tags:

views:

130

answers:

1
Q: 

What Determines the Default Setting of the x87 FPU Control Word?

What determines the default setting of the x87 FPU control word -- specifically, the precision control field? Does the compiler set it based on the target processor? Is there a compiler option to change it?

Using Microsoft Visual C++ 2008 Express Edition on an Intel Core Duo processor, the default setting for the precision control field is "01b", meaning double (53 bit) precision. I'm wondering -- why is the default not "11"b, or extended (64 bit) precision?

(I know I can change it using _controlfp.)

A: 

From a theoretical standpoint: probably the last thing that used it, or barring that, whatever Intel decided would be a good default.

From a practical standpoint: just set it to your desired precision before you begin work. Explicit defaults are usually better than implied defaults.

Broam  2010-03-31 15:09:39
But I'm asking, if I'm the 'first thing that uses it', how did it get set to start with? Why would the default be 53 bit and not 64 bit?
Rick Regan  2010-04-02 00:12:19
But you're **not** the first thing that uses it. There's probably BIOS code that's executing.
Broam  2010-04-05 14:39:20
That's one thing I'm trying to find out -- where it's initialized.
Rick Regan  2010-04-05 15:49:32
Since he's using Visual C++, I bet he is on Windows, which means that MS rather than Intel is responsible for the default since it should be part of the startup configuration for new processes. The BIOS was used really long ago when a Windows process starts.
erikkallen  2010-09-28 14:02:55
Depends on your definition of "first" :)
Broam  2010-09-28 15:39:27

related questions

Why do I see a double variable initialized to some value like 21.4 as 21.399999618530273?
ParseFloat function in JavaScript
Haskell FFI / C MPFR library wrapper woes
When to use Fixed Point these days
Confusing return statement
How to alter a float by its smallest increment (or close to it)?
Significant figures in the decimal module
Test if a floating point number is an integer
SQL server 2005 numeric precision loss
Why would I use 2's complement to compare two doubles instead of comparing their differences against an epsilon value?
Double and floats in C#
Converting floating point exceptions into C++ exceptions
How to manually parse a floating point number from a string
What is the fastest way to convert float to int on x86
Determine if a string is an integer or a float in ANSI C
cout prints "-0" instead of "0"
how to use "%f" to populate a double value into a string with the right precision
Convert from scientific notation string to float in C#
How is floating point stored? When does it matter?
round() in Python doesn't seem to be rounding properly
How do you generate a random number in C#?
Comparing IEEE floats and doubles for equality
Most effective way for float and double comparison
Convert Bytes to Floating Point Numbers in Python
A little diversion into floating point (im)precision, part 1