https://en.wikipedia.org/wiki/COSMAC_Elf https://en.wikipedia.org/wiki/CHIP-8
For example, https://developer.arm.com/documentation/den0013/0400/ARM-Pro...:
“The ARM architecture provides sixteen 32-bit general purpose registers (R0-R15) for software use. Fifteen of them (R0-R14) can be used for general purpose data storage, while R15 is the program counter whose value is altered as the core executes instructions. An explicit write to R15 by software will alter program flow.”
Similarly, the PDP-11 used R7 for that. https://en.wikipedia.org/wiki/PDP-11_architecture: “The CPU contains eight general-purpose 16-bit registers (R0 to R7). Register R7 is the program counter (PC)”
I’m not aware of any CPU where “every register could be the program counter”. Such a feature also feels weird to me from a design view because I think it complicates the hardware with few (code might be smaller for a limited set of jumps) benefits. Maybe you’re confusing that with the ability to jump to whatever address is in any of the general purpose registers?
EDIT: I just saw a comment mentioning the 1892. It indeed had that ability. https://en.wikipedia.org/wiki/RCA_1802#Registers_and_I/O:
“The SEP instruction can select any of the 16 registers to be the program counter. The SEX instruction can select any of the 16-bit registers to be the index register.”
There'll come a day when the final physical 6502, 80486 (etc etc) powers off for the final time, and it seems like these technologies would be worth preserving. (I'm sure this is much easier said than done, not trying to minimise the effort involved.)
Is there a project that does something like this?
Full die capture, full transistor level simulation.
Most of the others are being emulated in proportion to their importance to games, in arcade machines and consoles. Those emulations are going to be as cycle-accurate as required.
There are some issues around things like the Yamaha synth chips, which are mixed-signal and depend on analogue properties that can be difficult to emulate.
Note that 6502s are still being produced, albeit in a modern CMOS version.
Just calculating or shuffling data around was invisible. With that visualization I first realized how much stuff could be done with a 1MHz CPU.
Imagine these 8 bitters were mostly hardwired, with less than a million transistors.
The model number was decided long before the transistor-level design was finalized.
The OS was interesting too: https://en.wikipedia.org/wiki/OS-9
