The HP-12C is a financial calculator made by Hewlett-Packard (HP) and its successor HP Inc. as part of the HP Voyager series, introduced in 1981. It's HP's longest and best-selling product and is considered the de facto standard among financial professionals. There have been multiple revisions over the years. with newer revisions moving to an ARM processor running a software emulator of the original Nut processor. Critics claim that its 1980s technology is antiquated, but proponents point out that it is still the de facto and de jure in high finance. Functionality The HP-12C is HP's longest and best-selling product, in continual production since its introduction in 1981. Due to its simple operation for key financial calculations, the calculator long ago became the de facto standard among financial professionals. Its popularity has endured despite a relatively simple but iterative process such as amortizing the interest over the life of a loan, a calculation that modern spreadsheets can complete almost instantly, can take over a minute with the HP-12C. The 1977 October edition of the HP Journal contains an article by Roy Martin, the inventor of the simple method of operation used in HP financial calculators, which describes, in detail, the mathematics and functionality built by William Kahan and Roy Martin that is still used today. Models HP-12C There were at least eight hardware revisions of the HP-12C since 1981 (including one special issue). Over its lifespan, the proprietary CMOS HP Nut (originally the 1LF5, then 1LM2) processor's technology has been redesigned to integrate all the circuitry into a single chip (first the 1LQ9, then 1RR2) and to refresh the manufacturing process (as the foundry could no longer manufacture the necessary chips, having moved on to making higher-density chips). However, HP's market research found in the late 1980s that users did not trust results obtained too quickly and so the CPU speed was never improved from the original 884 kHz, but the speed could be increased by a user modification. In 2001 (from serial number CN11500001), the CPU was changed to a 3 V process (Agilent 2AF1-0001, later a Marvell 2AF1-0002) and the battery was therefore changed to a single CR2032 cell replacing the three LR44 cells previously used (F1637A). In 2008, HP modified the design to use an Atmel AT91SAM7L128 processor with ARM7TDMI core running a software emulator of the previous Nut processor, written by Cyrille de Brébisson, in order