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Strange value of the double type variable: -nan(0x8000000000000)

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Top Forums Programming Strange value of the double type variable: -nan(0x8000000000000)
# 8  
Old 05-31-2014
Code:
tmp1 = (simtime - seg->time) / currdisk->rotatetime;
currdisk->currangle += tmp1;

The above code can also cause NaN in the later calling.
So I step into to the assembly.
tmp1 = (simtime - seg->time) / currdisk->rotatetime;
Code:
0x0808fdf4  <disk_buffer_sector_done+559>:  fldl   0x80b2208
0x0808fdfa  <disk_buffer_sector_done+565>:  mov    -0x38(%ebp),%eax
0x0808fdfd  <disk_buffer_sector_done+568>:  fldl   (%eax)

...Here, the content of %eax is 0x80bdfeb, which is the address of seg->time.
After the above "fldl (%eax) " executed, the content of register st0 is 0x8000000000000000, which represents NaN.
So the NaN is propagated to the following instructions. This is the key issue.
Code:
0x0808fdff  <disk_buffer_sector_done+570>:  fsubrp %st,%st(1)
0x0808fe01  <disk_buffer_sector_done+572>:  mov    0x8(%ebp),%eax
0x0808fe04  <disk_buffer_sector_done+575>:  fldl   0xc4(%eax)
0x0808fe0a  <disk_buffer_sector_done+581>:  fdivrp %st,%st(1)
0x0808fe0c  <disk_buffer_sector_done+583>:  fstpl  -0x28(%ebp)

# 9  
Old 06-02-2014
Quote:
Originally Posted by 915086731
After the above "fldl (%eax) " executed, the content of register st0 is 0x8000000000000000, which represents NaN.
I don't really know this assembly language, but I looked up fldl and my source said it is a floating double load. The only reasons that I can think of for that to result in a NAN are:
1. There really was a NAN there to load
2. The address in question is not correctly aligned in core to be treated as a double.
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