Dear all,
can any one explain what is meant by pointer normalization
given here:-
can any one explain what is meant by pointer normalization
given here:-
-
-
Re: pointer normalization
sophia said:
It is possible for a pointer value to have more than one objectDear all,
>
can any one explain what is meant by pointer normalization
given here:-
>
http://c-faq.com/ansi/norml.html
representation. If p1 and p2 are two pointers to the same object, but with
different object representations , then p1 == p2 is required to yield 1, so
the implementation must (behave as if to) supply code to "normalise" the
pointer values used in the comparison - i.e. to convert one or the other
or both to a common form. (This was perfectly common in MS-DOS days.)
Note that the same requirement (of identifying the equality of those two
pointers) is not imposed on memcmp(&p1, &p2, sizeof p1).
--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999
-
Re: pointer normalization
On Apr 16, 7:59 am, Richard Heathfield <r...@see.sig.i nvalidwrote:Object in C means region of data storage isn't it ?sophia said:
>>Dear all,>>can any one explain what is meant by pointer normalization
given here:-
It is possible for a pointer value to have more than one object
representation. If p1 and p2 are two pointers to the same object, >but with different object representations ,
i am not getting your point,
same object with different representations ?
Comment
-
Re: pointer normalization
On 16 Apr, 13:33, sophia <sophia.ag...@g mail.comwrote:On Apr 16, 7:59 am, Richard Heathfield <r...@see.sig.i nvalidwrote:not possible on a sane architecture, but see the "Segmentati on">>can any one explain what is meant by pointer normalization
given here:->It is possible for a pointer value to have more than one object
representation. If p1 and p2 are two pointers to the same object,
but with different object representations ,
Object in C means region of data storage isn't it ?
>
i am not getting your point,
same object with different representations ?
section of http://en.wikipedia.org/wiki/Intel_8086
--
Nick Keighley
Comment
-
Re: pointer normalization
sophia said:
Consider MS-DOS's 20-bit pointers, where logical addresses are described byOn Apr 16, 7:59 am, Richard Heathfield <r...@see.sig.i nvalidwrote:>>sophia said:
>>>>Dear all,>>>>can any one explain what is meant by pointer normalization
given here:-
>It is possible for a pointer value to have more than one object
>representation . If p1 and p2 are two pointers to the same object, >but
>with different object representations ,
Object in C means region of data storage isn't it ?
>
i am not getting your point,
same object with different representations ?
two 16-bit values, called "segment" and "offset" respectively. To get the
physical address, we left-shift the segment value by four bits, and then
add the offset value.
Logical Logical Physical
segment offset address
address address
0001 1030 01040
0002 1020 01040
0003 1010 01040
0004 1000 01040
0005 00F0 01040
etc.
Five different pointer representations , all pointing to the same physical
object. All must compare equal when compared with ==. The implementation
is responsible for making this work.
--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999
Comment
-
Re: pointer normalization
In article <aY2dnetnsYENaJ jVRVnyjwA@bt.co m>,
Richard Heathfield <rjh@see.sig.in validwrote:
....A nitpick surely worthy of this group...>Consider MS-DOS's 20-bit pointers, where logical addresses are described by
>two 16-bit values, called "segment" and "offset" respectively. To get the
>physical address, we left-shift the segment value by four bits, and then
>add the offset value.
It's not "MS-DOS's 20-bit pointers". This is a feature of the 8086
processors (and its descendants, running in "real mode"). It is not a
function of the OS in any way.
Further note that the descendants still maintain this functionality; it
is just that it is rarely used. It is no longer necessary (at least up
to the 4G mark. I'm not sure what happens if you have a machine with
more than 4G RAM).
Comment
-
Re: pointer normalization
In article <fu4vs5$42d$1@n ews.xmission.co m>,
Kenny McCormack <gazelle@xmissi on.xmission.com wrote:And note that a *real* first-class nitpicker would point out that it's>In article <aY2dnetnsYENaJ jVRVnyjwA@bt.co m>,
>Richard Heathfield <rjh@see.sig.in validwrote:
>...>>>Consider MS-DOS's 20-bit pointers, where logical addresses are described by
>>two 16-bit values, called "segment" and "offset" respectively. To get the
>>physical address, we left-shift the segment value by four bits, and then
>>add the offset value.
>A nitpick surely worthy of this group...
not even *MS*-DOS, as if this functionality were someone unique to
and/or invented by Microsoft...
Comment
-
Re: pointer normalization
sophia wrote:Consider, for example, "real mode" on an x86 CPU. On that particular>
On Apr 16, 7:59 am, Richard Heathfield <r...@see.sig.i nvalidwrote:>sophia said:
Dear all,It is possible for a pointer value to have more than one objectcan any one explain what is meant by pointer normalization
given here:-
representation. If p1 and p2 are two pointers to the same object,
but with different object representations ,
Object in C means region of data storage isn't it ?
>
i am not getting your point,
same object with different representations ?
platform, addresses are represented by a 16-bit segment and a 16-bit
offset. (The physical address is segment*16+offs et.) Using this
particular architecture, the following segment/offset pairs all point
to the same physical address:
1234:0000
1230:0040
1200:0340
1000:2340
and even
1111:1230
0235:FFF0
--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
| kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer .h|
+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <mailto:ThisIsA SpamTrap@gmail. com>
Comment
-
Re: pointer normalization
Kenny McCormack wrote:Well if we're nitpicking.. :PIn article <aY2dnetnsYENaJ jVRVnyjwA@bt.co m>,
Richard Heathfield <rjh@see.sig.in validwrote:
...>>Consider MS-DOS's 20-bit pointers, where logical addresses are described by
>two 16-bit values, called "segment" and "offset" respectively. To get the
>physical address, we left-shift the segment value by four bits, and then
>add the offset value.
A nitpick surely worthy of this group...
>
It's not "MS-DOS's 20-bit pointers". This is a feature of the 8086
processors (and its descendants, running in "real mode"). It is not a
function of the OS in any way.
>
He didn't claim ms-dos 'owns or invented' the concept. He just offered
it as an example. MS-DOS did in fact have 20bit pointers because it ran
on 8086.
Comment
-
Re: pointer normalization
In article <480662d5$1@new s.acsalaska.net >, anoncoholic <no@no.netwrote :I doubt the MSDOS standards document uses the phrase "20 bit"...>Kenny McCormack wrote:>>In article <aY2dnetnsYENaJ jVRVnyjwA@bt.co m>,
>Richard Heathfield <rjh@see.sig.in validwrote:
>...>>>>Consider MS-DOS's 20-bit pointers, where logical addresses are described by
>>two 16-bit values, called "segment" and "offset" respectively. To get the
>>physical address, we left-shift the segment value by four bits, and then
>>add the offset value.
>A nitpick surely worthy of this group...
>>
>It's not "MS-DOS's 20-bit pointers". This is a feature of the 8086
>processors (and its descendants, running in "real mode"). It is not a
>function of the OS in any way.
>>
>Well if we're nitpicking.. :P
>He didn't claim ms-dos 'owns or invented' the concept. He just offered
>it as an example. MS-DOS did in fact have 20bit pointers because it ran
>on 8086.
Comment
-
Re: pointer normalization
Kenny McCormack wrote:
The system could employ PAE to use upto 64 Gb, though applications stillIn article <aY2dnetnsYENaJ jVRVnyjwA@bt.co m>,
Richard Heathfield <rjh@see.sig.in validwrote:
...>>>Consider MS-DOS's 20-bit pointers, where logical addresses are
>>described by two 16-bit values, called "segment" and "offset"
>>respectivel y. To get the physical address, we left-shift the segment
>>value by four bits, and then add the offset value.
A nitpick surely worthy of this group...
>
It's not "MS-DOS's 20-bit pointers". This is a feature of the 8086
processors (and its descendants, running in "real mode"). It is not a
function of the OS in any way.
>
Further note that the descendants still maintain this functionality;
it is just that it is rarely used. It is no longer necessary (at
least up to the 4G mark. I'm not sure what happens if you have a
machine with more than 4G RAM).
see only 4 Gb and remain flat model based.
Comment
-
Re: pointer normalization
On Apr 16, 5:47 am, Nick Keighley <nick_keighley_ nos...@hotmail. com>
wrote:Many sane architectures support virtual memory, by means of which youOn 16 Apr, 13:33, sophia <sophia.ag...@g mail.comwrote:
>>>On Apr 16, 7:59 am, Richard Heathfield <r...@see.sig.i nvalidwrote:can any one explain what is meant by pointer normalization
given here:->It is possible for a pointer value to have more than one object
representation. If p1 and p2 are two pointers to the same object,
but with different object representations ,>Object in C means region of data storage isn't it ?>i am not getting your point,
same object with different representations ?
not possible on a sane architecture, but see the "Segmentati on"
section ofhttp://en.wikipedia.or g/wiki/Intel_8086
can create aliases of the same object at different virtual addresses.
On some architectures, certain bits in an address determine whether,
for instance, the same address range is being accessed cached or
uncached.
There are also sane forms of segmentation, not simply based on
multiplying a segment by some constant and adding the offset.
Comment
-
Re: pointer normalization
In article <64de2778-0bab-4d92-aec1-de38c1c0f594@t5 4g2000hsg.googl egroups.com>,
Kaz Kylheku <kkylheku@gmail .comwrote:
It is possible for a pointer value to have more than one object
representation. If p1 and p2 are two pointers to the same object,
but with different object representations ,>>Object in C means region of data storage isn't it ?i am not getting your point,
same object with different representations ?>not possible on a sane architecture, but see the "Segmentati on"
>section ofhttp://en.wikipedia.or g/wiki/Intel_8086This is true, but not really relevant to the question. Such different>Many sane architectures support virtual memory, by means of which you
>can create aliases of the same object at different virtual addresses.
>
>On some architectures, certain bits in an address determine whether,
>for instance, the same address range is being accessed cached or
>uncached.
representations will not arise as a result of standard C operations.
An implementation may well provide a way to get an uncached pointer
to an object, but it is under no obligation to make that compare
equal to the cached version.
-- Richard
--
:wq
Comment
-
Re: pointer normalization
In article <fu63st$tsq$1@n ews.xmission.co m>,
Kenny McCormack <gazelle@xmissi on.xmission.com wrote:There's an MSDOS standards document?>I doubt the MSDOS standards document uses the phrase "20 bit"...
-- Richard
--
:wq
Comment
-
Re: pointer normalization
richard@cogsci. ed.ac.uk (Richard Tobin) wrote:
Erm, yes? I don't remember what colour it was, but IIRC burgundy.Kenny McCormack <gazelle@xmissi on.xmission.com wrote:>I doubt the MSDOS standards document uses the phrase "20 bit"...
There's an MSDOS standards document?
Techref for the hardware was blue. They came in binders.
Richard
Comment
Comment