National Semiconductor's SC/MP (pronounced scamp) for Simple Cost-effective Micro Processor, is an early 8-bit microprocessor which became available in April 1976. It was designed to allow systems to be implemented with the minimum number of additional support chips. SC/MP included a daisy-chained control pin that allowed up to three SC/MP's share a single main memory to produce a multiprocessor system, or to act as controllers in a system with another main central processing unit (CPU). Three versions were released over its lifetime, SCMP-1 through 3, the latter two also known as INS8060 and INS8070.
To lower cost, the system used a bit-serial arithmetic logic unit (ALU) and was thus significantly slower than contemporary designs like the Intel 8080 or MOS 6502 which had parallel ALUs. Another oddity was that the program counter could only access the lower 12-bits of the 16-bit address, and the upper 4-bits had to be set using special instructions. The result was that instructions accessed main memory as sixteen 4 kB "pages" and reaching memory outside those pages required multiple instructions.
The combination of slow speed and paged memory limited its attractiveness outside the embedded markets it was aimed at, and in this market it competed against the Fairchild F8 which had a number of useful additional features. The system saw relatively little use.
Design
ALU and main registers
The arithmetic logic unit (ALU) was accumulator-based, with a single 8-bit accumulator, AC. A second 8-bit register, "Extension", or E, could be used as a backup for AC. E could not be accessed directly by most instructions; one could not load data from memory directly into E for instance. Instead, the value in AC could be read from E, or AC and E could be swapped. E can also be used as the source value for the seven logical/arithmetic operations supported by the ALU. If E were not used for other operations, it could also used in a fashion similar to the mirror registers seen in designs like the Zilog Z80, with E acting as a quick way to store the value of AC during an interrupt.
A major purpose of E was to act as a buffer for the built-in serial I/O system. This was driven by the SIO instruction, which bit shifted the least significant bit in E to a latch which was connected to the SOUT pin on the processor. Repeated calls to SIO thus presented bits of the 8-bit E value one at a time to the output, or reversed this operation for reading from the SIN pin. Once a complete 8-bit value had been read or written, the value could be sent to memory by exchanging the value with A and then loading or storing A to memory. E could also be used as the source for logical operations on AC, so for instance, one could read a status byte in from the serial port and then AND that value with a mask in AC, to check for various status bits read from an external device.
The separate Status Register was 8-bits. The three least-significant-bits, 0 through 2, were connected directly to pins, providing an easy way to check external device's status. A further two pins, Sense A and B, were connected to bits 4 and 5. Sense A was normally used to service interrupts (see details below). Interrupts could be turned off by setting bit 3, Interrupt Enable (IE) to 0 with the Disable Interrupt instruction, DINT, and enabled with IEN, or by moving the value to and from A and manipulating it there. Bits 6 and 7 were conventional overflow and carry flags, as seen in most processors.
Addressing and index registers
Like most 8-bit microprocessors of the era, the SC/MP is based around a 16-bit address and an 8-bit data bus. To lower pin count, the external address bus contains only 12 pins, sharing 4 pins from the data bus to create a complete 16-bit address. Internally, the program counter is twelve bits wide, and there are separate instructions to set the upper four bits by copying an 8-bit value into the high byte. This provides a memory map of sixteen four-kilobyte pages, and program code could not refer to code or immediate or PC-relative data outside its own page. Thus the memory is arranged as 16 pages of 4 kB each. This leads to a curiosity of the design; when the program counter reaches the end of a page, it does not move to the next page, it returns to the start of the current page.