A processor, with 16-bit addressing, registers and data paths and a 1-bit serial ALU. The F100-L could only access 32K of memory (one address bit was used for indirection). It was designed by a British company for the British Military.
The unique feature of the F100-L was that it had a complete control bus available for a coprocessor. Any instruction the F100-L couldn't decode was sent directly to the coprocessor for processing. Applications for coprocessors at the time were limited, but the design is still used in modern processors, such as the National Semiconductor 32000 series.