Some of its features are: it was one of the first radiation-hardened microprocessors, and was used widely in satellites and space probes. It has a static CMOS design with no minimum clock frequency, so that it can be run at very low speeds and low power. It has an 8-bit parallel bus with bidirectional data bus and multiplexed address bus. It has a bit-programmable output port. It has four input pins which are directly tested by branch instructions. It has a flexible programmable I/O mode. The clock is a single-phase clock, with an on-chip oscillator. The register set is 16 registers, each 16 bits. The program counter can be any one of these, providing a simple way to implement multiple program counters, pointers, or registers.