MOS Technology 6502 and other 650x/651x processors are a family of inexpensive 8-bit microprocessors. 6502 and 6512 processors could address 64KB of memory, other CPUs in the family could address up to 8 KB. Stack size of the 6502 was limited to 256 bytes. The processors had only 6 registers, and only one of these registers - an accumulator - could be used for arithmetic and logic operations. The 6502 microprocessor had 13 addressing modes, some of these modes made extensive use of two 8-bit index registers. The processor didn't have I/O instructions, therefore 6502-based computers used memory mapped I/O.
All microprocessors in the 6502/650x family are software compatible between each other - the differences between CPUs are in hardware (different maximum size of addressed memory, clock oscillator is on-board or external, interrupt input options).
CMDu was one of the lesser known second sources. ------------------------- The 65816 was a 16 bit extension of the 6502.