Re: is Vax addressing sane today

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Sujet : Re: is Vax addressing sane today
De : mitchalsup (at) *nospam* aol.com (MitchAlsup1)
Groupes : comp.arch
Date : 26. Sep 2024, 19:11:56
Autres entêtes
Organisation : Rocksolid Light
Message-ID : <92da4e9b66e86a7cbdac8720a51ac7e0@www.novabbs.org>
References : 1 2 3 4 5 6 7 8
User-Agent : Rocksolid Light
On Thu, 26 Sep 2024 17:13:02 +0000, EricP wrote:

MitchAlsup1 wrote:
On Wed, 25 Sep 2024 16:54:18 +0000, EricP wrote:

IIRC it was enabled by default in all DEC languages, Fortran77, Pascal,
Ada, Cobol, and disabled by default for C. But it could be toggled with
a runtime library call.
>
Similar--but library call does not have to "gain privilege" to flip the
bit's state.
>
The library routine didn't need a privilege change.
Most architectures do not allow the unpriviledged to access or modify
PSW (except the IP via branch instructions and CC via arithmetic).
My 66000 ISA allows unprivileged to access and modify PSW state
that only effects how the application acts. So, unprivileged can
modify:
a) IEEE flags
b) exception enablement
c) rounding mode
but not
d) Translation Tables
e) ASID
f) Dispatcher
g) call stack pointer
..
Even though they are stored in the same cache line.
<snip>
>
The problem is mostly due to the fact that expressions are
*mixtures of signed and unsigned, checked and modulo arithmetic*.
If overflow checks are enabled by status flag then the program has to
keep switching between modes for individual arithmetic operations.
This leads to a slew of enable and disable instructions which could
be serializing.
And often are.

This is because array index value expressions are calculated using
signed,
checked arithmetic, then the result is range checked against the bounds.
However addresses are calculated using modulo arithmetic.
Since most OS define address 0 to be the start of user space,
and locate the OS at FF...FF, addresses are unsigned modulo numbers.
>
If checks are not disabled for the address calculation
and the address not calculated using modulo arithmetic,
it is easy to trigger false overflow exceptions with arrays
that do not have base-0 or base-1 array bounds as many compilers
use bias-base array buffer pointers.
>
This is why one wants separate instructions for ADD and ADDV - there is
no overhead to switching between modulo and checked linear arithmetic.
>
Second, if there is a control register, it becomes part of the ABI.
It can either be
- undefined on calls, in which case each routine must save the current
   flags state on *each* entry and set a value, and restore the original
   state on return,
- or defined to have a particular enable/disable value on call and
   callee's are required toggle it if needed but restore it to default
   for all calls and returns.
>
Third, there is no reason to have overflow as a dynamic enable/disable
because the kind of arithmetic, modulo or linear, is fixed by what the
programmer writes and does not change dynamically.
>
Dynamic overflow enable results in a continuous overhead managing it
which does not occur with explicit fault testing instructions.
Where it consumes valuable OpCode space which is sometimes not available
{{3-operand 1-result instructions are notoriously "tight" in encoding:
± on 2 operands, int/float/double, FMAC<->INSert, attached
constant,...}}

Date Sujet#  Auteur
5 Sep 24 * is Vax adressing sane today336Brett
5 Sep 24 +* Re: is Vax adressing sane today324John Dallman
6 Sep 24 i+- Re: is Vax adressing sane today1Lawrence D'Oliveiro
6 Sep 24 i`* Re: is Vax adressing sane today322Anton Ertl
6 Sep 24 i +- Re: is Vax adressing sane today1Lawrence D'Oliveiro
6 Sep 24 i +* Re: is Vax adressing sane today5MitchAlsup1
7 Sep 24 i i`* Re: is Vax adressing sane today4Anton Ertl
7 Sep 24 i i `* Re: is Vax adressing sane today3Anton Ertl
7 Sep 24 i i  `* Re: is Vax addressing sane today2John Dallman
7 Sep 24 i i   `- Re: is Vax addressing sane today1Anton Ertl
7 Sep 24 i +* Re: is Vax adressing sane today313John Levine
8 Sep 24 i i`* Re: is Vax adressing sane today312Anton Ertl
8 Sep 24 i i +* Re: is Vax adressing sane today304MitchAlsup1
8 Sep 24 i i i`* Re: is Vax addressing sane today303Lawrence D'Oliveiro
9 Sep 24 i i i +* Re: is Vax addressing sane today231MitchAlsup1
9 Sep 24 i i i i`* Re: is Vax addressing sane today230Brett
9 Sep 24 i i i i +* Re: is Vax addressing sane today7MitchAlsup1
10 Sep 24 i i i i i`* Re: is Vax addressing sane today6Niklas Holsti
11 Sep 24 i i i i i +- Re: is Vax addressing sane today1Lawrence D'Oliveiro
25 Sep 24 i i i i i `* Re: is Vax addressing sane today4Stephen Fuld
25 Sep 24 i i i i i  `* Re: is Vax addressing sane today3Michael S
25 Sep 24 i i i i i   `* Re: is Vax addressing sane today2MitchAlsup1
25 Sep 24 i i i i i    `- Re: is Vax addressing sane today1Niklas Holsti
10 Sep 24 i i i i `* Re: is Vax addressing sane today222Anton Ertl
10 Sep 24 i i i i  +* Re: is Vax addressing sane today4Michael S
10 Sep 24 i i i i  i`* Re: is Vax addressing sane today3Anton Ertl
10 Sep 24 i i i i  i +- Re: is Vax addressing sane today1Niklas Holsti
11 Sep 24 i i i i  i `- Re: is Vax addressing sane today1Michael S
11 Sep 24 i i i i  +* Re: is Vax addressing sane today7Lawrence D'Oliveiro
11 Sep 24 i i i i  i`* Re: is Vax addressing sane today6Michael S
11 Sep 24 i i i i  i `* Re: is Vax addressing sane today5David Brown
11 Sep 24 i i i i  i  +* Re: is Vax addressing sane today2Thomas Koenig
11 Sep 24 i i i i  i  i`- Re: is Vax addressing sane today1David Brown
11 Sep 24 i i i i  i  `* Re: is Vax addressing sane today2David Schultz
13 Sep 24 i i i i  i   `- Re: is Vax addressing sane today1David Brown
11 Sep 24 i i i i  +* Re: is Vax addressing sane today5John Levine
11 Sep 24 i i i i  i`* Re: is Vax addressing sane today4Thomas Koenig
11 Sep 24 i i i i  i +* Re: is Vax addressing sane today2Anton Ertl
11 Sep 24 i i i i  i i`- Re: is Vax addressing sane today1jseigh
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21 Sep 24 i i i i   +* Re: is Vax addressing sane today4MitchAlsup1
21 Sep 24 i i i i   i`* Re: is Vax addressing sane today3Lawrence D'Oliveiro
21 Sep 24 i i i i   i `* Re: is Vax addressing sane today2MitchAlsup1
21 Sep 24 i i i i   i  `- Re: is Vax addressing sane today1Lawrence D'Oliveiro
21 Sep 24 i i i i   +* Re: is Vax addressing sane today4Lawrence D'Oliveiro
21 Sep 24 i i i i   i`* Re: is Vax addressing sane today3MitchAlsup1
21 Sep 24 i i i i   i +- Re: is Vax addressing sane today1Niklas Holsti
21 Sep 24 i i i i   i `- Re: is Vax addressing sane today1Lawrence D'Oliveiro
21 Sep 24 i i i i   +* Re: is Vax addressing sane today30MitchAlsup1
22 Sep 24 i i i i   i+* Re: except what, is Vax addressing sane today18John Levine
22 Sep 24 i i i i   ii+- Re: except what, is Vax addressing sane today1Michael S
22 Sep 24 i i i i   ii+* Re: except what, is Vax addressing sane today8Lawrence D'Oliveiro
22 Sep 24 i i i i   iii`* Re: except what, is Vax addressing sane today7Chris M. Thomasson
22 Sep 24 i i i i   iii `* Re: except what, is Vax addressing sane today6MitchAlsup1
22 Sep 24 i i i i   iii  +* Re: except what, is Vax addressing sane today2Lawrence D'Oliveiro
22 Sep 24 i i i i   iii  i`- Re: except what, is Vax addressing sane today1MitchAlsup1
22 Sep 24 i i i i   iii  +- Re: except what, is Vax addressing sane today1Chris M. Thomasson
22 Sep 24 i i i i   iii  `* Re: except what, is Vax addressing sane today2John Dallman
22 Sep 24 i i i i   iii   `- Re: except what, is Vax addressing sane today1MitchAlsup1
22 Sep 24 i i i i   ii+* Re: except what, is Vax addressing sane today5Terje Mathisen
24 Sep 24 i i i i   iii`* Re: except what, is Vax addressing sane today4Lawrence D'Oliveiro
24 Sep 24 i i i i   iii `* Re: except what, is Vax addressing sane today3Chris M. Thomasson
24 Sep 24 i i i i   iii  `* Re: except what, is Vax addressing sane today2MitchAlsup1
16 Oct 24 i i i i   iii   `- Re: except what, is Vax addressing sane today1Chris M. Thomasson
22 Sep 24 i i i i   ii`* Re: except what, is Vax addressing sane today3Lars Poulsen
24 Sep 24 i i i i   ii `* Re: except what, is Vax addressing sane today2Lawrence D'Oliveiro
24 Sep 24 i i i i   ii  `- Re: except what, is Vax addressing sane today1Michael S
22 Sep 24 i i i i   i+* Re: is Vax addressing sane today7Lawrence D'Oliveiro
22 Sep 24 i i i i   ii`* Re: is Vax addressing sane today6MitchAlsup1
22 Sep 24 i i i i   ii +* Re: is Vax addressing sane today3Lawrence D'Oliveiro
22 Sep 24 i i i i   ii i`* Re: is Vax addressing sane today2MitchAlsup1
22 Sep 24 i i i i   ii i `- Re: is Vax addressing sane today1Lawrence D'Oliveiro
22 Sep 24 i i i i   ii `* Re: is Vax addressing sane today2Anton Ertl
24 Sep 24 i i i i   ii  `- Re: is Vax addressing sane today1MitchAlsup1
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24 Sep 24 i i i i   ii`- Re: is Vax addressing sane today1MitchAlsup1
25 Sep 24 i i i i   i+- Re: is Vax addressing sane today1MitchAlsup1
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24 Sep 24 i i i i   i +- Re: is Vax addressing sane today1Bill Findlay
24 Sep 24 i i i i   i `- Re: is Vax addressing sane today1Thomas Koenig
23 Sep 24 i i i i   +- Re: is Vax addressing sane today1Stefan Monnier
23 Sep 24 i i i i   `* Re: is Vax addressing sane today161Kent Dickey
24 Sep 24 i i i i    +* Re: is Vax addressing sane today11MitchAlsup1
24 Sep 24 i i i i    i+- Re: is Vax addressing sane today1Lawrence D'Oliveiro
24 Sep 24 i i i i    i`* Re: is Vax addressing sane today9Terje Mathisen
24 Sep 24 i i i i    i +* Re: is Vax addressing sane today4Michael S
24 Sep 24 i i i i    i i`* Re: is Vax addressing sane today3Terje Mathisen
24 Sep 24 i i i i    i i `* Re: is Vax addressing sane today2Michael S
24 Sep 24 i i i i    i i  `- Re: is Vax addressing sane today1MitchAlsup1
24 Sep 24 i i i i    i +* Re: is Vax addressing sane today3Stephen Fuld
24 Sep 24 i i i i    i i`* Re: is Vax addressing sane today2MitchAlsup1
25 Sep 24 i i i i    i i `- Re: is Vax addressing sane today1Stephen Fuld
12 Oct 24 i i i i    i `- Re: is Vax addressing sane today1Anton Ertl
24 Sep 24 i i i i    +* Re: is Vax addressing sane today3Terje Mathisen
29 Sep 24 i i i i    i+- Re: is Vax addressing sane today1Michael S
7 Oct 24 i i i i    i`- Re: is Vax addressing sane today1Kent Dickey
25 Sep 24 i i i i    +* Re: is Vax addressing sane today133MitchAlsup1
26 Sep 24 i i i i    i+- Re: is Vax addressing sane today1MitchAlsup1
28 Sep 24 i i i i    i`* Re: is Vax addressing sane today131Lawrence D'Oliveiro
28 Sep 24 i i i i    +* Re: is Vax addressing sane today3George Neuner
7 Oct 24 i i i i    +* Re: is Vax addressing sane today8Kent Dickey
12 Oct 24 i i i i    `* Re: is Vax addressing sane today2Anton Ertl
9 Sep 24 i i i `* Re: is Vax addressing sane today71Anton Ertl
8 Sep 24 i i `* Re: is Vax adressing sane today7Brett
8 Sep 24 i `* Re: is Vax adressing sane today2MitchAlsup1
6 Sep 24 +* Re: is Vax adressing sane today2MitchAlsup1
6 Sep 24 +- Re: is Vax adressing sane today1Lawrence D'Oliveiro
6 Sep 24 `* Re: is Vax adressing sane today8Anton Ertl

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