From: John (john_at_ucc.gu.uwa.edu.au)
Date: 2002-10-04 17:42:07
ruud.baltissen@abp.nl writes:
>Questions:
>- It says the first bit is used for the sign. So what is the function of $66
>???
There are two floating point formats used in the C64. "packed' has the
sign in the first bit, and occupies five(?) bytes. 'unpacked' has the
sign in a separate byte. It's easier for processing, but takes more
memory. Packed is used for variables, unpacked in the FACs.
>- If I understand well the other 31 bits hold the data representing the
>number right of the decimal point. But in what format?
It's been a long, long time. There should be one sign bit, some number
of exponent bits (probably stored in excess-N format), then the rest is
the fraction.
value = sign * 2^(exp-N) * 1.fraction
There are a few constants in the ROMs, such as 1.0, 10.0, PI, and a few
others. You should be able to work out how many bits the exponent and
fraction take, and what the exponent excess is.
John
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