RCA Spectra 7025: Another Mystery Computer

I was surprised when I found out that the Univac 9200 and 9300, in suppressing the use of a base register when the first bit of a 16-bit address field was zero, were copying the IBM System-360 Model 20.

I wondered if anyone else had cloned the 360-20.

So I looked at the brochure for the Spectra 70 at the time of its introduction in 1965, available at computerhistory.org. Four models were noted: 15, 25, 45 and 55.

The brochure claimed *only* that the models 45 and 55 had instruction sets identical with the 360 as far as non-privileged instructions were concerned. To handle interrupts more quickly, it had alternate sets of registers for use by interrupt service routines, and for other purposes.

So where did that leave the models 15 and 25? The Model 15 had a maximum memory capacity of 8K. But the Model 25 had a maximum memory capacity of 64K.

So it seems the 70-25 could not be a 360-20 clone. The 70-15 could be - but then again, it could have a completely different instruction set, originated by RCA. The 70-25 might have still been 360-like, just with some instructions omitted, too, of course.

John Savard Usenet Zone Free Binaries Usenet Server More than 140,000 groups Unlimited download