The National Semiconductor IMP-16 is actually a two chip
implementation, it's heart is being the IMP-00A.
The National Semiconductor IMP-00A bit-slice microprocessor was introduced
to market in the beginning of 1973, this is just the fourth microprocessor
(1st, Intel 4004, 2nd Intel 8008, 3rd
Rockwell PPS-4). The IMP-00A is the first bit-slice microprocessor,
predating the Intel 3002 and AMD 2901.
The IMP-00A was developed for the IMP series of minicomputers. The
IMP-16 Minicomputer was implemented using 4 IMP-00A’s and a IMP-16A CROM.
CPU devices are comprised on two major components the control unit (CU)
and the arithmetic and logic unit (ALU).
The CU controls the actions of the CPU and the ALU does the math
and logic functions. Bit-slice microprocessors are CPU’s and have been
split apart into CU and ALU functions. The IMP-00A is the ALU function.
The CU can be implemented with custom circuitry, but a sequencing devices
such as the IMP-16A is generally used to implement the CU.
Semiconductor named the IMP-00A a RALU (Register and Arithmetic Logic
Unit). The 4-bit IMP-00A was implemented using silicon gate PMOS
technology and was clocked at about 700KHz. The IMP-00A had 7 4-bit
registers, a status register, and 16 word stack. The IMP-00A could perform
ADD, AND, OR, and XOR operations at a rate of about 1 million per second.
To support the IMP-00A bit-slice processor, National Semiconductor
created a CROM chip. The CROM function was to provide storage for the
microprogram and control logic for the IMP-00A (this chip offered in
separate auction). Together the CROM and IMP-00A RALU comprise a functional CPU.
The IMP-16A CROM was used to implement the IMP minicomputer.
The CROM can hold 100 microinstructions. National Semiconductor created
6 CROM’s with pre-defined instruction sets for use with IMP-00A. There
were IMP-4A/521 (4-bit standard instruction set), IMP-8A/520 (8-bit
standard instruction set), IMP-16A/521 (16-bit standard instruction set),
and IMP-16A/522 (16-bit extended instruction set). These pre-defined
instructions set were influenced by the Data General Nova minicomputer.