Bit slice processors are modules used to build up larger processors. A larger processor of word size X is built up of several smaller modules, ALUs (Arithmetic and Logic Units), of word size usually X/16, X/8, X/4, or X/2; i.e., of 1, 2, 4 or 8 bits. Due to microlithography advances, this is now mostly obsolete technically, and of historical interest. Most bit slicing occurred from the mid-1970s to the mid-1980s, when integrated circuit chips were too small to hold all of the circuitry needed for a full larger processor (usually 16- to 32-bits), during the time of medium scale integration (MSI), and early large scale integration (LSI).