Re: Language design/VM design

Joachim Durchholz <joachim.durchholz@halstenbach.com.or.de> writes
...
>I assume you mean a type cast with "change" in the above. In that
>case, if you want pointer arithmetic and external/internal pointers,
>disallow casts between internal and external pointers.
>
>The AS/400 has exactly the same problem: they have C on a machine with
>external pointers to operating system objects. They have a tougher
>problem in that they have full ANSI C, so they cannot disallow type
>casts.
>I haven't looked too deeply into it, but I recall that the system checks
>that an external pointer is valid before dereferencing it. They may even
>have added a cryptrographic checksum to the pointer to prevent tampering
>(the AS/400 is paranoid enough about security to have such a thing, and
>external pointers are large enough, 60 bytes or more IIRC - I guess they
>include references to a user profile to allow access permission
>checking).


All AS/400 pointers are actually 8 bytes (128 bits); basically 64 bits
points to a machine space or object and the other 64 bits as the space
offset. This is true regardless of whether a pointer is to an "internal"
or "external" address. I believe there *could* also be some limited
checking to ensure that the 8 byte field is a pointer.


The AS/400 will check that a pointer is valid and that it references an
object to which you have access. Actually I have always wondered how
void casts are done, since the machine can't check what kind of object
is being accessed via the void pointer.


--
Jeremy Boden

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