Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!VTVM1.CC.VT.EDU!VALDIS
From: VALDIS@VTVM1.CC.VT.EDU (Valdis Kletnieks)
Newsgroups: comp.lang.asm370
Subject: STOSM/STNSM
Message-ID: <8911112126.AA11923@brazos.rice.edu>
Date: 10 Nov 89 17:48:24 GMT
References: <ASPRMH%UOFT01.BITNET@PSUVM.PSU.EDU>
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: IBM 370 Assembly Programming Discussion List <ASM370%UCF1VM.BITNET@ricevm1.rice.edu>
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Lines: 49

OK.. Here's the problem.  You often get stuck writing a subroutine
that cannot tolerate any interrupts, but you don't know for sure what
the system mask is when you get called.

Solution:

... leadin code here
MASKOFF  STNSM  MASKON+1,X'00'
        (protected code here)
MASKON   STOSM  MASKOFF+1,X'FF'

The STNSM will stick the current mask into the immediate field of the STOSM,
and then set the mask to zeros.  Then when you hit the STOSM, the 'FF' has
been replaced by the actual mask you were running on, and ORing in that
results in the original mask again.

Note that this code is *NOT* suitable for AP or MP configurations.
Anybody got a version that works in a multiprocessor environment?

Now, are you using TR or TRT to find the last non-blank?  And what's this
about BXLE? :-)

CS is pretty simple really - there's a good description on page A-40
of the S/370 PrincOp (GA22-7000-10).  All it does is fetch a value,
compare it, and if it's the same update.  It's used for serializing
code.  Example (cribbing from A-40.. :) - you have a lock word, that
if it's a 0 you can proceed, but if it's a 1 you have to wait.  The
person who gets the lock sets it to 1 when he enters, and to 0 when he leaves.

       SR     R1,R1     get a zero.
       LA     R2,1      get a one.
LOOP   CS     R1,R2,LOCKWORD  do it
       BC     4,LOOP    if somebody else got it, spin.
... locked code here
       ST     R1,LOCKWORD clear the flag

What the CS will do is:
       Fetch LOCKWORD
       Compare to R1
       If EQUAL, STORE R2 at LONGWORD
       else set condition code.

So if you come in and it's zero, you fall through and set to 1 all in one
shot, while if it's a 1 you just spin...

Of course, THIS case could have been done with TS.  See the example in
the PrincOp on how to do multiple bits and other extensions of the idea.

                                   Valdis