Check the link for a more detailed writeup

• x86 boot sector executes entirely in 16-bit real mode
• 20 character flag buffer with simple checking algorithm
• calculates the sum of absolute differences of each byte between two registers, one containing slightly modified flag input, the other with known constants
• compares the results with an array of expected results
• solved using Z3/python

Original writeup (https://github.com/DMArens/CTF-Writeups/blob/master/2017/CSAWQuals/reverse/realism-400.md).