[Info-vax] Kernel Hacks and Technical Debt (was: Re: Kittson question)

[Stephen Hoffman]

On 2015-01-01 10:10:22 +0000, glen herrmannsfeldt said:

> Many early Machintosh programs (or Mac OS) used high bits in address
> words, complicating the transition to virtual memory with more
> than 16MB addressing.

Objective C still has some of these shenanigans:
http://www.sealiesoftware.com/blog/index.html
https://mikeash.com/pyblog/friday-qa-2012-11-16-lets-build-objc_msgsend.html
http://www.friday.com/bbum/2009/12/18/objc_msgsend-tour-part-3-the-fast-path/

OpenVMS has some similar hackery lurking, too:
Negative values stored in certain system data structure fields (e.g. 
IRP$L_PID, or PCB$L_EFWM) can be system-space addresses, while positive 
values are something else.

Platform migrations are when these joyous little packages of technical 
debt usually burst forth onto the schedule in all their overtly-homely 
glory, too,


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