The National IMP-4 (originally National GPC/P) is a family of multi-chip 4-bit bit-slice microprocessor developed by National Semiconductor and introduced in 1974. Units could be combined to implement systems with 4 to 32-bit words. The IMP-8 and IMP-16 families were made using the IMP-4 logic. The IMP-4 had 2 chips: a control unit and an ALU, some systems used the RALU with custom control circuitry. The RALU was the first bit-slice microprocessor - predating both the 3000 and the am2900.

Family Members
Part (Commercial)	Part (Military)	Description
IMP-4A/520	IMP-4A/420	Bundled Kit, CROM + RALU
IMP-4A/521	IMP-4A/421	Control and Read-only Memory (CROM)
IMP-00A/520	IMP-00A/420	Register and Arithmetic Logic Unit (RALU)

2nd sources

Rockwel was the only 2nd source for the IMP-4 series. Some USSR clones are known to exist.

Design

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