[TUHS] Bootstrapping UNIX - how was it done

[Tom Lyon via TUHS]

Same deal with virtual machines under VM/370, except one issued the IPL CP
command.
'IPL 00C' booted from the virtual card reader - I used this way too many
times during the UNIX on the 370 bringup.

On Thu, Mar 26, 2026 at 8:57 AM Paul Winalski via TUHS <tuhs at tuhs.org>
wrote:

> On the IBM System/360 and S/370 one set the dials on the system console to
> the 3-hex-digit address of the boot device and pressed the IPL (Initial
> Program Load) button.  On S/370 models 115 and 125, which did not have
> blinkenlight consoles but instead a video console, one selected IPL from
> the console menu options.
>
> This caused the system to send a "Read IPL" channel command word (CCW) to
> the selected channel and controller, which then read one record off of the
> device into hardware memory location 0.  The IPL record was 24 bytes long.
> The first 4 bytes were loaded into the Program Status Word (PSW) register
> and specified the address of the first instruction to be executed.  The
> second 4 bytes were a CCW to load further instructions and data from the
> boot device.  One could boot from any device that supported the Read IPL
> CCW function.  This included disk, drum, and data cell devices, magnetic
> tape, and punch cards.
>
> -Paul W.
>
> On Thu, Mar 26, 2026 at 7:29 AM bruner--- via TUHS <tuhs at tuhs.org> wrote:
>
> > Most of the PDP-11's I used had boot ROMs, but I do recall at least one
> > machine where the boot procedure was
> >
> > Set HALT/RUN to HALT
> > Press START
> > LOAD ADDR 777404
> > DEP 5
> > LOAD ADDR 0
> > Set HALT/RUN to RUN
> > Press START
> >
> > The reset cleared all of the controller registers, so this read 64Kwords
> > from RK05 drive 0 into memory starting at 0. A little overkill for
> reading
> > one 512-byte block...
> >
> > --John
> >
>