Re: understanding floating point types

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sun, 22 Aug 2010 23:29:54 -0700 (PDT), Ada novice wrote:

> On Aug 22, 9:34�pm, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
> 
> Thanks for the explanation. If a model number is a number which can be
> accurately representable by a computer, then I don't understand the
> term machine number.

The machine number is used by the compiler to implement the model number.
The properties of the machine numbers depend on the hardware and the
compiler, for example it may have binary mantissa and hexadecimal exponent.
It may have IEEE 754 non-numeric elements like NaN. It can be an analogue
computer with a DAC on the ALU's input and ADC on the output. It can be an
oracle who gives answers. The machine number is a floating-point model of
that thing.

> On my computer I can have Long_Long_Float (18
> digits). In this case is the model number precision not equal to the
> machine number precision?

The model number of 18 decimal digits precision requires at least 61 binary
bits of mantissa. That is S'Mantissa. This is independent on any hardware.

The machine number of Intel FPU has 64 bits. The attribute
S'Machine_Mantissa is this implementation-dependent number of bits.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de