🚀 go-pugleaf

In article <14833@amdcad.UUCP> phil@amdcad.UUCP (Phil Ngai) writes:
>In a Unix system I am designing, I am considering catering to bad
>code. That is, like the VAX I propose to make location 0 contain a
>readable 0. I think that code which gets ported to a Sun machine often
>has to have this kind of thing cleaned up. ....
[followed by many articles, mostly against allowing zero in there,
with a few allowing the option.]

1) At MIPS we left 0 out of the process image, for all the reasons
that others have cited, although the next version has a ld option
to move text to 0, just in case.  However, on at least some machines you might
be able to write a SIGSEGV catcher that stepped past the offending
instruction [which after all, can't be right], or maybe even stuck
a zero in the right register. Doing this implies the need to
semi-interpret the offending instruction and bump the PC past it.
This is anything from trivial to difficult, depending on the machine,
but is at least a possibility for someone who doesn't control the
OS on their machine, or has one whose architecture doesn't have
a zero location in user space at all.

2) The discussion reminds me of a similar effect that happened somewhere
around 1970.  At Penn State, we were switching from a 360/67 to
a 370/165.  360's required data items to be placed on natural boundaries
[words on words, halfwords on halfwords, etc].  If you disobeyed you
got a specification exception.  370's don't make that restriction.
People at our computer center, as I recall, actually requested a price
from IBM to get our 370 modified to restore the boundary requirement,
because they'd found that in the (large number of) bugs they'd reported to IBM,
SOMETHING LIKE 30-40% HAD FIRST SHOWN UP AS SPECIFICATION ERRORS!
(We didn't get this feature, but people tried.)
-- 
-john mashey	DISCLAIMER: <generic disclaimer, I speak for me only, etc>
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