A Mask ROM is read only memory which is build from the silicon mask which in very simplistic terms is the image of the chips to be printed to make the cpu. Thes

A Mask ROM is read only memory which is build from the silicon mask which in very simplistic terms is the image of the chips to be printed to make the cpu. These 'images' are called masks, giving the Mask ROM. These rom's are cheap, simple and small but can in no way be changed since that would require changing the chip. Sometimes it includes also non masked rom which cannot be altered. Regular rom can be many things, rewritable like (E)EPROM or flash, disks like cd(-rom) or chips like PROM (which is write once, read many).