The module's programs run directly from ROM, so they leave the calculator's memory free for the user.

Although the TI-59 is Turing-complete, supporting straight-line programming, conditions, loops, and indirect access to memory registers, and although it supports limited alphanumeric output on the printer only, writing sophisticated routines is essentially a matter of planning machine language and using a coding pad.

Unlike the SR-52, the TI-58 and TI-59 do not have the factorial function built-in, but do support it through the software module which was delivered with the calculator.

Here is the same program written for TI Compiler:[3] In comparison to its contemporary main competitor, Hewlett-Packard HP-67, the TI-59 has about twice the memory.

The TI-59 was the first programmable pocket calculator where the manufacturer provided a system for sharing memory between data registers and program storage.

The TI-59 can store programs and data on small magnetic cards when the calculator is turned off and quickly reloaded when needed.

Once read by the cardreader, the card can then be stored, as shown, in a slot between the top of the keyboard and the display, thus providing a notation indicating both the name of the program currently loaded and the purpose of each of the five label buttons A-E and their secondary functions A'-E' within the loaded program.

Additional modules - for such applications as real estate, investment, statistics, surveying and aviation - were sold separately.

Alphanumeric text (64 characters total, including space, 0-9, A-Z and 25 punctuation and mathematical symbols) can be output as well as numbers.

In the early model PC-100A, a switch inside the battery charger compartment allows use with the earlier SR-52 and SR-56 calculators as well as the TI-58/59 series.