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The 6502 microprocessor is an 8-bit CPU with an 8-bit ALU and a 16-bit address bus capable of direct access to 64KB of memory space.  The 6502 is an 8-bit CPU in the purest sense. Unlike the Z80, the 6502 does not have any 16-bit instructions and cannot pair it's registers. To work with a 16-bit number you will need to split it in two and work with each half individually. Like the Z80, the 6502 is a little-endian CPU, meaning it stores reads 16-bit values with the least significant byte first, followed by the most significant byte. The 6502 has 151 instructions, which are composed of 56 distinct opcodes across various addressing modes.