A microprocessor natively translates and executes instructions of both the x86 instruction set architecture (ISA) and the Advanced RISC Machines (ARM) ISA. An instruction formatter extracts distinct ARM instruction bytes from a stream of instruction bytes received from an instruction cache and formats them. ARM and x86 instruction length decoders decode ARM and x86 instruction bytes, respectively, and determine instruction lengths of ARM and x86 instructions. An instruction translator translates the formatted x86 ISA and ARM ISA instructions into microinstructions of a unified microinstruction set architecture of the microprocessor. An execution pipeline executes the microinstructions to generate results defined by the x86 ISA and ARM ISA instructions.