I was just reading some history of HP calculators
HP-35, HP-45 were doing math with 10 decimals (no internal precision)
HP 13 internal decimals were a response to TI full-page ad (p. 145)
Type in you telephone numbers.
Now, take the logarithm
Now, hit the exponential key
Do you get your numbers back ?
You do on our calculator ...
Why decimal math  ? (page 137, For humans, decimal is best)
HP34C (HP41C too ?) NiCad leaking batteries (page 148)
History of HP12C (page 151)
Why HP15C so rare (page 158)
The HP 12C was successful enough that they were willing to take my advice about building the 15C, but not take my advice about how many to build. They wanted a third of my figure, and Harms did half again what they wanted, and that’s what they were doing.
 I like decimal calculator with its WYSIWYG numbers
Binary calculator is also OK (say, doing it in Lua)
What bugs me is mixing them together (binary + fake decimal)
This is how my Casio(s) do roundings ... total mess
_expression_ fx-115ms fx-260solar
1e10 + .50 - 1e10 0 0
1e10 + .79 - 1e10 0 0
1e10 + .80 - 1e10 0 0.8
1e10 + .94 - 1e10 0 0.9
1e10 + .95 - 1e10 1 0.9
1e10 + .99 - 1e10 1 0.9