PDP-11 addressing question and a model round-up

From: Eric Smith <eric_at_brouhaha.com>
Date: Mon Feb 14 19:24:31 2005

Pete wrote:
> early versions of the KDF11-A had 18-bit ODT while later versions,
> and all KDF11-Bs had 22-bit ODT.

I wrote:
> Are you sure that there was a difference in the ODT? I thought the
> ODT was the same in all F11 chipsets, and the difference was purely
> a matter of whether the KTF11 MMU was installed, and whether the
> module wired the top four address lines to the bus.

Pete wrote:
> No, I'm *fairly* sure there was a difference in the ODT. Perhaps
> Sellam can tell us for a change; if he has an 11/23 (KDF11-A) with
> 18-bit ODT (his MXV21 responds to 777170) and also an 11/23-plus
> (KDF11-B) with 22-bit ODT (his MXV21 responds to 17777170 but not to
> 777170) he can tell.

Wouldn't that be explained just as easily if the ODT was identical,
the logic that generates BBS7 ignores the top four address ines on the
18-bit-only processor module?

Looking at the KDF11-A and KDF11-B schematics leaves me confused as
to exactly how BBS7 is generated. Possible the F11 chipset provides
it, but if so, I suspect that the F11 is "told" whether to use 18-bit
or 22-bit physical addresses by one of the undocumented bits of
"fixed data" that the microcode can read from the module. That's
how it gets the boot mode, but those bits are documented.

Interestingly enough, the KDF11-B user manual gives 18-bit addresses
for all the ODT examples, even though the -B has 22 bit physical
addresses. Perhaps the F11 ODT only uses 18-bit physical addresses?
The F11 chip provides a special indication of ODT cycles.

Aha! Section 5.8 of the manual says:
    During normal address cycles, BBS7 L is asserted if BSIO H is
    asserted by the F11 data chip or the MMU chip. During ODT address
    cycles, the ODT logic peformans an ODT relocation cycle to clear
    BBS7 L if either ODTA17(1)H or ODTA16(1)H is clear.
If this documentation is correct, ODT always uses only 18-bit addresses.
I don't have a KDF11-B to check.

I'm not sure if my KDF11-A is an 18-bit or a 22-bit version. Anybody
know what etch and circuit revisions introduced 22-bit capability?

Eric
Received on Mon Feb 14 2005 - 19:24:31 GMT