Contains the CPU submodel, where applicable. Allegro defines at least the
following CPU family types (see include/allegro/system.h for a more
CPU_MODEL_PENTIUM, CPU_MODEL_K5, CPU_MODEL_K6
CPU_MODEL_POWERPC_x, for x=601-604, 620, 750, 7400, 7450
You can read this variable after you have called check_cpu() (which is
automatically called by allegro_init()). Make sure you check the cpu_family
and cpu_vendor so you know which models make sense to check.