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]

IMP-16 registers

15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00 (bit position) Main registers AC0 Accumulator AC1 Accumulator AC2 Acc/Base AC3 Acc/Base Program counter PC Program Counter Stack STK (16 entries) Status Flags Register (FR) L OV CY GF GF GF GF GF GF GF GF GF GF GF GF GF Status

IMP 16A

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. Like the Nova, the smallest unit of addressable and writable memory is the 16-bit word; byte-level writes are not supported. IMP-16 could address up to 64K words. An extended instruction set option offered byte support but was limited to the lower 32K words of memory. ^[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was used in the IMP-16P microcomputer and Jacquard Systems' J100 but saw little other use.^[4][5] The IMP-16 was later superseded by the PACE and INS8900 single-chip 16-bit microprocessors, which had a similar architecture but were not binary compatible. It was also used in the Aston Martin Lagonda, thanks to National Semiconductor's chairman Peter Sprague being a major shareholder in Aston Martin at the time.^[6]