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

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was used in the IMP-16P microcomputer and Jacquard Systems' J100 but saw little other use.[4][5] The IMP-16 was later superseded by the PACE and INS8900 single-chip 16-bit microprocessors, which had a similar architecture but were not binary compatible.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]

References

  1. ↑ "Use of RALU flags".
  2. ↑ "IMP-00A/520 MOS/LSI register and arithmetic logic unit (RALU)" Archived 2016-03-04 at the Wayback Machine. p. 1
  3. ↑ "IMP-16 Programming and Assembler Manual" (PDF). bitsavers. National Semiconductor. Retrieved 26 December 2021.
  4. ↑ "IMP-16C/200 IMP-16C/300 Microprocessors, IMP-16P Microcomputer Product Descriptions" (PDF). 1974.
  5. ↑ Surdan, Esther (November 21, 1977). "Jacquard Systems Starts Small But Thinks Big". Computerworld. XI (47): 66. Retrieved 3 November 2022.
  6. ↑ https://sprague.com/peter-sprague/aston-martin/
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