Santa Clara, 1974. National Semiconductors introduced the ISP-8A/500 (INS 8050) SC/MP 8-bit microprocessor. Intel had of course led the way with the 8008 and 8080, but these were not single-chip solutions. Motorola introduced the 6800 in 1974, and that is IMO a better processor, but nonetheless, my first computer was a Scamp.
|Number of chips||One (Note 1)||Three: CPU, Clock Generator, System Controller||Two (Note 2)|
|Supply Voltage||+5, -7||+5||+5V, -5V, +12V||+5V|
|Clock Frequency (cycle time)||900kHz (11us)||2MHz (5us)||2MHz (2us)||1MHz (2us)|
|Address Bus Width||12 bits (note 1)||16 bits||16 bits|
|Number of Instructions||46||75||72|
|Introduced||1974||1977||April 1974||Aug 1974|
|Cost at introduction||$10 (in quantity)||$17.76 (single unit)||$360||$360|
Note 1: Provision is made for a pseudo-16-bit address bus. The upper four bits can be written to an external register (upping the chip count to 2) using a special command. In this way, 16 pages of 4k each can be addressed. The Program Counter is effectively 12 bits wide.
Note 2: The 6800 needs an external bi-phase clock which is normally supplied by a 6871 clock generator.
The first SC/MP has some crazy power requirements, +5V and -7V. Well, it's not that crazy really, referenced to +5V it's -5V and -12V, so it's an upside-down 8080 that doesn't need a third voltage.
It features an 8-bit data bus and a sort-of 16-bit address bus -- the program counter will roll-over on 12-bits (4096), there are separate instructions to alter the program counter to set the upper 4-bits, which are subsequently output on the address bus along with status signals. This provides a memory map of 16 pages each of 4 KB. Internally it provided five registers plus the program counter, but no stack pointer.
Part III: SC/MP, early advanced multiprocessing (April 1976)
The National Semiconductor SC/MP (Single Chip/Micro Processor (sometimes "Small Costeffective Micro Processor), nicknamed "Scamp") was a typical 8 bit processor intended for control applications (a simple BASIC 2.5K ROM was added to one version). It featured 16 bit addressing, with 12 address lines and the 4 upper lines borrowed from the data bus (it was common to borrow lines (sometimes all of them) from the data bus for addressing - however only the lower 12 index register/PC bits were incremented (4K pages), special instructions modified upper 4 bits). Internally, it included four index registers (P1 to P3, plus the PC/P0) and two 8 bit registers. It had no stack pointer or subroutine instructions (though they could be emulated with index registers). During interrupts, the PC and P3 were swapped. It was meant for embedded control, and many 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 (as well as other intelligent devices, such as DMA controllers) could be hooked up to the bus. 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, but the bit-serial CPU was slow (even simple instruction took 5-7 cycles, while memory access was 2 cycles, which allowed them to share a memory bus without saturating it (up to three), as opposed to a 6502 which could share memory with at most one other CPU, and only then because of the way the CPU clock was used). However this feature was almost never used for multiprocessing.
In addition to I/O ports like the 8080, the SC/MP also had instructions and one pin for serial input and one for output.
National semiconductor also produced the IMP series (originally cards, later microprocessors in 4, 8, 12, and 16 bit versions) and 16-bit PACE. The SC/MP was eventually replaced with the COP4 (4 bit) and COP8 (8 bit) embedded controllers, with only two index registers, but adding stack support.
http://www.computinghistory.org.uk/det/39408/SC-MP-Keyboard-Kit-User-s-Manual/ http://www.dos4ever.com/SCMP/SCMP.html http://www.retrotechnology.com/herbs_stuff/sing_bds.html
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