 Unlike the X1, the arithmetic unit of the X8 included floating point arithmetic, with a 41-bit mantissa and 12-bit exponent (which adds up to 53 bits rather than 54; the reason is that there are two copies of the mantissa sign bit).The system is most notable as the target processor for Edsger Dijkstra's implementation of the THE multiprogramming system.This includes the invention of semaphores, enabled by a specific instruction in the X8 instruction set.