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).