casio fx20
this model is the missing link between the older machines
like the fx10, which had an incomplete function
set, to the more mainstream fx29 and fx39 style
models.
it really is quite a nice little calculator, much smaller than you
would expect. it fits neatly in one hand. you can almost use it with the
same thumb. this palmsized style reminds me of the hp 20
series, like the hp25. when pocket sized
really meant it.
quaintly, it has the small zero digit of early casio machines like fx10
and the memory 8r. like the older machines, this
one operates only in degrees and also hms>h is provided but not the
other way.
interestingly, natural log shares the same button as x^y. clearly
x^y
is implemented by continuation from ln by
a multiply and e^x. not a bad way to save a button assuming you wont be
testing y for integer.
improving on the fx10 are trig inverses; arcsin, arccos and
arctan, a
memory (mc, mr, m+), change sign and, at last, proper exponential number
representation. ie. x*10^y. displayed as usual with a 2 digit exponent.
curiously, the exponent can only range to +/39 inc. missing features are;
1/x, pi, and the antique a^n operation.
some thought has gone into the keyboard layout, the x^y and ln sharing
feature and the two = buttons, one is simple = and the other =M+. when
you're not using the memory, you can hit either button making quick
calculations quicker. 

calculation speed is an improvement over previous models, rather than flicker
and twitch for some time, only the right most digit flickers during scientific
calculation and not for too long.
the machine only works to eight digits with no guard. trigs display 7 digits,
e^x, six and ln, 8. basic calculations and square root are also 8. you cant seem
to enter more than 6 digits with an exponent because after the sixth digit is
entered, exp is ignored.
trigs stop at the magic 1440 degrees, except for cosine. for some reason, cos
accepts input up to 1530. a bug? maybe but the results returned are correct. 