strange number corruption on pdp11/34

Don North ak6dn at
Tue Jan 27 16:56:15 CST 2015

Could be something wrong with MUL, DIV, ASH, ASHC that may be used in the print
routine, but would not be used during basic OS operation. XXDP in particular is
designed to run on a very basic CPU so it does not normally use any optional

I'd start with running the 11/34 CPU tests.


-----Original Message-----
>From: Johnny Billquist <bqt at>
>Sent: Jan 27, 2015 2:42 PM
>To: cctalk at
>Subject: Re: strange number corruption on pdp11/34
>On 2015-01-27 21:24, Noel Chiappa wrote:
>>      > From: Jacob Ritorto
>>      > I'm getting all zeroes nearly any time there's supposed to be a large
>>      > number output to my terminal.
>> It sounds like the CPU may be too broken to run diagnostics. Why don't you
>> try writing some small programs to test various bits and pieces of the CPU
>> (e.g. add instructions, etc), and toggle them in, and try and find what's
>> busted that way? (Although for 'add', if the thing's that broken, I'm amazed
>> it even boots.)
>> Here's an example:
>> It's probably too long to toggle in, but you could steal pieces from it
>> and try them.
>> I'd probably try out the shift and multiply instruction to start with; trying
>> to figure out what's probably not used during booting, but could be used
>> during number printing, those are two good bets.
>> Anyway, there's no point to running diagnostics until you get that first
>> problem fixed.
>If it comes as far as booting RSX as well as XXDP, there can't be 
>anything seriously wrong with the CPU...
>	Johnny
>Johnny Billquist                  || "I'm on a bus
>                                   ||  on a psychedelic trip
>email: bqt at             ||  Reading murder books
>pdp is alive!                     ||  tryin' to stay hip" - B. Idol

More information about the cctalk mailing list