HP Saturn

Saturn
	Photo of the exposed die of the HP 1LT8 Clarke SoC which contains the embedded Saturn microprocessor manufactured by HP for use in the HP 48SX
Designer	Hewlett-Packard
Bits	Hybrid 64-bit GPR, 20-bit address bus, 4-bit datapath, 4-bit minimum word size
Introduced	1984
Version	"Level 2"
Design	CISC
Type	Combination of a load–store architecture with some exceptions which fall into the register–memory architecture family
Encoding	Variable
Branching	Carry bit, "sticky" bit and other flag registers
Endianness	Little-endian
Page size	No MMU or paging available
Extensions	Only extensions for emulated ARM-based "virtual" CPUs
Open	Proprietary
Predecessor	Nut
Registers
	Nine 64-bit GPR and "scratch" registers, two 20-bit "data pointer" registers and other miscellaneous registers
General-purpose	Four 64-bit GPRs
Floating point	No FPU registers
Vector	No vector registers

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 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.

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