[TUHS] Bootstrapping UNIX - how was it done

[Paul Winalski via TUHS]

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
>