Single stepping 6800 is possible
To quote the 6800 datasheet:
To debug programs it is advantageous to step through programs instruction by instruction. To do this, HALT must be brought high for one MPU cycle and then returned low as shown at point B of Figure 13. ...
Looking at the schematic for the Altair 680, the address LEDs are tied directly to the switches. They simply display how the switches are set. Probably wanted to stay as simple as possible.
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