CODEWITHSUNDEEP
assemblyvisual-c++x86inline-assembly
Zhibo Shen
Zhibo Shen

Reputation: 145

Get linear address of FS:[0] in 32-bit protected mode / MSVC inline asm

I used this instruction in Visual C++ inline assembly

lea eax, FS:[0]

Why did eax get a zero?

And how do I get the linear address of FS:[0]?

Upvotes: 3

Views: 893

Answers (2)

Ross Ridge
Ross Ridge

Reputation: 39621

Assuming FS points to the Windows Thread Information Block (TIB), also known as the Thread Environment Block (TEB), you get the linear address of the TIB by reading the 32-bit value at fs:[0x18]. The best way to do this in Visual C++ is to use the __readfsdword intrinsic:

TEB *teb = (TEB *) __readfsdword(0x18);

Upvotes: 6

Brendan
Brendan

Reputation: 37212

The LEA instruction ("Load Effective Address") is badly named (e.g. should probably be called LEO/"Load Effective Offset") because it only calculates the offset within a segment.

Upvotes: 3

Related Questions