The SC/MP is finally alive!

river river at zip.com.au
Tue Apr 5 16:44:02 CDT 2005


Hi,

Yes, for it's time (1976) the SC/MP had the logic that allowed it to share
the bus with other processors etc. The 8080 chip could also do this, but
this device required 3 separate power supplies and you needed three chips
to get the CPU working properly. The 6800 also offered similar multiprocessing
abilities and, like the SC/MP was a single chip system, but I'm not sure if it
came out before or after the SC/MP.

The SC/MP I (ISPA/500) was a PMOS device and required +5 and -7 volt power
rails. The SC/MP II (ISPA/600) was NMOS and required only +5 rail (same as the
6800), it also had three of it's CPU control lines inverted (as compared to the
SC/MP I) and though it could take 4 times the speed of the clock of the SC/MP I,
due to internal clocking it ran only twice as fast.

What made the SC/MP popular back in those days was the cost. It was about a
quarter of the cost of the 8080 and 6800 processors.

The SC/MP, like some other early processors, such as the 2650, had it's memory
divided into pages, and the Program Counter couldn't access the entire memory
unless some specific registers or jump/load instructions were used.

The simplicity of the SC/MP instruction set (only 46 instructions) is it's strength, yet
also a part of its weakness (ie, page memory, no dedicated stack, no compare
instructions, no rotate/shift left, cumbersome interrupt, etc).

Half the fun is getting the hardware all going, and the rest of the fun is to lock horns
with it's instruction set and do some fancy programming.

I'll take a few pics of the system (including my homebuilt 8085 system) and let you know 
when they are on the web.

Finally, it's good to see some other old SC/MP dudes around and also others who like
to build and program their systems from scratch.

river


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