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(Very late to the game, but this seemed like an interesting addition): It's not a register at all, it's a quirk of the Intel instruction encoding. When using a ModRM byte to load from memory, there are 3 bits used for the register field to store 8 possible registers. But the spot where ESP (the stack pointer) "would" be is instead interpreted by the processor as "a SIB byte follows this instruction" (i.e. it's an extended addressing mode, not a reference to ESP). For reasons known only to the authors, the GNU assembler has always represented this "zero where a register would otherwise be" as a "%eiz" register. The Intel syntax just drops it.