The Saturn family of microprocessors was developed by Hewlett-Packard in the 1980s for programmable scientific calculators/microcomputers. The original Saturn chipset was first used in the HP-71B hand-held BASIC computer, introduced in 1984. Later models of the family powered the popular HP-48 series of calculators, among others.
Architecture
The Saturn architecture is nibble-based; that is, the core unit of data is 4 bits, which can hold one binary-coded decimal (BCD) digit. The Saturn has four general-purpose and five scratch registers that are 64 bits wide. Data in the general-purpose registers can be accessed on nibble boundaries and used for calculations, whereas the scratch registers allow only load and store operations. The 64 bits (16 nibbles) can hold BCD-formatted coded floating point numbers composed of a sign nibble, 12 mantissa digits and a 3-digit exponent stored in 2's complement format (±499). 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. For optimum memory usage efficiency, the Saturn's addresses are also nibble-based. The three pointer registers (including the program counter) and address data paths are 20 bits wide, therefore the Saturn architecture can address 1 M nibbles = 512 K bytes. Beyond that size (e.g. in the 48GX), bank switching is used.
Chipsets and applications
The original Saturn CPU gave its name to the entire architecture. Later chips had their own code names:
| CPU codename | Used in calculator models | Properties |
| Saturn | HP-71B, HP-18C, HP-28C | 640 kHz |
| Bert | HP-10B, HP-20S, HP-21S | 640 kHz, 10 KB ROM, 256 Bytes RAM, LCD driver |
| Sacajawea | HP-14B, HP-22S, HP-32S, HP-32SII | 640 kHz, 16 KB ROM, 512 Bytes RAM, LCD driver |
| Lewis | HP-17B, HP-19B, HP-27S, HP-28S | 1 MHz, 64 KB ROM, LCD driver, memory controller, IR control |
| Clarke | HP-48SX, HP-48S | 2 MHz, LCD controller, memory controller, UART and IR control |
| Yorke | HP-48GX, HP-48G, HP-38G, HP-39G, HP-49G | 3.68 MHz, LCD controller, memory controller, UART and IR control |
(The CPU codenames are inspired by members of the Lewis and Clark Expedition, the first United States overland expedition to the Pacific coast and back, of 1804–1806.)
