On Tuesday, 4 July 2017 at 12:32:26 UTC, Patrick Schluter wrote:
In times of lore, BCD floats were very common. The Sharp Pocket Computer used a BCD float format and writing machine code on them confronts one with the format. The TI-99/4A home computer also used a BCD float format in its Basic interpreter. It had the same properties as the float format of the TI calculators, I.e. 10 visible significant digits (+ 3 hidden digits) and exponents going from -99 to +99.
If you look at the instruction set for 6502 (and probably similar 4-8bit CPU's) they literally don't deal with anything other than 8bit add/subtraction & other basic binary operators. Without multiplication or division all of that has to be simulated in software. And only needing 10 instructions to do just about everything, well...
BCD of course has a big advantage built-in: Because it's all base10, converting BCD to a string and printing it is very fast (as well as precise). And with 1 byte reserved for exponent, raising/lowering the level is also very very easy and goes a very large range.
There's also a series of algorithms for calculating some of the more complex functions using small tables or an iteration of shift & add which is implemented on calculators allowing a simpler (and reduced) instruction set or fewer transistors to make calculators work (CORDIC). It's actually pretty fascinating to read about.
BCD could still be an option though... I could probably write one; Although with doubles avaliable, you probably don't need it.
