National Semiconductor's SC/MP family of 8-bit microprocessors was
introduced at the beginning of 1976. SC/MP abbreviation stands for
Simple Cost-effective MicroProcessor, and is pronounced as SCAMP.
The SC/MP processor can address up to 64 KB memory, although
certain types of memory accesses cannot cross 4 KB memory block
boundaries. For example, the CPU cannot cross 4 KB memory block
boundary when executing continuous sequence of instructions.
Instruction set of the INS8060 is quite simple and consists of 46
one- and two-byte instructions. Two-byte instructions include an
opcode and one byte immediate or displacement value. It's not
possible to specify 12- or 16-bit memory address as part of an
instruction, therefore to access any arbitrary memory location the
CPU needs to load memory address into one of pointer registers, and
then use this register to access the memory. Despite of its
simplicity, the instructions set includes fairly advanced
Auto-indexing addressing method, which is used to access any memory
location referenced by one of pointer registers, and adjust the
pointer register afterwards. There are also a few interesting
instructions, such as DLY (delays program execution by certain number
of microcycles), XPPC (exchange pointer register with program
counter), or ILD/DLD (load data from memory and increment/decrement
memory after that). Because the processor doesn't have stack memory
and doesn't have built-in support for subroutines, the above
mentioned XPPC instruction is especially useful for emulation of
subroutine calls. The INS8060 processor supports interrupts.
Like many microprocessors of its time, the SC/MP requires multiple
CPU cycles (microcycles) to execute one instruction. About half of
SC/MP instructions take from 5 to 9 microcycles to execute, and
remaining half requires 10 or more microcycles. SC/MP CPU with 1 MHz
external crystal has 2 microsecond microcycle time, hence the fastest
instructions can be executed in 10 microseconds, which translates
into 100,000 operations per second. Register-to-register arithmetic
and logical instructions require 6 microcycles, which equals
approximately 83,000 operations per second. Memory-to-register
arithmetic instructions are much slower - the SC/MP executes 28,000
of such operations per second. This is significantly slower than
other microprocessors of its time (Intel 8080, MOS technology 6502,
etc).
National Semiconductor SC/MP processors were manufactured in
40-pin DIP package. There were no second-source manufacturers
of SC/MP microprocessors.
