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wfjm.w11/doc/ECO-028-ps_init.md

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ECO-028: PS init fix (2016-12-26)

Scope

  • Was in w11a from the very beginning
  • Affects: all w11a systems

Symptom summary

The PS was at start-up and after CPU reset initialized to 000340, thus psw.pri = 7, thus with interrupts disabled. The real 11/70 actually started with 000000 and psw.pri = 0, thus with interrupts enabled.

Background

The PS reset behavior in the w11a was simply copied from the simh simulator in 2007, which at this time initialized the PS with 000340. Only in 2013 (!) it was realized by Bob Supnik that this was incorrect. See the full story in the exhaustive commit message for simh commit f0d41f1.

Fixes

PS now initialized with 000000 to stay in line with the real 11/70 and SimH.

Side effects

Hopefully none. All ibus devices come up with interrupts disabled, so there shouldn't be any spurious interrupts at start-up.

Hindsights

Even a very plausible behavior like "disable all interrupts at power on" can be the incorrect one.