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jack2



Joined: 06 Jul 2008
Posts: 33
jack2 21 Dec 2008, 16:57
I need to load an unsigned integer into fpu but can't figure out how,
suppose we have an integer = FFFFFFFFFFFFFFFF, what adjustments to the fpu is needed to get the unsigned value?
Post 21 Dec 2008, 16:57
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asmcoder



Joined: 02 Jun 2008
Posts: 784
asmcoder 21 Dec 2008, 18:06
[content deleted]


Last edited by asmcoder on 14 Aug 2009, 14:54; edited 1 time in total
Post 21 Dec 2008, 18:06
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LocoDelAssembly
Your code has a bug


Joined: 06 May 2005
Posts: 4624
Location: Argentina
LocoDelAssembly 21 Dec 2008, 18:13
Quote:

signed/unsigned is not property of value, but of handling type.
cmp: unsigned - check CF, signed - check SF/OF


On FPU it is!
Post 21 Dec 2008, 18:13
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jack2



Joined: 06 Jul 2008
Posts: 33
jack2 21 Dec 2008, 18:38
I found this snippet on the web
Code:
;esp points to unsigned int64
fild qword ptr [esp]
test byte ptr [esp+7], 80h
jz _ret
fadd dword __pow_2_64
_ret:  ret
__pow_2_64:       db 0, 0, 0x80, 0x5F

any better way?
too bad there's no fuild Laughing
Post 21 Dec 2008, 18:38
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
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revolution 21 Dec 2008, 23:28
There is no other way. What you found "on the web" is as good as it gets.
Post 21 Dec 2008, 23:28
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bitRAKE



Joined: 21 Jul 2003
Posts: 4073
Location: vpcmpistri
bitRAKE 22 Dec 2008, 00:49
Ah, the decadence - to branch senselessly...
Code:
      push $3FFF + 63
     push -1
     fld tbyte [rsp]
...or, in 32-bit mode...
Code:
  push $3FFF + 63
     push -1
     push -1
     fld tbyte [esp]
Very Happy

(need to correct the stack of course)
Post 22 Dec 2008, 00:49
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
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revolution 22 Dec 2008, 00:52
Hardly the same thing! You are pushing a fixed value.
Post 22 Dec 2008, 00:52
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bitRAKE



Joined: 21 Jul 2003
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bitRAKE 22 Dec 2008, 00:55
Whatever, PUSH works with memory and registers, too - silly.
Post 22 Dec 2008, 00:55
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revolution
When all else fails, read the source


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revolution 22 Dec 2008, 00:56
Not if the value is below 2^63.
Post 22 Dec 2008, 00:56
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bitRAKE



Joined: 21 Jul 2003
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bitRAKE 22 Dec 2008, 00:58
Your processor must work differently than mine.
(See Vol.1 8-19, Figure 8-13)
Post 22 Dec 2008, 00:58
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revolution
When all else fails, read the source


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revolution 22 Dec 2008, 01:02
You need to properly shift the mantissa and adjust the exponent if the number is does not have the high most bit set. Else you get a malformed float.
Post 22 Dec 2008, 01:02
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
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revolution 22 Dec 2008, 01:03
If you want to quote Intel manuals then look two pages further in at table 8-3. You get an unnormal for values <2^63
Post 22 Dec 2008, 01:03
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bitRAKE



Joined: 21 Jul 2003
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bitRAKE 22 Dec 2008, 01:11
So, it doesn't work on your processor?
Post 22 Dec 2008, 01:11
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revolution
When all else fails, read the source


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revolution 22 Dec 2008, 01:14
Your code above will work because it is a fixed value (2^64-1). But for the general case it will produce unsupported FPU values for numbers < 2^63.
Post 22 Dec 2008, 01:14
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bitRAKE



Joined: 21 Jul 2003
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bitRAKE 22 Dec 2008, 01:15
Yet, it works on your processor.
Post 22 Dec 2008, 01:15
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revolution
When all else fails, read the source


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revolution 22 Dec 2008, 01:17
What is it? The general case or the fixed value?
Post 22 Dec 2008, 01:17
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bitRAKE



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bitRAKE 22 Dec 2008, 01:19
The general case works.
Post 22 Dec 2008, 01:19
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bitRAKE



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bitRAKE 22 Dec 2008, 01:20
Okay, it just appears to work.
Post 22 Dec 2008, 01:20
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revolution
When all else fails, read the source


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revolution 22 Dec 2008, 01:21
Are you sure it works on all CPUs? Have you tested AMD, Intel, Via, Cyrix, Harris, etc.?
Post 22 Dec 2008, 01:21
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
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Location: In your JS exploiting you and your system
revolution 22 Dec 2008, 01:32
Very well, I tested it. CPU: Intel Pentium M.
Code:
include 'win32ax.inc'

start:
        push $3FFF + 63
        push -1
        push -1
        fld tbyte [esp]

        push $3FFF + 63
        push (1 shl 31)-1
        push -1
        fld tbyte [esp]

      fadd st0,st1    ;<---- gives an invalid FPU operation exception (I)

.end start
Doesn't work for unnormals on my CPU.
Post 22 Dec 2008, 01:32
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