simh & hardware - pdp-11 psw in supervisor mode?

Brad Parker brad at
Sun Oct 24 13:33:42 CDT 2010

  I have a low-level pdp-11 question...

I'm confused about writing to the PSW on cpu's which support user & 
supervisor mode.  My
read of the docs is that in user mode you should not be able to write 
the "mode" bits of the PSW.

(or, perhaps more accurately, you should not be able to *clear* any mode 
bits from user space)

I have a little diagnostic which doesn't work as I though it should 
under simh and I thought I'd
ask what others think...

Basically, simh allows code running in "user mode" to write the PSW even 
when (I claim) it
should not.   I have not tried this on a real 11/44 or 11/34 yet, but I 

Should simh allow this?  In the test blow the "clr @#PSW" is successful 
when run
on simh and I think it should basically be a nop...

(which begs another question - should it be a nop? or a exception?)

A side question might be "the psw is not protected from writes, except 
by using
the mmu" - is this true on all models?  or just some?  The 11/40 manual 
that it *is* protected.  But 11/73 docs seem to say the opposite and 
imply using the mmu.

diagnostic follows:

     .TITLE test17
PSW=177776    ;processor status word
     .word    200
     .word    0007
     mov    #200, r5    ;we should be in kernel mode here
     clr    @#PSW        ;kernel mode
     mov    #500,sp        ;sp=500 in kernel mode
     mov    #140000,@#PSW    ;user mode
     mov    #700,sp        ;sp=700 in user mode
     trap    377        ;should move us to kernel mode
     clr    @#PSW        ;back to kernel mode

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