IMP-16

The IMP-16, by National Semiconductor, was the first multi-chip 16-bit microprocessor, released in 1973. It consisted of five PMOS integrated circuits: four identical RALU chips, short for register and ALU, providing the data path, and one CROM, Control and ROM, providing control sequencing and microcode storage. The IMP-16 is a bit-slice processor; each RALU chip provides a 4-bit slice of the register and arithmetic that work in parallel to produce a 16-bit word length.^[1][2]

Each RALU chip stores its own 4 bits of the program counter, several registers, the ALU, a 16-word LIFO stack, and status flags. There were four 16-bit accumulators, two of which could be used as index registers. The instruction set architecture was similar to that of the Data General Nova.^[3] The chip set could be extended with the CROM chip (IMP-16A / 522D) that implemented 16-bit multiply and divide routines. The chipset was driven by a two-phase 715 kHz non-overlapping clock that had a +5 to -12 voltage swing. An integral part of the architecture was a 16-bit input mux that provided various condition bits from the ALUs such as zero, carry, overflow along with general purpose inputs.

The microprocessor was used in the IMP-16P microcomputer but saw little other use.^[4] The IMP-16 was later superseded by the PACE and INS8900 single-chip 16-bit microprocessors, which had a similar architecture but were not binary compatible.

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• IMP-16C board at the Selectric Typewriter Museum