The cpuburn programs are designed to load x86 CPUs as heavily as possible for the purposes of system testing. They have been optimized for different processors. FPU and ALU instructions are coded an assembler endless loop. They do not test every instruction. The goal has been to maximize heat production from the CPU, putting stress on the CPU itself, cooling system, motherboard (especially voltage regulators) and power supply (likely cause of burnBX/MMX errors).