You're correct; slot 6 for instance is $C600. If you crashed to the system monitor you could boot a disk by entering C600G (with the 'G' standing for 'go to').

IIRC the disk controller had firmware that loaded the first 256 byte sector from disk into memory.

If you crashed to the monitor, you could hit Ctrl+B and get back to BASIC, then type IN#6 to boot the disk.

Yeah. It was neat. But it rebooted in under a second so a complete crash was no biggie.

RAM wasn't even cleared so usually no (or limited) data loss.

I thought it was PR#6 (redirect output) to boot from the disk controller in slot 6. I wonder what redirecting input would do.

That was it at the AppleSoft BASIC prompt (or IN#6). But the parent poster commented on how to do it from assembly.

There is an even quicker way from the monitor:

6 CTRL+P

Will instantly divert output to slot 6. (and boot the disk if there is hardware there)

Both worked to start a boot from the disk controller in that slot.