this was a breakthrough model in 1972; the first pocket
sized scientific calculator.
at the time, a scientific calculator was a desktop machine and some
doubted that a pocket-sized device had a place in the market. when it was
announced, demand totally outstripped supply and the model, almost
overnight, killed the 100 year old slide rule
industry.

*our object in developing the hp-35 was to give you
a high precision portable electronic slide rule. we thought you'd like
to have something only fictional heroes like james bond, walter mitty or
dick tracy are supposed to own.*

*the hp-35 has far more computing power than
previous pocket calculators. its ten digit accuracy exceeds the
precision to which most of the physical constants of the universe are
known. it will handle numbers as small as 10^-99 and up to 10^99, and
automatically places the decimal point for you. it is the first pocket
calculator to provide you with transcendental functions like logarithms
and sines and cosines. the operational stack and the reverse
"polish" (lukasiewicz) notation used in the hp-35 are the most
efficient way known to computer science for evaluating mathematical
expressions.*

inside there are 3 rom chips each of 256, 10 bit codes, totalling only
768 instructions. programming this to perform the whole set of scientific
functions was no easy task, plus it had to be quick enough.

the methods used are *cordic* (coordinate rotation digital
computer) and were first described by volder in 1959. these methods use an
iteration which can yield any of multiply, divide, sin, cos, arctan, log
and square root depending on different inputs. see my cordic
feature.

heres more on
the hp35 and the simulator.