internal number representation

[David Hayden]

On the HP 48, the sign nibble is encoded with 0 meaning positive and 9 meaning negative.

The 3 nibble of the exponent are encoded in BCD, but if the value is negative, then it's encoded as 1000-ABS(exponent). Put another way, if value in the exponent nibbles is more than 499, then take the value, subtract it from 1000 , negate it and that's the value. For example, if the exponent nibbles contain 600, then actual exponent is -(1000-600) = -400.

See pages 104-105 here: http://www.hpcalc.org/details/1693