Texas Instruments 9900 processors
Introduced in 1976 and based on the Texas Instruments 990 minicomputer CPU, the 9900 was one of the first true 16-bit microprocessors.
It had a 15-bit address bus, a 16-bit data bus, and three internal 16-bit registers (PC, WP, and ST). One unique feature, though, was that all general purpose user registers were actually kept in external memory. A single workspace register (WP) pointed to the 16 register set (each register being 16 bits wide) in RAM, so when a subroutine was entered or an interrupt was processed, only the single workspace register had to be changed - unlike some CPUs which required dozens or more register saves before acknowledging a context switch.
This was feasible at the time because RAM was often faster than the CPUs. A few modern designs, such as the INMOS Transputers, use this same design using caches or rotating buffers, for the same reason of improved context switches. Other chips of the time, such as the 650x series had a similar philosophy, using index registers, but the 9900 went the farthest in this direction.
Source: Wikipedia, the free encyclopedia.