National Semiconductor PACE
National Semiconductor's IPC-16A/520 PACE, short for "Processing and Control Element", was the first commercial single-chip 16-bit microprocessor. PACE had four general-purpose accumulators, with an instruction set architecture loosely based on the earlier IMP-16 architecture, which in turn had been inspired by the Data General Nova minicomputer.
PACE was slightly faster than the IMP-16, and offered a "byte mode" for more convenient processing of 8-bit data. Some PACE instructions were restricted to operation on only the first accumulator, AC0, rather than allowing use of any accumulator as on the IMP-16.
The PACE was followed by the INS8900, which had the same architecture but was implemented in NMOS for easier electrical interfacing.