Fairchild PPS-25: 4-bit CPU for 25-digit precision
10 hours ago
- #VintageComputing
- #CPUs
- #Fairchild
- The Fairchild PPS-25 was a 4-bit CPU designed for 25-digit precision, introduced in 1971.
- It operated at 400KHz with a 62.5μs word cycle time and 2.5μs bitrate, using PMOS technology.
- The system was composed of multiple chips, including the 3805 ALU and 3806 Function/Timing chips.
- The 3805 featured a 25-bit accumulator with a 6-bit mask for flexible operations.
- The 3806 included timing, control logic, status registers, and branch logic.
- Support chips included the 3808 and 3809 registers, and up to 25 256×12-bit serial ROMs for instructions.
- The PPS-25 was used in applications requiring high precision, such as calculators, navigation systems, and engineering tools.
- Notable uses included the Cybernetic Mathiputer and King Radio Corp's navigation computer for airplanes.
- Fairchild discontinued the PPS-25 by 1974, shifting focus to the F8 processor.
- The design influenced other CPUs, such as HP's Saturn and Nut processors.