[Info-vax] Ignorance promenade: On the nature of patching VMS

[Bob Gezelter]

On Friday, December 13, 2013 6:15:25 PM UTC-5, Subcommandante XDelta wrote:
> I really have to take an angle grinder to the scale, rust and verdigris
> 
> on my VMS expertise, it is on the to-do list; my TDL is longer than
> 
> Methuselahs beard and just as hairy, alas.
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> Question:
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> Is it possible that certain adustments to deep levels of VMS could not
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> be achieved by the issuing of a patch, but would require the complete
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> recompilation of the VMS kernel image?
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> 
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> (or whatever the correct terminology is)
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> Whilst it is a question framed from genuine ignorance, it is also a
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> trick question.
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> The viability of certain VMS activism strategies are contingent upon the
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> answer.
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> 
> Thank you.

Subcommandante XDelta,

Actually, the elimination of binary "patching" is not so much a switch in architecture from VAX to Alpha, but the advent of compiled languages in system-level products. 

When writing in traditional assembler, it is not difficult to determine precise addresses to modify (frequently inserting a out-of-line transfer or call if more space is needed). When using a compiled language, such micro-level detail becomes difficult to manage for the purposes of patching. It is far simpler to replace the component image with a new version.

The kernel-level OpenVMS components are divided into "execlets", which are individually loaded at boot time. Could one replace an execlet with a modified version? Of course, the answer is yes, and replacing an execlet does not require a global re-link (otherwise, software corrections on the level of replacing execlets would be infeasible).

A philosophical note: Studying the mechanisms used to maintain the system will often illuminate ways in which the system can be modified. OpenVMS is particularly consistent in this way, there are few, if indeed, any, special gimmicks used for maintenance that are not utilities used in the normal course.