can't run dos MBR

guys.I read MBR from c disk to a memory such as 0:0200H,and then i debug it.however,when i arrive 0:7c18H REPZ 0:7c19H MOVSB i get a message 'CPU error:incorrect opcode.Killing process.' i dont know how to deal with it. any help开发者_如何学C will be appreciated.thanks.

REPZ MOVSB is a string move operation typically. Your MBR is likely looking to read a string it expects to be in memory causing it to fail.