10
votes
What does adding 4.294967296e9 to a double do?
I actually found the answer while writing this question!
This number is exactly UINT_MAX + 1, stored as a double. So it seems this code converts a unsigned integer to a double. The fild instruction ...
- 241
9
votes
Accepted
XMM register instructions and their c equivalents
You likely won't get an exact reproduction because cvtdq2pd takes the lower 64 bits of the second operand but since we're limited to 32 bits because we're using eax here, there are probably better(?) ...
- 684
6
votes
IDA decompiler macro
COERCE_TYPE(x) is the same thing as *(TYPE *)&x. Hex-Rays uses COERCE_... macros when &x is illegal. For example:
COERCE_DOUBLE(__PAIR__(i1,i2))
Is the same as *(double *)&__PAIR__(i1, ...
- 111
4
votes
Accepted
Moving integer to xmm register
To move a number into an XMM register, you'll first need to move that number into a memory address since you can't move an immediate into an XMM register (meaning, you can't do something like mov XMM1,...
- 2,234
4
votes
Accepted
what is COERCE_FLOAT in ida Hex-Rays' C++ pseudocode?
I assume it's just a simple cast.
v29 = *reinterpret_cast< float* >( &v30 );
It would be easier if you'd share the generated assembly instead of the pseudo code.
- 56
3
votes
Accepted
(bad) opcodes of objdump
You are correct, and objdump is actually wrong to disassemble such instruction, only to mark it as (bad) in a later stage.
Here is how it works:
db a5 4e 9c 95 68 lock (bad) [rbp+0x68959c4e]
db ...
- 2,354
3
votes
Accepted
Encoding of 64-bit double
This appears to be a fixed-point (rather than a floating-point) format.
If you treat the 64-bit values as signed integers and divide by 4398046511104.0, you will get the decimal values you show.
e.g. ...
- 2,503
2
votes
Pointer arithmetic in C generated by ida / hexrays decompiler
These casts are done to make the pointer arithmetic that's happening more clear.
When you add an integer to a pointer, the integer internally gets multiplied by the size of the object the pointer is ...
- 12.9k
2
votes
Reverse engineer a blackbox function operating on single or double precision floats for emulation
My starting point here wouldn't be to look at generic emulation approaches as these themselves produce approximations to the function being emulated. (If an approximation is good enough then why not ...
- 2,503
2
votes
Accepted
determine proprietary 16-bit floating-point format
Maybe it is the temperature in Kelvin: 311 - 80/2 = 217K = -2.15°C
Or the offset is different than 80/2. A 16-bit floating-poitn format, especially a different one from IEEE-754 is highly unlikely. ...
- 136
2
votes
Encoding method of float
This isn't a complete answer but is a bit more than fits in a comment.
There's definitely a pattern in the powers of 2. They all have exactly 4 bits set. The high bit is always 1 and the lower 15 ...
- 2,503
1
vote
How this Float “4-bytes” is converted to Short_signed “2-bytes”?
One word, precision
By definition, a normal is a vector with a length of 1.0f which means that each component of the vector will always fall between -1.0f and +1.0f, and UV are texture coordinates ...
1
vote
Weird IDA/Decompiler floating point output
I have added some comments to the disassembly you posted, so it's easier to understand.
sub esp, 8 ; stack frame
fld [esp+8+arg_0] ; load value of arg0 to st
fist [esp+8+var_8] ...
- 3,369
1
vote
Accepted
Viewing 32-bit floats in Internal Call disassembly from .NET 2.0 DLL in x64dbg
After a little digging I got it. Right click your Dump to change data types between address and float. You have to follow the address of the address pointed to by esp-4 to get the parameters.
- 121
1
vote
Accepted
How to multiply an SSE float with a hardcoded value using MULSS?
Although you indeed cannot use mulss with an immediate value like you've pointed out, you are allowed to pass an 32bit offset as mulss's second operand:
Multiplies the low single-precision floating-...
- 11.7k
1
vote
Accepted
Understanding FCOMP instruction and extracted value from address operand
FCOMP Compares the fpu register ST0 with a constant
the constant is a QWORD (Meaning DOUBLE , FLOAT , Etc 8+ bytes wide )
ollydbg can show both the ST0 register and decipher the contents of the ...
- 16.3k
1
vote
Understanding FCOMP instruction and extracted value from address operand
Understanding the assembly line
FCOMP QWORD PTR DS:[402CB0]
It checks for a qword sized operand. That is 8 bytes while your picture only shows about 3.
This comparison is also a floating point ...
- 11.7k
1
vote
IDA decompiler macro
coerce_* functions are generally a little more coercive that a simple casting, I would assume their meaning as follows:
#define VALUE_SIZE (sizeof(int))
struct VALUE_TYPE {
char contents[...
- 320
1
vote
IDA decompiler macro
I'd read this (and probably it is a mistake) as treating binary content of the variable as a variable of another type, something like this:
coerce float is *((float*)&var), where var was of the ...
- 8,428
1
vote
Analyse a piece of code using FloatingPoint instructions and calculations
sub esp, 8
movsd xmm0, ds:MEMORY1 ; mov 128-bit memory
movsd [esp], xmm0
Fetch MEMORY1. This will be our denominator in fmod().
mov eax, [ebp+ItirationCounter_var_60]
mov [ebp+...
- 4,701
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