General DEC diagnostic questions

Pete Turnbull pete at
Sun Oct 22 09:23:11 CDT 2006

Jay West wrote:
> I assume these are probably silly questions, but hey, it's just not 
> clear to me :)
> In the PDP11 diagnostic handbook (1988) on bitsavers, some of the 
> diagnostics mention setting CSR at 174, and SWR at 176 for various 
> options to a given diagnostic. I am assuming that SWR is the console 
> switch register.

That's the original meaning, but many PDP-11s don't have a real switch 
register, so most XXDP diagnostics use location 176 as a psuedo-SWR. 
Almost all the diagnostics start at 000200, so 000176 is just the 
location immediately preceding the program.

  I thought on a /34 this was off in location 777050 (or
> something like that). So do I put those options in the keypad and hit 
> LSR (which I thought put them in 777050) or do I need to put them in 176 
> before starting the diag?

If the diagnostic is asking you about 176, put the contents (desired 
flags and/or unit number) there.

> Also, is there something special about 174 that I'm not getting?

Not really, it's just the location immediately preceding 176, and many 
diagnostics which test peripheral controllers allow you to set a value 
there, to tell the diagnostic where the device CSR really is, in case 
it's a floating address, or you want to specify a second unit's CSR 
instead of the first unit, or the device is configured with a 
non-standard address.

> Some of the docs say a particular diagnostic prompts "SWR=000000 
> NEW="... I assume the diags are querying if I even HAVE a switch 
> register, and if I do, they don't print that question. Correct?

Not quite, it's saying it doesn't see a hardware SWR (or it's zeroed, or 
it didn't bother looking) and location 176 -- the pseudo-SWR -- is 
zeroed, and it's prompting you in case you want to set a different value 
there.  You would typically do that if you wanted to set the 
loop-on-error, halt-on-error, extended messages or other flags, or set a 
particular unit number (eg for testing multiple disks on a single 

Pete						Peter Turnbull
						Network Manager
						University of York

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