2020年12月9日 · The compiler could have moved this load earlier as well, but chose not to. But add %dl, (%rax) is an RMW on (%rax), so the dl data isn't needed until a load from (%rax) has that data ready. Having the RAX address ready early is more valuable than the DL data because the address is being used for another load, not ALU -> store.

See the question and answer for x86_64 registers rax/eax/ax/al overwriting full register contents as well as Why do most x64 instructions zero the upper part of a 32 bit register which provide some explanation about how the 64 bit register operation differ from the 32 bit register operation.

2016年12月20日 · mov %rax, %eax # AT&T syntax (INVALID!) mov eax, rax ; Intel syntax (INVALID!) but not identical. This last case is actually an invalid instruction because there is an operand-size mismatch. If it were valid, the instruction would be copying the contents of the 64-bit rax register into the 32-bit eax register. Of course, this is impossible.

2013年12月13日 · uint64_t rax = 0, rbx = 0; __asm__("" : "=a"(rax), "=b"(rbx) ::); /* make rax and rbx take on the current values in those registers */ Note that you don't need any actual instructions -- the constraints tell gcc that after doing nothing, the …

2017年7月7日 · The registers starting with r as in rax, rbx, etc, are the 64-bit registers introduced with the AMD64 extension to the existing 32-bit x86 ISA. That ISA extension was subsequently adopted by Intel and is often known by the more neutral name x86-64 .

2019年7月10日 · You’re right that there appears to be an extra level of indirection here, since (%rax) already indicates the memory at the address contained in rax. The asterisk is an idiosyncrasy of the gnu assembler syntax for indirect call instructions. It does not indicate an additional indirection. In Intel syntax, this would be “call [rax]”. –

hmm eax == 32bits ax == 16bits ah|al == 8bits, it's always like this, x64 adds new registers, back in the 16 bit days we only had ax & al & ah then when 32 bit addressing came round it was added in a way that didn't really effect how you address the 16bit or 8bit registers, the new registers in x64 (64 bit registers that overlap, eax that overlaps ax, etc) start with an r so rax rbx and so on.

2021年6月28日 · Remember that the RAX return-value register isn't written by the function until after you've jumped to it, so yes of course it's fine to use RAX as a temporary before a tailcall, even to a function with a return value. That's not a problem. You mess with RAX, then the function runs, then it returns to your caller.

My program works correctly most of the time, but about every 800 runs or so I get an unexplained segfault within the 3rd party library at the instruction indicated below. It would appear that the segfault occurs due to a null pointer dereference ( %rax is being dereferenced, but it is an invalid dereference because %rax is zero).

I asked about reading those registers. Now I wrote a code to read (just for now on) RAX and RBX. I'm using CodeBlocks pm 64-bit Windows 7 with MinGW as a compiler and I'm working on an x86-64 CPU. When I tried to compile the below code, I got those errors: Error: bad register name `%rax' Error: bad register name `%rbx' And the code: