An earlier logic chip used as a building block for CPUs. Bit slice processors used arithmetic logic units (ALUs) that typically came in 4-bit increments, although 1- and 2-bit devices were also made. Connected to a control unit, the ALU slices were strung together to make larger processors (8-bit, 16-bit, etc.). They included inputs and outputs for borrow and carry bits (addition and subtraction require carrying to and borrowing from the digit on the left). In the early days of microprocessors, bit slice processors enabled larger CPUs to be built from off-the-shelf components, and products were made by AMD, Intel and National Semiconductor in the mid-1970s. Most notable was AMD's 2900 family of integrated circuits used in CPUs from Digital and others, which included the Am2901 4-bit slice ALU. See ALU.