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What does XOR EAX EAX?
xor eax, eax is a faster way of setting eax to zero. This is happening because you’re returning zero. The in instruction is doing stuff with I/O ports. Basically reading a word of data from the port specified dx in and storing it in al .
Does x86 have a zero register?
Most RISC architectures have a “zero register” which always reads as zero and cannot be written to. While the x86/x64 architectures do not have an architectural zero register it seems likely that the Sandybridge processor has a physical zero register.
What happens if you XOR something with itself?
This is clear from the definition of XOR: it doesn’t matter which way round you order the two inputs. This means that XOR operations can be chained together and the order doesn’t matter. This means that any value XOR’d with itself gives zero.
Does MOV clear register?
mov does break the dependency chain on the old value of the register (regardless of the source value, zero or not, because that’s how mov works).
What is test EAX EAX?
Anding a value with itself gives the same value, so test eax, eax sets the flags based on whatever eax contains. ZF is set when the result of an operation is zero. jne jumps when ZF is not set. So the jump will be taken when strcmp returns nonzero, meaning the strings are unequal.
What is the purpose of zero flag?
Along with a carry flag, a sign flag and an overflow flag, the zero flag is used to check the result of an arithmetic operation, including bitwise logical instructions. It is set to 1, or true, if an arithmetic result is zero, and reset otherwise.
What is rip pointer?
The instruction pointer register (%rip) points to the next instruction to execute; it cannot be directly accessed by the programmer, but is heavily used as the base for position-independent code addressing.
What is MOV PLC?
Move Instruction (MOV) The MOV instruction moves a Source value to a Destination location. The symbol for the move command is illustrated above. The MOV command. is an output instruction that moves a copy of a value from a Source to a desired Destination.
What happens when output to EAX on x64?
On the x64, any operation that outputs to a 32-bit register automatically zero-extends the result. In this case, outputting to eax has the effect of truncating the result to 32 bits. Return values are passed in the rax register.
What’s the difference between EAX and EAX XOR?
xor eax, eax is a faster way of setting eax to zero. This is happening because you’re returning zero. The in instruction is doing stuff with I/O ports. Basically reading a word of data from the port specified dx in and storing it in al. It’s not clear why it is happening here. Here’s a reference that seems to explain it in detail.
How does the zero extend work on the x64?
On the x64, any operation that outputs to a 32-bit register automatically zero-extends the result. In this case, outputting to eax has the effect of truncating the result to 32 bits. Return values are passed in the rax register. In this case, the result is already in the rax register, so the function returns.
How to perform a logical exclusive OR ( XOR )?
* In64-bitmode,r/m8cannotbeencodedtoaccessthefollowingbyteregistersifaREXprefixisused:AH,BH,CH,DH. Performs a bitwise exclusive OR (XOR) operation on the destination (first) and source (second) operands and stores the result in the destination operand location.