Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!oliveb!tymix!antares!jms
From: jms@antares.UUCP (joe smith)
Newsgroups: comp.arch
Subject: Re: Self Modifying Code
Summary: Writing in routine preserves registers
Keywords: writing return address to routine front
Message-ID: <113@antares.UUCP>
Date: 21 Jul 88 06:36:39 GMT
References: <5254@june.cs.washington.edu> <76700032@p.cs.uiuc.edu> <361@scolex> <2317@pt.cs.cmu.edu> <60742@sun.uucp>
Reply-To: jms@antares.UUCP (joe smith)
Organization: Tymnet QSATS, San Jose CA
Lines: 22

The PDP-10 still uses JSR to write the return address at the front of the
routine, but only when absolutely necessary.  Normal calls us the
recursive/reentrant mechanism of pushing the return PC on the stack, but
the debugger (DDT) can't do that.  Since programs can use anywhere from
0 to 16 different stack pointers, there is no such thing as "the" stack.
The usual use for JSR is to save all accumulators in memory before setting
up a private stack for processing the breakpoint.  This even works in
exec mode on an SMP (symmetric multi processor) system as long as each
CPU maps the same virtual address of the return PC word to different
physical addresses.

The main point I'm trying to make is that the PDP-10 gives us the option
of storing the return PC in memory, in an accumulator, or on the stack,
allowing the programmer to choose whichever option is best suited to the
task at hand.  Having a small impure section of the program is reasonable
under certain circumstances.

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