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Comment by DrTung | Hacker News Reader

PDP 11/34 had a bootrom and an octal keyboard, which is kind of cheating.

I grew up using the older PDP 11/45 where you had to key in (using binary toggles) a boot program. Or, the way I did it, manually enter write address and byte count to the RK05 disk controller (memory mapped I/O) then writing a "01" Go command to RK05's command register and watch the RK05 disk activity light flicker briefly. Voila, RT11 was loaded. (This worked because on the PDP 11/45 DMA transfers from disk to memory worked also in CPU Halt mode.)

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Boot ROMs for the early PDP-11s could be pretty nifty in their own right though. This is the first version of I believe the RX01 floppy disk boot code. 32 sixteen-bit words: https://www.cca.org/blog/20120222-Diode-Matrix.shtml User-editable!

By the time of the PDP-11/34 things were much more integrated though. Mask IC ROMs. The front panel/interactive serial debugger were actually controlled by an Intel 8008 microprocessor. You needed a computer just to boot the computer even then.

Binary toggle switches? Luxury. In my day we didn’t have binary toggle switches. We had to quickly touch raw wire leads to each other in the right sequence, flawlessly, every time. And we were damn grateful of it.

I couldn't find any good videos of toggling in a bootloader on a real PDP-11, but here is one on a PiDP-11 emulator: https://www.youtube.com/watch?v=31vrju0BMQc

well this is disappointing..."binary toggles" had me picturing a single 3-way toggle switch where you manually entered ones and zeroes.

That is exactly what is going on but you don't need 3 way toggle.

Down is 0 and up is 1, you load all of the values in like a shift register but in parallel, then write/act upon them.

Yes the data bits were 2-way toggles, but one important switch was a 3-way toggle: the Examine (press down) and Deposit (press up) switch.

On the 11/70, Deposit was the only switch that toggled up (other than the light test switch). That is a nice safety feature so you don't accidentally toggle Continue or other action switches.

Right...but surely you see how that's different from what I described.

If your -45 had core memory, the bootloader could be toggled in one then left in place for subsequent reboots after power off.

We had a PDP 11/4 with core memory that ran a TruDrill CNC PCB drill. The bootloader was in ROM and if the core memory became corrupt you had to reload it with the same paper tape reader that read the G-code.

My first computer job was programming a PDP-11 which ran a Casio emulator. Our PDP-11 was mounted very low beneath a floppy drive cabinet, and each morning the elderly programmer would lie on the dusty floor to toggle in the boot sequence which then loaded from paper tape. When that became my job, I somehow figured that if the toggles were left at the first address, then hitting the start key would allow the core memory to take over. Everyone was convinced that I was a computer genius.

DMA transfers should work on all PDP-11 that kept memory on the same bus as I/O (most of them, iirc, I think separate side bus for memory happened with VAX Q-Bus models?)

I had to do this a couple of times for our 11/40 (also running RT11) - the was a printed cheat sheet of the OCD (octal codded decimal) you had to toggle in then hit the load to advance to the next location.

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