Re: Re: Assembler Clarification

The return address is stored in a register? No recursion... right?

Is there a stack?


On Sun, May 9, 2010 at 2:16 PM, peter.thingsted <[email protected]> wrote:

>
>
> The first thing a program needs to do is to save the contents of the
> general registers (16 of them ) in memory.
>
> The general registers are to be thought of as separate electronic pieces of
> storage - outside the main storage.
>
> You need to save the contents of these registers when your program starts
> running, and restore them after your program has finished doing whatever it
> is programmed to do.
>
> One register (14) is especially important to preserve, because it contains
> the return address of the calling program, i.e. the program that was running
> just before your assembler program. So the very last instruction in your
> program should be a BR 14, i.e branch to the machine instruction stored in
> the memory location that register 14 poinded to when YOUR program took over.
>
> You can play with one register for good effects just before your BR 14
> instucton. You can put a value in register 15, e.g. a big fat zero by coding
> SR 15,15. This signals that all ended well. It is customary (but not
> required or in any way enforced) that you can put various powers of 2 into
> reg 15 to signal any number of abnormal situations thet your program
> detected.
>
> The latter here is part of what we call the linkage conventions. Note that
> they are conventions not enforced by any part of MVS - but you best know
> these conventions otherwise your program wil not cooperate well with all the
> other programs that taken together constitute the MVS sustem.
>
> Rgs,
>
> Peter
> mvsaudit.org
>
>
> --- In [email protected] <turnkey-mvs%40yahoogroups.com>, Ron
> Hudson <hudson.ra@...> wrote:
> >
> > Hey Wizards of Assembly Language..
> >
> > It looks to me like one of the first things a program does in setting up
> > it's own environment
> > is tell the CPU where to put the registers? Are do the registers actually
> > reside in memory?
> > Does each program get it's own set of registers? Gee that would make
> context
> > changes
> > really easy..
> >
> > (Not starting assembly language just yet but interested...)
> >
> > Thanks.
> >
>
>  
>