National Semiconductor SC/MP
National Semiconductor's Simple Cost-effective Micro Processor or SC/MP (pronounced scamp) is an early microprocessor which became available in early 1974. It features a 16-bit address and an 8-bit data bus. The program counter is twelve bits wide, and there are separate instructions to set the upper four bits of the program counter, which are subsequently output on the address bus, along with status signals. This provides a memory map of sixteen four-kilobyte pages. Internally it provided five registers plus the program counter, but no stack pointer.
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SC/MP unusual features
An advanced and unusual feature for the time, is the ability to release the buses, so that they can be shared by multiple processors. The linked datasheet shows an implementation with three SC/MP in a multi-processor configuration.
SC/MP increments the program counter before fetching the instruction, so that on reset it actually starts executing instructions from location 0001. This also needs to be taken into account for calculating displacements, since the offset will be added to the program counter which will be still pointing to the location of the displacement and not the next instruction.
To minimize chip count in control applications it has dedicated serial input and output pins to allow implementation of serial communications in software without the need for a UART (this feature was removed in the later SC/MP III).
Implementations
ISP-8A/500 SC/MP-1 Clocked at 1 MHz, first implementation (P Channel MOS technology)
INS 8060 ISP-8A/600 SC/MP-2 Clocked at 4 MHz (internally 2 MHz) first N Channel MOS version (single +5V supply)
INS 807x SC/MP-3 Clocked at 4 MHz (internally 2 MHz) included variations with up to 4 KB ROM (optional onboard BASIC (NIBL))
Uses
The SC/MP was also used as the basis of a single board microcontroller produced by Science of Cambridge (later Sinclair Research Ltd) called the MK14. Montgomery Elevator Co of Moline IL (later purchased by KONE, Inc) used the SC/MP as the basis for its first micro processor based elevator controller released in 1975. There are still many of these units running in buildings across the U.S.A.