PDP-8/I vs 8/E EAE

Kyle Owen kylevowen at gmail.com
Tue Mar 2 14:08:42 CST 2021

Has anyone noticed a difference in DVI overflow behavior on the PDP-8/I EAE
versus the PDP-8/E EAE? The 8/E EAE claims to be 8/I compatible in Mode A,
and I think I agree, for the most part. At least, it's compatible for the
parts that matter.

When a DVI instruction results in overflow, the EAE immediately returns
with the link set. The results in AC and MQ seem to have no relevance, but
they appear to differ between the 8/E and 8/I.

For instance, running the 8/I MUY/DVI diagnostics under SimH fails due to
the following:

sim> lo maindec/maindec-8i-d0ba-pb.bin
sim> d sr 40
sim> g 201

DIVERR    L       C(AC)          C(MQ)          C(MB)

PROB      0   000000000000   000000000000   000000000000

GOOD      1   111111111111   000000000000   000000000000
BAD       1   000000000000   000000000001   000000000000

SCA           000000000000

HALT instruction, PC: 01512 (JMP I 1506)

The link is set, but obviously MQ and AC do not match.

Running the same diagnostic on an 8/I works fine.

I can't imagine a scenario outside of diagnostics where this behavior would
impact the software, but it does seem curious nevertheless that the DVI
approach to handling overflow differs slightly between EAEs on the 8/I and


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