4-bit Bit-Slice Arithmetic Logic Unit for 32-bit RSFQ Microprocessors

A 4-bit bit-slice arithmetic logic unit (ALU) for 32-bit rapid single-flux-quantum microprocessors was demonstrated. The proposed ALU covers all of the ALU operations for the MIPS32 instruction set. It processes bit-sliced 32-bit data that are divided into eight slices of 4 bits. The bit-slice approach simplifies the circuit structure and reduces the hardware cost. The ALU uses synchronous concurrent-flow clocking and consists of eight pipeline stages. It was implemented using the 1.0-μm Nb/AlO
_x
/Nb nine-layer advanced process 2 (ADP2) with a critical current density of 10 kA/cm
²
, and fabricated by National Institute of Advanced Industrial Science and Technology (AIST). It consists of 3481 Josephson junctions with an area of 3.09 × 1.66 mm
²
. It achieved the target frequency of 50 GHz and a latency of 524 ps for a 32-bit operation, at the designed DC bias voltage of 2.5 mV, via precise control of interconnect delays and clock distribution. Furthermore, it achieved a throughput of 6.25 × 10
⁹
32-bit operations per second. All the correct ALU operations with measured DC bias voltage margins of around 10% at 50 GHz were successfully obtained. The proposed ALU can be used for any 4n-bit processing.