<processor> (Nicknamed "Scamp") A typical 8-bit microprocessor from National Semiconductor released in April 1976. It was intended for control applications (a simple BASIC in a 2.5K ROM was added to one version). It featured 16 bit addressing, with 12 address lines and 4 lines borrowed from the data bus (it was common to borrow lines from the data bus for addressing). Internally, it included three index registers (P1 to P3) and two 8-bit general-purpose registers. It had a PC, but no stack pointer or subroutine instructions (though they could be emulated with index registers). During interrupts, the PC was saved in P3. It was meant for embedded control, and these features were omitted for cost reasons. It was also bit serial internally to keep it cheap.
The unique feature was the ability to completely share a system bus with other processors. Most processors of the time assumed they were the only ones accessing memory or I/O devices. Multiple SC/MPs could be hooked up to the bus, as well as other intelligent devices, such as DMA controllers. A control line (ENOUT (Enable Out) to ENIN) could be chained along the processors to allow cooperative processing. This was very advanced for the time, compared to other CPUs.
In addition to I/O ports like the Intel 8080, the SC/MP also had instructions and one pin for serial input and one for output.