Prints "ope" because the repe cmpsb stopped when it hit the 'n' (the first place where [rdi]!=[rsi]). mov rcx,10 ; rep repeats up to this many times mov rsi,B ;С o Z S P A rep / repe / repz / repne / repnz - Repeat instruction M Format : rep repeat the specified string instruction until the conditions are met . (The REPZ and REPNZ prefixes are synonymous forms of the REPE and REPNE prefixes, The REP prefixes apply only to one string instruction at a time. REP, REPE (repeat while equal), and REPNE (repeat while not equal) are prefix that are applied to string operation. Each prefix cause the string instruction Each prefix causes the associated string instruction to repeat until the count register (CX) or the zero flag (ZF) matches a tested condition. Repeats a string instruction the number of times specified in the count register ((E)CX) or until the indicated condition of the ZF flag is no longer met. The REPE/REPZ - Repeat Equal / Repeat Zero Usage: REPE REPZ Modifies flags: None Repeats execution of string instructions while CX != Opcode. instruction. Description. F3 6C. REP INS r/m8, DX. Input (E)CX bytes from port DX to ES:[(E)DI]. F3 6D. REP INS r/m16,DX. REPE, REPZ, REPNE, and REPNZ are operational repeat prefix coded before the string instructions SCAS and CMPS, which change the ZF.