SC/MP
<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.
(1994-11-16)
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