I think I may have found the computer that was the primary influence in the design of the 6800. This is going to require more research, but the Wikipedia page on Honeywell's H-316 describes the processor, but doesn't give any more information on how much of the design was inherited from the original DDP-116 that Honeywell got from Computer Control Company:
Accumulators A and B? 16-bit, but check.
Index? Check.
I'm thinking it looks closer than either the PDP-8 or PDP-11, both of which are usually cited as influences, but have much larger register sets of general purpose registers rather than the 6800's limited pair of small accumulators, single index register, stack pointer, and program counter (instruction pointer).
The 68000 (one more zero!) clearly shows influences from the PDP-11, and the 6809 borrows indexing modes from the PDP-11, but the 6800 does not look like a PDP in any way that is not attributable to both implementing Turing machines in register-memory architecture.
[JMR20220703:
I've been looking around again, and I think the Data General Nova is another CPU that the 6800 might have been fairly directly influenced by --
- two accumulators (again, 16-bit),
- and, okay, two, not one, index registers.
- The 6800's constant byte offset could be inspired by the Nova's constant offset indexed addressing mode,
- and the 6800's direct page is similar to the Nova's page zero.
Two index registers -- the 68HC11 descendant of the 6800 finally got a second index register, well more than a decade after the introduction of the 6800.
It is fairly clear that the Nova was part of the inspiration for the original ARM architecture, I think, and I know several people who agree with me about that.
]
This needs more research, however. I'll try to update this stub when I have better information.
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