Negative floating point to binary conversion

Question

I am using the AR Drone SDK to program the drone. In the docs, is says this:

The number -0.8 is stored in memory as a 32- bit word whose value is BF4CCCCD(16), according to the IEEE-754 format. This 32-bit word can be considered as holding the 32-bit integer value -1085485875(10). So the command to send will be AT*PCMD=xx,xx,-1085485875,xx,xx.

How are they arriving at -1085485875 for the decimal representation of the binary conversion? It doesnt make sense to me. Using this page: http://kipirvine.com/asm/workbook/floating_tut.htm and this page: http://www.madirish.net/240 and this page http://cs.furman.edu/digitaldomain/more/ch6/dec_frac_to_bin.htm , this is what I came up with:

decimal value = -0.8

binary value of decimal (-0.8) = -.11001100110011001100110

biased exponent (move above decimal over once to right) = 127 - 1 = 126 = 01111110

sign, exponent, mantissa (mantissa: drop off the 1 to left of decimal point, pad to 23) = 1 01111110 10011001100110011001100

10111111010011001100110011001100 = 3209481420 (10) BF4CCCCC (16).

The docs says its -1085485875 (10) and BF4CCCCD (16)

What am I doing wrong here???

Thanks.

Edit:

Since I am writing an AR DRONE application with node/js, if I use these node.js functions:

var buffer = new Buffer(4);
buffer.writeFloatBE(-0.8, 0);
return -~parseInt(buffer.toString('hex'), 16) - 1;

I get the correct result as per the documentation. I just don't get what I am doing wrong when I write it out long hand. I want to understand what is going on. Any help is really appreciated.

Thanks.

UPDATE:

So, I've figured out (from the above javascript code) if I apply the bitwise NOT operator to binary first, it then produces the correct number. My question is, why does this need to be applied? After doing this, you'll have to determine if its negative if the first bit is a zero not a one. Whats the point of this?

Answer 1

-0.8 = -0.110011001100110011001100 11001100110011001100110011001100110011... (the space is between bits 24 and 25 -- bit 25 is the rounding bit). This rounds to 24 bits as 0.110011001100110011001101. Here's how it looks in IEEE single-precision format:

sign  biased exponent    trailing 23-bits of mantissa
 \/        \/                      \/
 1      01111110         10011001100110011001101

That's 10111111010011001100110011001101, or BF4CCCCD in hex. If you view that as a twos-complement an integer, it's -1085485875.