The TI-59 was an early programmable calculator, manufactured by Texas Instruments from 1977. It was the successor to the TI SR-52, quadrupling the number of "program steps" of storage, and adding "ROM Program Modules" (an insertable ROM chip, capable of holding 5000 program steps). Just like the SR-52, it had a magnetic card reader for external storage. One quarter of the memory was stored on each side of one card. It was one of the first LED calculators with the capability and flexibility to take on many real-world calculation challenges, and quickly became popular with professionals in many fields.
The TI-58 (May 1977), and later TI-58C (1979), were cut down versions of the TI-59, lacking the magnetic card reader and having half the memory, but otherwise identical. Although the TI-58C used a different chip than the TI-58, the technical data remained identical. The "C" in a TI (or HP) model name indicated that the calculator had a constant memory (or continuous memory, respectively) allowing retention of programs and data when turned off.
These calculators used a parenthesized infix calculation system called "Algebraic Operating System" (AOS), where, compared to the postfix RPN system used by other scientific calculators (such as Hewlett-Packard), the operator entered calculations just as they were written on paper, using up to 9 levels of parenthesis.
The calculator could be powered from an external adapter or from its internal NiCd rechargeable battery pack (although the battery had to remain present when using the external AC adapter to avoid damage to the calculator circuitry).