For what it's worth, I've used a HP 200LX to confirm that some undocumented details (found by disassembling microcode) work as expected: REP BOUND

For what it's worth, I've used a HP 200LX to confirm that some undocumented details (found by disassembling microcode) work as expected: REP BOUND only compares against lower bound REP MUL result in AH (byte) or SP (word) REP IMUL result in CH (byte) or BP (word) REP IMUL reg,r/m,i16 first operand ignored, 32 bit result in DX:AX REP IMUL reg,r/m,i8 first op ign., AH=result bit 23..16, AL=bit 7..0 AAM 00H no "divide by zero" exception, AH=FFh, AL unchanged So it's certainly based on the 80186 microcode, as none of these instructions do the same on any other x86 chip.