Allen had to write the loader in machine code, which was toggled in on the Altair console. The BASIC interpreter itself was loaded from paper tape via the loader and a tape reader. The first BASIC program Allen ran on the Altair was apparently "2 + 2", which worked - i.e. it printed "4" I'd like to have such confidence in my own code, particularly the I/O, which must have been tricky to emulate on the Dec10.
Well, I found it a bit hard on my Dec10-based emulator. I never got the memory-mapped stuff to work properly - I just mocked up some of the I/O instructions. But it was actually a spare-time project, intended to let my students do stuff like sorting, searching in strings, so I didn't feel too guilty. It had an assembler, debugger and other stuff. And it was portable - completely standard FORTRAN77!
And I was just thinking - wouldn't it be cool to write a Dec10 emulator on a modern CPU?
Actually, one of my programmer colleagues did try to buy our Dec10 when it was decommissioned, with all peripherals, and install it in his garage. Power supply and wife were major obstacles.
> which must have been tricky to emulate on the Dec10
I don't see why it would be tricky. I don't know how Allen's 8080 emulator on the PDP-10 worked, but it seems straightforward to emulate 8080 I/O.