K12MIT on PDP-12

Vincent Slyngstad vincent.slyngstad at gmail.com
Sat Nov 27 00:51:01 CST 2021

On 11/26/2021 1:43 AM, Christian Corti via cctalk wrote:
> I've found the bug. Charles Lasner broke PDP-12 support (and probably 
> also older PDP-8 models like 8/I) with his additions to support the 
> DECmate family. The location is at 0206, labelled SETBAUD. This should 
> be a NOP on standard systems, but is overlayed by an IOT to set the baud 
> rate on a DECmate 1. This results in a 6413 instruction (because I have 
> assembled it for the second serial port on 40/41). This instruction 
> always skips (as the transmitter done flag is always set), thus jumping 
> into a 4411 that is the address of a CRLF to be printed but executed as 
> a JMS into nirvana.

It took a bit of puzzling to follow, but I think I see.  There are three 
possibilities; the the DMI should have a "NOP", and the DMII should have 
"MSB". DM2LIS correctly has the MSB patch for DMII, and DM1LIS has a NOP 
as it perhaps should.

The problem is that the default for "KL8" interface uses an "RSBIOT".  I 
don't think that will work for anything but the KL8J/M8655, though it 
should be harmless on the KL8E/M8650.

I'd argue that the correct thing would be to detect an 8/e/f/m/a and 
patch in "RSBIOT".  The default "family of eight" thing should be the 
"NOP", same as the DM1.

> If patching location 0206 to 7000 (NOP), K12MIT will run on a PDP-12.


> Sending files works, but receiving seems to be broken. Most of the 
> times, the sending Kermit aborts the transfer to the PDP-12 with a 
> out-of-window-NACK error (confirmed by looking at the protocol debug 
> log). K12MIT sends a NACK to a previous packet that it already ACK'ed 
> *after* receiving the next data packet.

Alas, I don't know enough about Kermit protocol to comment on that.


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