[COFF] What Happened to Interdata?

[Paul Winalski]

On 7/26/23, Greg 'groggy' Lehey <grog at lemis.com> wrote:
>
> That's not the way I remember it.  RCA (and UNIVAC with them) and
> Interdata had instruction sets that were close to the IBM instruction
> set, but my recollection was that they were different enough that IBM
> software wouldn't run on them.  That's a different situation from
> Amdahl, which was almost completely compatible.

As I recall it, the RCA Specra 70 was compatible with the IBM System
360 instruction set.  It thus could run S/360 user applications.  And
maybe OS/360?  The downfall came with S/370.  IBM withheld the
privileged portions of the architecture (including dynamic address
translation [DAT; virtual memory]) and so when OS/VS and DOS/VS came
out they wouldn't run on Spectra 70.  RCA was forced to develop their
own OS.  Over time key IBM applications such as CICS depended on new
features in OS/VS and DOS/VS.  It wouldn't surprise me if many of
those dependencies were gratuitous.  So RCA was stuck in an endless
cycle of having to add new (D)OS/VS features to their own OS.  The big
IT customers who form this market valued stability more than the lower
price for the Spectra vs. S/370, and so RCA lost its market and
eventually gave up.

By the time Amdahl founded his own company, the privileged side of the
S/370 architecture had been revealed.  The S/370 architecture allows
for a restricted set of model-dependent behavior and features in areas
such as control register semantics and machine check handling.  The
Amdahl CPUs were no different from the S/370s as one model of S/370 is
from another.  (D)OS/VS keeps this model-dependent code in separate
modules from the bulk of the OS.  Amdahl only had to provide their own
set of model-dependent modules.  A smaller--and tractable--task
compared to what had faced RCA and Interdata at the start of the S/370
era.

-Paul W.