Re: Memory control -- RPG400-L

I've always labored under the impression that the AS400 was able to keep a
program from overstepping its bounds, memory-wise; i.e. I cannot write an
HLL program that will modify the memory allocated by another program,

What is a parameter? It's a pointer to the memory of another program thatyou can change. So, yes, you can definitely change the memory thatbelongs to another program!

or at least, certainly not by another job.

You can also modify another program's space -- after all that's part ofthe idea of single level store. One big memory space for everything.However, it's extremely unlikely that you'd do this by mistake.

I also have this idea that it's not possible for a program to modify itsown executable code, only the data portion of its memory.

That's true, unless you do some special stuff to circumvent it, the systemwon't let you point a space pointer (a pointer to data) to executablecode. It needs a different pointer type (a system pointer, or procedurepointer or whatever) to do that. If you read Leif's e-book on MIprogramming, you can learn how to work around this stuff -- but you'dnever do it by mistake.

I guess my question is, what are the system-enforced memory boundaries for a
program?  The program's data segment?  Any memory available to the job it's
running in?

Most of the time if you're doing it by mistake you'll either getun-allocated memory, or data storage from another program in the job.But, do remember, that the operating system does store some of it'sper-job stuff in your job's memory. (Though, the really critical stuff isprotected.)

I've already seen people completely screwup the database manager of theoperating system by overwriting some of it's internal memory.

99% of the time, you'll screw up your caller's memory or you'll hitunallocated space.

The point, though, is don't just assume that the memory you hit will besafe to use. If it tells you it's passing 3 parameters, don't try to usethe 4th. We want to keep our repuation as a stable platform!