I honestly don’t know how the processor counts the instruction length, so it was only pure speculation on my part as to why the limit is 15. Maybe they naively check for the 4-bit counter overflowing to determine if they’ve reached a 16th byte? Maybe they do offset by 1 (b0000 is 1 and b1111 is 16) and check for b1111? I honestly have no idea, and I don’t think we’ll get an answer unless either (1) someone from Intel during x86’s earlier years chimes in, or (2) someone reverses the gates from die shots of older processors.