memory (4GB) in linear fashion, so basically the segment registers are not needed. They are still registers, so they can of course be used for various other assembly operations.
How many segment registers are there?
The 8086 has four special segment registers: cs, ds, es, and ss. These stand for Code Seg- ment, Data Segment, Extra Segment, and Stack Segment, respectively. These registers are all 16 bits wide. They deal with selecting blocks (segments) of main memory.
What is endbr64?
It stands for “End Branch 64 bit” — or more precisely, Terminate Indirect Branch in 64 bit.
How many registers are in 8086?
Besides the eight 16 bit registers, the 8086 CPUs also have eight 8 bit registers. Intel calls these registers al, ah, bl, bh, cl, ch, dl, and dh. You’ve probably noticed a similarity between these names and the names of some 16 bit registers (ax, bx, cx, and dx, to be exact).
What does FS [ register + value ] mean in Assembly?
It is undecidable by the code snippet you provided. would return the ‘Stack Base’ in EAX – ‘ (FS+1)+ (0+3)’ = real-FS: [4] = ‘Stack Base’ location. Addition: For completeness: why it doesn’t assemble has been mentioned in the comments by ‘Ross Ridge’: It’s MASM syntax and not NASM syntax.
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.
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.
What does FS stand for in Windows 10?
FS is used to point to the thread information block (TIB) on windows processes . one typical example is (SEH) which store a pointer to a callback function in FS:[0x00].