From:	MERC::"uunet!WKUVX1.BITNET!goathunter" 11-FEB-1993 19:36:32.19
To:	Alpha-IDS@WKUVX1.BITNET
CC:	
Subj:	Summary:  PCB address in UWSS rundown handler

Greetings all,

Two weeks I sent an inquiry about how to obtain the PCB address in the
rundown handler in a User Written System Service in OpenVMS AXP.  This
was needed in order to get to the AST listhead in order to remove
pending inner mode ASTs.  Here is a summary of the replies received
with my additional comments.

Reply #1
From: Ehud Gavron 602-570-2546 <GAVRON@IT.SUNQUEST.COM>
>Well seeing as it's called in process context, how about
>       movl    g^ctl$gl_pcb,r4
>Works for me.

     This is certainly the easiest and may or may not be necessary
     according to reply #3 from Digital below.  Since it doesn't hurt
     then the safest thing to do is go ahead and use this.


Reply #2
From: mahan@TGV.COM (Patrick L. Mahan)
>Well the entry point to rundown handlers are JSB entry points on the Alpha.
>You should be using the following to define your entry points to the PLV
>vector.
>KERNL_RNDWN::
>       .JSB_ENTRY      Input=R4
>       [rundown handler code]
>       Rsb
>This will allow you to access the PCB via R4.

     This appears to be in disagreement with reply #3 from Digital
     below which I've been assured is a 'definitive' answer from the
     actual developer.


Reply #3
From: dtn 471-5411 02-Feb-1993 1732 <rabahy@amcfac.enet.dec.COM>
>  The rundown routines are invoked with a CALLS instruction on Alpha.
>  R4 does contain a pointer to the PCB, as it did on VAX, but... I haven't
>  found any documentation that user written system services can depend on
>  this.  I'm looking into that further.  At any rate, it IS loaded from
>  CTL$GL_PCB shortly before the calls.


Reply #4
From: "Jeff Needle, VAXworks, DTN 223-8389, MSO1/M21 (pole E1)"
      <needle@plugh.enet.dec.COM>
>There's been a change for OpenVMS AXP whereby the image rundown handlers
>automatically delete all ASTs with P0 addresses.  According to the image
>activator maintainer, "It seems like the polite thing to do and made life a lot
>easier for some layered products."
>This is done for all modes including kernel and exec.  The goal was to prevent
>crashes due to pending inner mode ASTs.
>By the way, this is an Alpha AXP change and is not present on the VAX.

     So I may not even need the whole rundown handler at all!  Oh
     well...


Reply #5
From: ISVNET::ASCI_BEN
>    I believe R4 still points to the PCB, but in any event, you can
>    issue the FIND_CPU_DATA macro which requires an output register,
>    followed with a move of CPU$L_CURPCB.
>       FIND_CPU_DATA R0
>       MOVL    CPU$L_CURPCB(R0),R4     ;R4 -> PCB

     Beware of using this in future AXP SMP environments.  To quote
     from the VAX/VMS IDS pg 1016:  "Note that use of this macro is
     restricted to code that executes in kernel mode with memory
     management enabled.  Furthermore, the code must run at an IPL
     above 2 between invoking the macro and using the returned
     address.  Code running in process context at IPL 2 or below is
     subject to rescheduling and subsequent execution on another
     processor whose per-CPU data area is at a different address."


So I reckon what we'll end up using (if we use the rundown handler at
all) is something like:
        .ENTRY  RUNDOWN_HANDLER, ^M<R4>
        MOVL    G^CTL$GL_PCB, R4
         |
        RET
Thank you all for your help.  I hope I can return the favor someday.

Cheers, Bill Law  :-)
Law@UCBeh.San.UC.Edu