C compiled for the 6502 would use the zero page (first 256 bytes of RAM) as registers, and use the actual registers just to run instructions of a higher-level abstract machine that, e.g., understood 16-bit numbers. Kind of like Xerox Alto, in that particular way; on the Alto, only device drivers were coded native.
There was a C compiler that kept one of the 6502 index registers zero at all times, just to have a zero handy.
This use of a little interpreter to provide a higher-level instruction architecture to program to was really, really common in the 50s and 60s. The Apollo AGC computer that landed on the moon was mostly programmed that way. It seems surprising that with memory so tight, they would use up so much of it for the virtual machine interpreter, but instructions for that could be much more compact than native code. It made a slow computer even slower, but they all felt fast back then.
Steve Wozniak burned a little interpreter like that into the Apple ][ ROM, just smart enough for dumb jobs like copying blocks of memory. It used a reserved fragment of the zero page as its registers.
There was a C compiler that kept one of the 6502 index registers zero at all times, just to have a zero handy.
This use of a little interpreter to provide a higher-level instruction architecture to program to was really, really common in the 50s and 60s. The Apollo AGC computer that landed on the moon was mostly programmed that way. It seems surprising that with memory so tight, they would use up so much of it for the virtual machine interpreter, but instructions for that could be much more compact than native code. It made a slow computer even slower, but they all felt fast back then.
Steve Wozniak burned a little interpreter like that into the Apple ][ ROM, just smart enough for dumb jobs like copying blocks of memory. It used a reserved fragment of the zero page as its registers.