 The PC-286 and early PC-386 versions were termed the M Series due to the MCA bus used, and had a similar case design to that used by later models of the PC-186.Later RM PCs using the 386 processor used an ISA bus and were shipped in a particularly ("S" for) slimline desktop case with only two 3.5" drive bays.The processor typically ran at 16 or 25MHz.