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From: VALDIS@VTVM1.CC.VT.EDU (Valdis Kletnieks)
Newsgroups: comp.lang.asm370
Subject: Re: help for new assembly'er
Message-ID: <9102271741.AA03538@ucbvax.Berkeley.EDU>
Date: 27 Feb 91 17:32:59 GMT
References: <chinet!laird%GARGOYLE.UCHICAGO.EDU@OHSTVMA.IRCC.OHIO-STATE.EDU>
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Reply-To: IBM 370 Assembly Programming Discussion List
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On Wed, 27 Feb 91 04:32:34 GMT Laird J. Heal said:
>Since registers zero, one and two can be modified by everything from
>SVCs to TRTs, use them for scratch areas.  R15 is your entry address
>but you should save it off - R12 is typically used for this and thus
>as the code base register.  R14 is the return address but can be used
>within your code as can R15, as the OS never really relies on them.
>R13, on the other hand, should always point to your Register Save
>Area, unless you are working with DOS/VSE/POWER, in which case I
>commend your soul to the appropriate power.

Please note that at least in OS/MVT and VS/1 and (presumably) the
various MVS variants, and the OS Simulation routines under VM/CMS,
that the various QSAM I/O macros (such as GET and PUT), involve
a  BALR  14,15  - so these registers should *not* be used except as
scratch unless you *know* that any system macros you use don't modify
these registers.  Incidentally, macros like this are the *REASON* you
usually use R12 as the base rather than R15....


                                  Valdis Kletnieks
                                  Computer Systems Engineer
                                  Virginia Polytechnic Institute