AMD 2901 and 2903 processors
AMD 2901 bit-slice processor family includes 2901 and 2903 4-bit microprocessors slices, 2909 and 2911 microprogram sequencers, 2910 microprogram controller and other support chips. The 2901 processor consists of 16 4-bit registers, 4-bit ALU and associated decoding/multiplexing circuits. The ALU accepts 9-bit microinstructions that specify source operands, ALU function and the destination register. The 2901 ALU can perform 8 different functions (they are encoded into 3 bits within the microinstruction): addition, subtraction and logic operations. Multiple 2901 bit-slice processors could be combined together to build microprocessors with any data width (in 4 bits increments).
Enhanced version of 2901, AMD 2903 has 9 new special ALU functions used for implementation of multiplication, division and normalization operations. The number of arithmetic and logic ALU functions in 2903 was increased to 15.