On x86 (32-bit), a no-operation (nop) can be encoded as a CPU instruction 0x90 (among other choices).
0x90 can also be interpreted as
xchg eax, eax is not a nop, as it clear the upper-half of the rax register; hence, it must be encoded as 0x87 0xc0.
xchg rax, rax could be translated into a nop.
rasm2 allows to easily experiment with different assembler engines for x86 (.nz is default):
rasm2 -a x86.nz -b 64 "xchg eax,eax" // .nz .. handmade assembler 87c0 rasm2 -a x86.nz -b 32 "xchg eax,eax" 90 rasm2 -a x86.nasm -b 64 "xchg rax,rax" // using NASM, notice the extra override byte 0x48 4890 rasm2 -a x86.as -b 64 "xchg rax,rax" // using GNU assembler 90
At least the following libraries/tools get this wrong:
As you might have guessed, these are my Hacktoberfest 2022 contributions.