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signed bits...

Dan Silva
Ranch Hand

Joined: Sep 05, 2007
Posts: 86
Hi,
I'm having a terrible time understanding how to know if a bit is signed or not. does it have to do with the furthest left digit, or what? Thanks.
Bert Bates
author
Sheriff

Joined: Oct 14, 2002
Posts: 8815
    
    5
note: this is an interesting topic in general, BUT remember it's ONLY on the 1.4 exam, not the 5.0 or 6.0 exams.


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Campbell Ritchie
Sheriff

Joined: Oct 13, 2005
Posts: 38519
    
  23
No such thing as a signED bit.

There are two meanings for the term "Sign bit."

  • Floating-point numbers. Here the name is accurate.
  • Whole numbers, byte, short, int and long, but not char. Here the use of "sign bit" is inaccurate, because the bit has a value.

  • Floating point:
    You get bits 0-22 (23 bits reading from the right), which represent the fractional part of, say, 1.01010101010101010101010. Count carefully and I ought to have got 24 bits in there; you can actually get the equivalent of 24 bits' precision into the space of 23. That is for a float; a double uses 53 bits' precision squeezed into 52 bits.

    The next 8 bits (float, 11 for a double) represent the exponent. You know you can have 1.23*10^45 and you usually write it as 1.23E45. Well, you can get the same with floating-point numbers. If you miss out 00000000 and 11111111 as those 8 bits, you get a total of 254 different values from 00000001 to 11111110. If you subtract 01111111 (127) from those numbers when you work out the exponent, so you can multiply that binary fractional number above by anything from 2^127 to 2^-126. 00000000 is reserved for smaller numbers to increase the range and 11111111 for infinity and NaNs.
    Now you have the magnitude of the number.

    We get to the leftmost bit, no 31. If it is 0 the whole number is positive and if it is 1 the number is negative.

    Integers: These don't have a true sign bit. They work in two's complement. Imagine a byte which has 8 bits. Imagine bit 0 means 1, bit 1 means 2, bit 2 means 4, bit 3 means 8, bit 4 means 16, bit 5 means 32, bit 6 means 64 and bit 7 means minus128. This isn't actually how two's complement works, but it is near enough to work out the values of your numbers.
    Some people call the leftmost bit the sign bit, but it actually has a value.
     
    With a little knowledge, a cast iron skillet is non-stick and lasts a lifetime.
     
    subject: signed bits...