[sje]

I think you're right about an inherent limitation in the 6800 concerning single step operation. I haven't been able to find any example circuits that will handle controlling a 6800 in step mode, either per instruction or per bus cycle. Other than the Altair 680, none of the early 6800 based personal computers (SWTPC, Sphere, etc.) had a front panel.

What is interesting is that the MOS 6502 is easy to step; the circuit needs only a pair of flip flops and a couple of switches. And the 6502 is pretty much the same as the MOS 6501 with a different pin-out; the 6501 itself was an almost drop-in replacement for the Motorola 6800, but the 6501 withdrawn from the market by MOS due to legal threats by Motorola. So where did the single step capability in the 6502 come from if it wasn't in the 6800?