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Re: "Using" statement?

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John W Kennedy <jwkenne@attglobal.net> wrote: 

> On 2012-07-06 13:52:45 +0000, hancock4@bbs.cpcn.com said:
> 
> > Many assembler programs begin with a USING * statement.  Could someone
> > explain it?
> > 
> > Thanks.
> 
> "USING *" is actually obsolete in properly written z/Architecture code 
> that segregates code from data and uses only branch-relative 
> instructions,

USING is not obsolete in any serious code, whether or not it is
properly-written, reentrant, etc. If you are considering "USING *" a special
case then I'll go further than you and say USING * is never required, use a
label. 

You need to understand USING to get addressability to system control blocks
and other DSECTs regardless of whether you have the branch relative and
immediate facility. And you need it to understand 99.999% of the code that
already exists that does use USING.

USING associates a user-defined base register to be used by the assembler in
generating references to the area. Check bitsavers.org for the Assembly
language course book and you will get a good explanation of all this. In
fact, without understanding USING you can never read or write IBM assembler
code.

> general register r has the address of * in it. In primitive S/360 code, 
> you might have:
> 
>          BALR  12,0
>          USING *,12

You also need to do this in certain other types of programs that have no
standard method of entry.

> Under the usual operating systems, a routine is entered by BALR 14,15, 
> so GR15 will always have the address of the routine loaded in it, so 
> you can begin with
> 
>          USING *,15

Not true of SVCs and certain other system extension routines. And you don't
need a USING for *,15 anyway, you should load your chosen base register with
the contents of R15 and then the next issue you mentioned:


> Of course, you must be careful to have
> 
>          USING something else
>          DROP  15
> 
> soon after since GR15 will usually quickly be reused.

becomes irrelevant.

> But, as I say, if you are writing code expressly for z/Architecture, 
> and use only the Branch Relative family of instructions, and put all 
> your data in a separate CSECT or DSECT (or PSECT), you do not need a 
> base register for your code CSECT. That's why the Branch Relative 
> instructions were invented -- so that you have one more general 
> register to use for other purposes.

This is fine advice for "Hello, World!" programs but for any real
programming you will need to understand base addressing and DSECTs and the
assembler USING directive since they are foundations of IBM assembler. And
you will need to understand USING if you are ever going to need to read,
understand and maintain the gigantic body of code written because virtually
none of that code uses the branch relative and immediate facility, not
vendor code and not IBM code. Compilers will use it increasingly, but it is
uncommon to patch compiled code unless you work for one of 3 compiler
vendors targeting z/OS (one of those is IBM). If you want to code in
assembler you need to code like an assembler programmer.

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"Using" statement? hancock4@bbs.cpcn.com - 2012-07-06 06:52 -0700
  Re: "Using" statement? John W Kennedy <jwkenne@attglobal.net> - 2012-07-06 11:24 -0400
    Re: "Using" statement? hancock4@bbs.cpcn.com - 2012-07-06 10:15 -0700
    Re: "Using" statement? Fritz Wuehler <fritz@spamexpire-201207.rodent.frell.theremailer.net> - 2012-07-12 07:31 +0200
      Re: "Using" statement? John W Kennedy <jwkenne@attglobal.net> - 2012-07-12 22:09 -0400
    Re: "Using" statement? Pete Nelson <plnelsonoma@yahoo.com> - 2012-07-12 08:36 -0700
      Re: "Using" statement? John W Kennedy <jwkenne@attglobal.net> - 2012-07-12 22:13 -0400
        Re: "Using" statement? Fritz Wuehler <fritz@spamexpire-201207.rodent.frell.theremailer.net> - 2012-07-13 14:12 +0200
          Re: "Using" statement? John W Kennedy <jwkenne@attglobal.net> - 2012-07-13 13:57 -0400
            Re: "Using" statement? Anonymous <noreply@breaka.net> - 2012-07-15 05:54 -0400
              Re: "Using" statement? -  JEmebius <jemebius@xs4all.nl> - 2012-07-30 22:09 +0100
                Re: "Using" statement? -- JEmebius <jemebius@xs4all.nl> - 2012-07-30 22:13 +0100
                Re: "Using" statement? --- JEmebius <jemebius@xs4all.nl> - 2012-07-30 22:19 +0100
                Re: "Using" statement? --- woodag@trap.ozemail.com.au (Andy Wood) - 2012-07-31 06:22 +0000
                Re: "Using" statement? --- reply JEmebius <jemebius@xs4all.nl> - 2012-08-01 02:44 +0100
                Re: "Using" statement? - Fritz Wuehler <fritz@spamexpire-201207.rodent.frell.theremailer.net> - 2012-07-31 20:46 +0200
                Re: "Using" statement? - reply JEmebius <jemebius@xs4all.nl> - 2012-08-01 03:10 +0100
                Re: "Using" statement? - reply Fritz Wuehler <fritz@spamexpire-201208.rodent.frell.theremailer.net> - 2012-08-07 16:36 +0200
                Re: "Using" statement? - reply Nomen Nescio <nobody@dizum.com> - 2012-08-07 21:33 +0200
                Re: "Using" statement? - reply Clark F Morris <cfmpublic@ns.sympatico.ca> - 2012-08-07 18:13 -0300
                Re: "Using" statement? - "Michel Castelein" <arcis@advalvas.be.without.this.no.spam.tail> - 2012-08-03 11:13 +0200
                Re: "Using" statement? - Anonymous <nobody@remailer.paranoici.org> - 2012-08-03 13:05 +0000
                Re: "Using" statement? - Allodoxaphobia <knock_yourself_out@example.net> - 2012-08-03 13:29 +0000
                Re: "Using" statement? - WAB JEmebius <jemebius@xs4all.nl> - 2012-08-04 00:03 +0100
                Re: "Using" statement? - WAB Allodoxaphobia <knock_yourself_out@example.net> - 2012-08-04 03:14 +0000
                Re: "Using" statement? - Nomen Nescio <nobody@dizum.com> - 2012-08-06 15:15 +0200
  Re: "Using" statement? alistair.j.l.maclean@gmail.com - 2012-08-19 05:58 -0700
    Re: "Using" statement? John W Kennedy <jwkenne@attglobal.net> - 2012-08-19 13:15 -0400
      Re: "Using" statement? Nomen Nescio <nobody@dizum.com> - 2012-08-19 22:20 +0200
        Re: "Using" statement? John W Kennedy <jwkenne@attglobal.net> - 2012-08-19 18:30 -0400

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