On 2009-11-10, Rhodri James <rho...@wildebst.demon.co.uk> wrote:
> On Sun, 08 Nov 2009 19:45:31 -0000, Terry Reedy <tjre...@udel.edu> wrote:
>
>> I believe the use of tagged pointers has been considered and so far
>> rejected by the CPython developers. And no one else that I know of has
>> developed a fork for that. It would seem more feasible with 64 bit
>> pointers where there seem to be spare bits. But CPython will have to
>> support 32 bit machines for several years.
>
> I've seen that mistake made twice (IBM 370 architecture (probably 360 too,
> I'm too young to have used it) and ARM2/ARM3). I'd rather not see it a
> third time, thank you.
MacOS applications made the same mistake on the 68K. They
reserved the high-end bits in a 32-bit pointer and used them to
contain meta-information. After all, those bits were "extra" --
nobody could ever hope to actually address more than 4MB of
memory, right? Heck, those address lines weren't even brought
out of the CPU package.
Guess what happened?
It wasn't the decades-long global debacle that was the MS-DOS
memory model, but it did cause problems when CPUs came out that
implemented those address lines and RAM became cheap enough
that people needed to use them.
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