Comment by bogomipz | Hacker News Reader

Comment by bogomipz | Hacker News Reader

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Isn't the LGDT just an address though? You provide a starting address and a length offset that's it. Why can't that be expressed in C? Does it live in a register not visible to the compiler?

You mentioned: "For instance, a function call in C is an abstraction that defines both transfer of control (which the CPU knows about) and persistence of local state (saving and restoring registers, stack pointer etc., which the CPU knows nothing about). "

Doesn't the CPU know about this "persistence of local state"? I mean it pushing and popping that state form the stack right? I'm curious what you mean there.

What's a "naked function" in this context? I think of naked function as one that doesn't have a return statement.

Cheers.

LGDT is an instruction that Loads the Global Descriptor Table [1].

The CPU doesn't know about the persistence of local state in that it doesn't know (in this case), the significance of anything that is being pushed onto / restored from a frame around a call. It knows that a "PUSH R9" writes the value of R9 onto the region in the stack being currently pointed to by the stack pointer (RSP). It however doesn't know that this is being done because, the current frame has a live value in R9, which, per the ABI, the function being called is allowed to trash, as it is considered a volatile, callee saved register. Like I'd said earlier, these are just agreed upon conventions that might even change across compilers.

I'd used the term "naked" the way the VC++ compiler uses it. A "naked" function is one without a prolog or an epilog [2].

[1] https://pdos.csail.mit.edu/6.828/2008/readings/i386/LGDT.htm

[2] https://msdn.microsoft.com/en-us/library/21d5kd3a.aspx

I see, that makes sense. Thanks for the clarification and the links!

No, LGDT is actually an instruction (like MOV or ADD). You'll be hard-pressed to get a C compiler to emit that without using inline assembly.

http://x86.renejeschke.de/html/file_module_x86_id_156.html

Thanks

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