National Semiconductor SC/MP

Encyclopedia : N : NA : NAT : National Semiconductor SC/MP

The SC/MP from National Semiconductor was an early microprocessor, becoming available in early 1974. The name SC/MP was an acronym for: "Simple Cost-effective Micro Processor".

It featured a 16-bit address and an 8-bit data bus. The program counter will roll-over on 12-bits (4096) unless you manually alter the program counter to set the upper 4-bits, which was subsequently output on the address bus along with status signals. This provides a memory map of 16 pages each of 4k bytes. Internally it provided five registers plus the program counter, but no stack pointer.

SC/MP Unusual features

An advanced and unusual feature for the time, was the possibility 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 configuation.

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.

Implementations

NSC 8060 Clocked at 1MHz, first implementation

NSC 807X Clocked at 4MHz (internally 2MHz) included an onboard BASIC (NIBL)

External links

http://xgistor.ath.cx/scmp.htm More info and simulators (Link dead)
http://www.bitsavers.org/pdf/national/_dataSheets/SCMP_dataSheet.pdf National Semiconductor datasheet
http://members.aol.com/mk14emu/index.htm Excellent resource with extensive information about SC/MP including an emulator for newer SC/MP II based on Science of Cambridge/Sinclair MK14
http://www.sasktelwebsite.net/jbayko/cpu2.html#Sec2Part3 Another summary of SC/MP
http://www.generalrobotics.com/downloads/8073_ref_man.pdf NSC 807X datatsheet

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