Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!decwrl!ucbvax!gmdzi.UUCP!icking
From: icking@gmdzi.UUCP (Werner Icking)
Newsgroups: comp.lang.asm370
Subject: Re: SAVE area offset zero
Message-ID: <9008211315.AA29393@ucbvax.Berkeley.EDU>
Date: 21 Aug 90 10:46:06 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: IBM 370 Assembly Programming Discussion List
              <ASM370@OHSTVMA.BITNET>
Distribution: inet
Organization: The Internet
Lines: 49

TROTH@RICEVM1.RICE.EDU (Rick Troth) writes:


>        In the save area ... registers 14 thru 12 are placed at offset 12,
>which means R14 goes to +12, R15 goes to +16, R0 goes to +20, R1 goes to +24,
>and so on.  Offset 4 points to the next SAVE area, right?  And offset 8 to
>the previous SAVE area, right?

>        So what does offset zero refer to?  If you say "R13 goes there",
>I won't shoot you.  Pointing to itself would provide validity checking,
>but is that what offset zero does?  If not,  then what?

We - a group of people programming 360-code in the late 60-th and beginning
70-th - used offset zero for the length of the save area when writing re-
enterable programms. The first 18 words of the save area were used for
the length, the chaining and registers 12 to 14, the remainder for the variables
of the programme. With this layout a supervisor programm was able to remove
a programme and all its data from memory by cutting the save area chain in
reverse order and releasing the memory as stated by the length field.

A typical programm entry was like this:

       USING  *,15
       B     BEGIN
       DC    AL1(..),C'name copyright date etc.' .. means length of this
BEGIN  STM   14,12,12(13)
       LA    0,DSA#
       GETMAIN  ...       allocate memory for the new save area
       ST    0,0(,1)      note the length
       MVC   4(4,1),4(13) propagate forward pointer
       ST    13,8(,1)     set the backward pointer
       ST    1,4(,13)     set new forward pointer
       LR    13,1         activate the new save area
       ...
DSA    DSECT
       DS    18F          standard register save area
TEMP8  DS    D            DW work area
       ...
DSA#   EQU   *-DSA

In reality all this was "hidden" in macros and subroutines. E.g. the GETMAIN
was in a subroutine, doing a conditional GETMAIN, returning to the caller
of the programme (not the subroutine!) a return code of -4  if the GETMAIN
was not successful, ...

--
Werner Icking          icking@gmdzi.gmd.de          (+49 2241) 14-2443
Gesellschaft fuer Mathematik und Datenverarbeitung mbH (GMD)
Schloss Birlinghoven, P.O.Box 1240, D-5205 Sankt Augustin 1, FRGermany