It's not that magic. It's the address of the (RF disc) IPL in the "UNIX ROM". It allows the operator to bootstrap Warm UNIX into single-user mode by just pres

It's not that magic. It's the address of the (RF disc) IPL in the "UNIX ROM". It allows the operator to bootstrap Warm UNIX into single-user mode by just pressing LOAD then START without adjusting the front panel switches. Bootstrapping Warm UNIX into multi-user mode is achieved by just toggling one front panel switch (#15) in between LOAD and START. It's all described in the accompanying boot(7) manual page.