The [[6502]] processor has 13 [[Addressing Mode|Addressing Modes]], which affect how the arguments for instructions are accessed. Not  For example, the Load Accumulator <code>LDA</code> instruction can load the accumulator from different sources:  LDA #$05 ; Immediate addressing mode - load the accumulator with the number 5 LDA $05 ; Zero-page addressing mode - load the accumulator from memory address 5 LDA $0805 ; Absolute addressing mode - load the accumulator from memory address 0x8005 (32773). These are the available addressing modes on the 6502 processor; note that not all addressing modes are valid for every instruction.:

Data is accessed using a 16-bit address specified as a constant, to which the value of the X Y register is added (with carry).

An 8-bit zero-page address and the X register are added, without carry (if the addition overflows, the address wraps around within page 0). The resulting address is used as a pointer to the data being accessed. Note that, effectively, this makes the X register an index into a list of pointers. Also note that pointers are two bytes long, so the X register should be an even number when accessing a list of pointers (otherwise you'll get half of one pointer and half of another).