If this option is on, IDA will check that unused fields of instructions are filled by zeroes. If they are not, it will refuse to disassemble the instruction. If this option is on, IDA will simplify instructions and replace them by clearer pseudo-instructions. For example, bis $31, $1, $2 will be replaced by mov $1, $2 If checked, IDA will use mnemonic names of the registers, like s0, s1, t0, v0...