 When the CPU cache was moved inside the CPU, the CPUs implemented "fixed-range MTRRs" which cover the first mebibyte of memory to be compatible to what PC-BIOSes provided at that time.These are used to control the cache policy needed for VGA accesses and all other memory-accesses done while the system is in real mode.Above 1 MiB, CPUs provide a number of "variable-range MTRRs", which can be freely placed and even overlap.