The Saturn family of 4-bit (datapath) microprocessors was developed by Hewlett-Packard in the 1980s first for the HP-71B handheld computer and then later for various HP calculators (starting with the HP-18C). It succeeded the Nut family of processors used in earlier calculators. The original Saturn chip was first used in the HP-71B hand-held BASIC-programmable computer, introduced in 1984. Later models of the family powered the popular HP 48 series of calculators. The HP48SX and HP48S were the last models to use genuine Saturn processors manufactured by HP. Later calculator models used Saturn processors manufactured by NEC. The HP 49 series initially used the Saturn CPU as well, until the NEC fab[nb 1] could no longer manufacture the processor for technical reasons in 2003. Therefore, starting with the HP 49g+ model in 2003, the calculators switched to a Samsung S3C2410 processor with an ARM920T core (part of the ARMv4T architecture) which ran an emulator of the Saturn hardware in software. In 2000, the HP 39G and HP 40G were the last calculators introduced based on the actual NEC fabricated Saturn hardware. The last calculators based on the Saturn emulator were the HP 39gs, HP 40gs and HP 50g in 2006, as well as the 2007 revision of the hp 48gII. The HP 50g, the last calculator utilizing this emulator, was discontinued in 2015 when Samsung stopped producing the ARM processor on which it was based.
The Saturn hardware is a nibble serial design as opposed to its Nut predecessor, which was bit-serial. Internally, the Saturn CPU has four 4-bit data buses that allow for nearly 1-cycle per nibble performance with one or two buses acting as a source and one or two acting as a destination. The smallest addressable word is a 4-bit nibble which can hold one binary-coded decimal (BCD) digit. Any unit of data in the registers larger than a nibble, in which the end of said data unit falls on a nibble boundary and the start of said data unit starts at nibble zero (and also in some cases where said data unit's starting position falls on a nibble boundary with certain register fields eg. "M" or "X"), and which can be up to 64-bits, can be operated on as a whole, but the Saturn CPU performs the operation serially internally on a nibble-by-nibble basis.