# What does normalizing with hidden bit really mean?

I have a question related to representing numbers in base 2 with floating point. For example, if I have such a number $$0.000011 \cdot 2^3$$ then is its normalized form this? $$1.1\cdot 2^{-2}$$ Generally speaking about normalizing, normalizing with hidden bit, does it imply that the first number of mantissa should always be zero, or does hidden bit just mean that a "one" should be taken left to the point?

Generally speaking, normalized means "put in scientific notation." That just means, the mantissa should never start with 0, and should be less than the base. In binary that means the mantissa must be "1". Since the mantissa of a normalized binary floating point number is always 1, we don't need to store the 1. The first mantissa bit is hidden in the sense that it always exists, but we don't actually store the bit, because we know its value is 1.

So your normalized result ($1.1 \times 2^{-2}$) is correct, and it is correct because you've moved the first 1 to the left of the binary point.

• How is it that "In binary ... the mantissa must be '1' "? I didn't understand this ? Can you please explain ? Thank you. – Ivan Gandacov Mar 2 '15 at 19:36
• The mantissa must start with "1", because leading zeros don't matter. Well, there is still the problem with representing zero, as there is no way to normalize it. – TEMLIB Mar 2 '15 at 20:36
• JohnG: when you write a number in scientific notation in base 10 the first digit is something in the range $[1, 9]$. In base 8 the first digit would be something in the range $[1, 7]$. In general, the first digit must be in the range $[1, (b-1)]$, where $b$ is the base. But in binary $b = 2$, so the range is $[1, 1]$. That is: the first digit of the mantissa is always "1". (Except in the case of representing 0, Nan, Inf, and denormals, as @TEMLIB points out.) – Wandering Logic Mar 3 '15 at 0:05

The "hidden bit" refers to concrete representations, where you represent the -2 exponent as a bit field in memory storage, and the .1[00000...] as another bit field. For example in IEEE 32-bit format, it is:

00111110110000000000000000000000
+^^^^^^^^_______________________


With the sign, exponent (in bias format), and the hidden-bit mantissa.