The Free On-line Dictionary of Computing (30 December 2018):

MOS Technology
MOS Technologies

    A microprocessor design company started by some
   ex-Motorola designers, shortly after the Intel 8080 and
   Motorola 6800 appeared, in about 1975.  MOS Technology
   introduced the 650x series, based on the Motorola 6800
   design, though they were not exact clones for legal reasons.

   The design goal was a low-cost (smaler chip) design, realized
   by simplifying the decoder stage.  There were no instructions
   with the value xxxxxx11, reducing the 1-of-4 decoder to a
   single NAND gate.  Instructions with the value xxxxxx11
   actually executed two instructions in paralell, some of them
   useful.

   The 6501 was pin-compatible with the 6800 for easier market
   penetration.  The 650x-series had an on-chip clock oscillator
   while the 651x-series had none.

   The 6510 was used in the Commodore 64, released September
   1981 and MOS made almost all the ICs for Commodore's pocket
   calculators.

   The PET was an idea of the of the 6500 developers.  It was
   completly developed by MOS, but was manufactured and marketed
   by Commodore.  By the time the it was ready for production
   (and Commodore had cancelled all orders) MOS had been taken
   over by Rockwell (Commodore's parent company).  Just at this
   time the 6522 (VIA) was finished, but the data sheet for it
   was not and its developers had left MOS.  For years, Rockwell
   didn't know in detail how the VIA worked.

   (2001-03-31)