Specifically on 6502-derived processors, you have the following registers: Accumulator, X and Y indexes, Stack Pointer, Program Counter, Flags. Of these, only the Program Counter is double-width.
Additionally, the zeroth "page" of memory (the lowest 256 bytes) are accessible with a dedicated addressing mode which saves space and time in the program, and can be used as a form of cache.
Additionally, the zeroth "page" of memory (the lowest 256 bytes) are accessible with a dedicated addressing mode which saves space and time in the program, and can be used as a form of cache.