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The IMP-16, by National Semiconductor, was the first multi-chip 16-bit
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
, released in 1973. It consisted of five PMOS
integrated circuits An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Transistor count, Large ...
: four identical RALU chips, short for register and
ALU ALU, Alu or alu may refer to: Computing and science ;Computing *Arithmetic logic unit, a digital electronic circuit ;Biology * Alu sequence, a type of short stretch of DNA *'' Arthrobacter luteus'', a bacterium Organizations * Abraham Lincoln ...
, 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. 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 In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ...
was similar to that of the Data General Nova. 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. The IMP-16 was later superseded by the PACE and INS8900 single-chip 16-bit microprocessors, which had a similar architecture but were not binary compatible.


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External links


IMP-16C board at the Selectric Typewriter Museum
IMP-16 16-bit microprocessors Computers using bit-slice designs