[Info-vax] The Road to V9.0

[Simon Clubley]

On 2019-10-13, Dave Froble <davef at tsoft-inc.com> wrote:
>
> Question for you John ...
>
> Are these problems you're correcting actual poor / bad coding?  I'm 
> getting the impression that for most it's just in the conversion to LLVM.
>

This is not poor coding as I understand it.

This appears to be about different abstraction levels in VMS interfaces
versus (for example) Linux interfaces.

>From what I can see, things which are an hidden implementation detail
in the ABI on Linux (and hence are invisible to the Linux code being
compiled) are visible at source code level in VMS and hence need to be
implemented in a way which does not break existing code.

An example I have used previously might make this clear, although this
is not directly related to the current example John posted:

In traditional VMS control blocks, the address of a memory location is
a raw 32-bit longword. It is _NOT_ an abstracted language level pointer
to a memory location.

You can't move the traditional VMS control blocks to a fully 64-bit
address space without implementing a new VMS control block with addresses
that are 64-bit wide. This is something which is very visible at source
code level.

OTOH, on Linux, when you move between 32-bit and 64-bit the work involved
is a lot easier and, within the application, quite a lot of this can be
handled transparently by the compiler which knows that the width of a
pointer is now 64-bits instead of 32-bit and generates the correct code
for you automatically.

This is not fully automatic (for example, if you are using some external
interface which is only 32-bit aware), but within your code it's a _lot_
easier to handle these kinds of problems at source code level on Linux
than it has traditionally been on VMS.

This is the basic difference between an API designed to support assembly
language applications (VMS) versus APIs designed to support languages
which have additional levels of abstractions (C on Unix).

This memory pointer issue is one example of how the VMS data structures
and APIs simply tend to have a lower level of abstraction at source code
level.

Simon.

-- 
Simon Clubley, clubley at remove_me.eisner.decus.org-Earth.UFP
Walking destinations on a map are further away than they appear.