Question

Bonus Problem: Someone stored his/her personal password generation algorithm in the on-chip ROM of an 8051 MCU. However, the board is somehow damaged and he cannot recall the data using typical UART communication between the device and the computer. Now he/she got the machine code stored in the ROM as shown below and his/her password would be stored in 00h after code execution, please help him/her find out the correct password 0111010001111001110001001101001100010011000100111111010100000000

Answer 1

First we get all the binary in the form of a nibble and convert is to Hexadecimal to get the instruction.

• 0111-0100   74H   MOV A,#immd and 0111-1001   79H is the immd value.
• 1100-0100   C4H   SWAP A this instruction exchanges the low-order and high-order nibbles within the accumulator. so A= 97H
• 1101-0011   D3H   SETB,C sets the value of Carry flag to 1 so C=1
• 0001-0011   13H   RRC,A ,this instuction basically rotates the bits in the accoumulator to right by 1 bit where the bit0 is moved to carry flag and the bit of carry flag is moved to bit7,bit7 to bit6 and so on. so after this operation the value in accumulator 97H(10010111) becomes   CDH(11001011) and the C flag has 1.The new value at A=CDh
• 0001-0011   13H   RRC,A, we will perform the same rotation like the previous instruction on CDh (11001011) so the new value is E5H (11100101) and C flag has 1.New value at A = E5H
• 1111-0101   F5H   MOV direct,A and 0000-0000    00h is the location where we move the value of the accumulator. ie at location 00h we have the value = E5h

So out password is (E5)_hex which is (11100101)₂