from
The Free On-line Dictionary of Computing (8 July 2008)
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)