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Index > Compiler Internals > "db (-1) shr 1" results into "value out of ra

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edemko



Joined: 18 Jul 2009
Posts: 549
edemko
Tomasz, provide some proof - i do not understand.
Some life situation...
Post 08 Sep 2010, 10:42
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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7734
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Tomasz Grysztar
07FFFFFFFFFFFFFFFh+1
  • with 64-bit signed calculation, this causes overflow error
  • with 64-bit "composite" calculation it is allowed, however if you use the result for further calculations, it may get interpreted as negative value
  • with 65-bit signed calculation this is allowed and is still unambiguously positive value

-8000000000000000h-1
  • with 64-bit signed calculation, this causes overflow error
  • with 64-bit "composite" calculation it is allowed, because there is no way to tell it from "8000000000000000h-1", which should be allowed - it yields the positive result
  • with 65-bit signed calculation this is allowed, but will overflow when you try to put it into 64-bit container; it is also correctly distinguished from "8000000000000000h-1", which produces a positive value that can fit into 64 bits.

Code:
dq 8000000000000000h-1 ; error with 64-bit signed, assembles correctly with 64-bit composite and 65-bit signed
dq -8000000000000000h-1 ; error with 64-bit signed and 65-bit signed, assembles (though not really correctly) with 64-bit composite    


PS I have added the "calculator" feature to the fasm's IDEs (fasmd/fasmw) under Ctrl+F6 combination for the purpose of checking out what the result of some your calculations would be, so if you have trouble with some of the fasm's quirks, you can at least quickly check out what your expression would become.
Post 08 Sep 2010, 11:37
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edemko



Joined: 18 Jul 2009
Posts: 549
edemko
There is another way when bit65 says "-" or "+" and modulo values stored in preceding 64 bits.
Such method widens number spectrum.
Positive and negative values lay same magnitude differing in their signs only.
There will be no ambiguity.
Number operations must be changed then too.

Look how easy multiplication then is:
Code:
result = abs1 * abs2                   ;this goes to 64bits

sign   = (abs1.sign + abs2.sign) and 1 ;this goes to bit65
       = abs1.sign xor abs2.sign

sure, 00b + 00b and 01b = 00b          ;+ * + = +
      00b + 01b and 01b = 01b          ;+ * - = -
      01b + 01b and 01b = 00b          ;- * - = +

eq    0 xor 0 = 0
      0 xor 1 = 1
      1 xor 1 = 0
    

etc
etc
Post 08 Sep 2010, 20:01
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17344
Location: In your JS exploiting you and your system
revolution
xor is then same as add (and sub) if you ignore the carries. GF(2^n) uses xor as the additive/subtractive operation.


Last edited by revolution on 08 Sep 2010, 20:08; edited 1 time in total
Post 08 Sep 2010, 20:08
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Tomasz Grysztar



Joined: 16 Jun 2003
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Location: Kraków, Poland
Tomasz Grysztar
edemko wrote:
There is another way when bit65 says "-" or "+" and modulo values stored in preceding 64 bits.
Such method widens number spectrum.

No, it does not - it actually has narrower number spectrum than regular 65-bit encoding, because it allows range -(2^64-1) to 2^64-1, as it is not able to encode the -2^64 number. It loses this one encoding, because it has both the negative and positive zero, and so it is able to encode one number less than the standard scheme (which is a truncated 2-adic numbers scheme).
Post 08 Sep 2010, 20:08
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edemko



Joined: 18 Jul 2009
Posts: 549
edemko
Seems you are right.
2-complementation implies easy computation through addition when subtraction needed, bothers mult. and div., ensures logical evaluation: shl/shr etc as no number conversion needed.

Eh.
I've found the solution - use qwords Smile
Post 08 Sep 2010, 20:31
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edemko



Joined: 18 Jul 2009
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edemko
revolution: commonly i avoid big theories, your the only sentence is more comprehensive those texts are
Post 08 Sep 2010, 20:33
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edemko



Joined: 18 Jul 2009
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edemko
Tomasz.
Soon or later there will be an overflow.
Bit 65 postpones it.
User allowed inputting composite nums which can be overflow condition once.
Will fasm say: "the expression can not be evaluated" ?
Why not to leave it and count the way CPU does.
Post 09 Sep 2010, 03:44
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Tomasz Grysztar



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Location: Kraków, Poland
Tomasz Grysztar
edemko wrote:
Soon or later there will be an overflow.
Bit 65 postpones it.
The 65-bit signed precision is really the minimum if we want to allow 64-bit numbers in the composite signed+unsigned range analogously to 8-bit/16-bit/32-bit/48-bit numbers. Just as TASM's 33 bits were the minimum to do it for 32-bit numbers. Since fasm doesn't allow to define single values of larger sizes, it is not needed to have more bits. If we needed to define 128-bit numbers, 129-bit signed range would be right choice, etc.

edemko wrote:
Why not to leave it and count the way CPU does.
And what do you mean by that? Allow all kinds of overflows and never do any range checking? That is not a good idea.

edemko wrote:
commonly i avoid big theories
You should be careful, sometimes you may miss something important. Have you ever had some Knuth books in your hand? I highly recommend such a reading.
Post 09 Sep 2010, 07:43
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Tomasz Grysztar



Joined: 16 Jun 2003
Posts: 7734
Location: Kraków, Poland
Tomasz Grysztar
revolution wrote:
xor is then same as add (and sub) if you ignore the carries. GF(2^n) uses xor as the additive/subtractive operation.

revolution, if you are interested in such topics, you may find this related:
http://www.emis.de/journals/UIAM/PDF/46-21-30.pdf
(the bitwise XOR and AND are mentioned there, though under a different nomenclature).
Post 09 Sep 2010, 08:38
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edemko



Joined: 18 Jul 2009
Posts: 549
edemko
Tomasz wrote:

And what do you mean by that? Allow all kinds of overflows and never do any range checking? That is not a good idea.

Will next fasm deny this "dq 18'446'744'073'709'551'615 + 18'446'744'073'709'551'615"?
In my vision that seems a non-needed service.
qword->dword/word/byte and vice-versa important.

Tomasz wrote:

Have you ever had some Knuth books in your hand?

No.
Book looks good.
Post 09 Sep 2010, 10:35
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17344
Location: In your JS exploiting you and your system
revolution
Tomasz Grysztar wrote:
bitRAKE wrote:
Why not make FASM 2.0 arbitrary precision?
That obviously would be something nice. It still may be decided, I'm not starting fasm 2.0 in any nearest future. You know, they are going to release Duke Nukem Forever next year, so then I may get a chance to beat them in terms of development time. Wink
Duke Nukem Forever Has A Release Date: May 3, 2011
Post 22 Jan 2011, 08:27
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