Having only a single index register and no addressing mode indexing against 16-bit addresses in memory seems to have been widely considered a primary reason that the Motorola 6800 fared poorly in benchmarks against its contemporiries, the Intel 8080 and the MOS 6502. (It was, indeed, an explicit design goal of the 6502 to mitigate this problem with the 6800.)
I thought I had seen a microprocessor or microcontroller object-compatible with the 6800 that added an additional index register. However, I no longer seem to be able to find a reference for this, if indeed I'd really seen it in the first place.
Does anybody know of a microprocessor or microcontroller from the '70s or '80s, manufactured by Motorola or anyone else, that added another index register and/or an additional addressing mode that would mitigate this problem?
To be clear, processors such as the 6805 and 6809 that do not run 6800 object code do not qualify. (The 6805 was "mostly" like a 6800—much closer than the 6809—and did fix the problem by adding a new indexing mode with a 16-bit offset to which X was added, like the 6502, but it also reduced the size of the X register to 8 bits.)