The tan(x) function has a period of π radians, with asymptotes at ±π/2; it effectively calculates sin(x)/cos(x), and the latter goes to zero at those points. So a function which evaluates properly over that interval can be used for any angle, by first reducing the angle to the range supported.
However, accurately calculating the tangent function near the ...
The restrictions on the range of arguments the transcendental instructions are able to handle is a direct result of hardware resource limitations in these early floating-point units. The primary source for the implementation details of the transcendental instructions in the 8087 is:
Rafi Nave, "Implementation of transcendental functions on a numerics ...
I’ve wondered about this for a long time — manufacturers don’t seem to communicate FPU transistor counts as readily as CPU counts.
The best I’ve found so far is a claim on coprocessor.info that the 80387 contained approximately 120,000 transistors (quite a bit more than the 8087’s 45,000 transistors).
Well, it could easy be a socket for a 387 type FPU. Size and number of pins would fit a 387 (or some pin compatible Cyrix FastMath) as PLCC carrier. On the other hand it's rather unusual to place it far from the main CPU, seen in the lower left.
But without more information it's hard to say. Maybe some sharp close up can reveal markings supporting this?
I remember that FPTAN didn't give the actual tg(x), but two results and you had to divide one by another in order to get tg(x) (and they were not sin(x) and cos(x) as one would hope).
Probably that's why it was "Partial".