The Saturn family of 4-bit (datapath) microprocessors was developed by Hewlett-Packard in the 1980s first for the HP-71B handheld computer, released in 1984, and later for various HP calculators (starting with the HP-18C).

The HP 49 series initially used the Saturn CPU until the NEC fab[nb 1] could no longer manufacture the processor for technical reasons in 2003.

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.

Saturn ALU instructions support variable data width, operating on one to 16 nibbles of a word.

There is also an 8-level, circular, LIFO 20-bit hardware return stack named RSTK used when a subroutine call instruction is issued.

Additionally, the Saturn CPU is equipped with a 16-bit software status register named ST and a 1-nibble hardware status register named HS, which notably, contains the SB or "sticky bit" flag indicating whether a binary 1 was right shifted off of a GPR.

When an interrupt occurs, the CPU finishes executing the current instruction, saves the program counter to the hardware return stack (RSTK) and jumps to address 0x0000Fh, where the preceding value is in nibbles.

The use of BCD instead of straight binary representation is advantageous for calculators as it avoids rounding problems that occur on the binary/decimal conversion.

The virtual CPU / emulator code names were inspired by the prototype "New-Yorke" Saturn-based 8 MHz SoC that never made it to production.