The 6502 that powers the Nintendo stores its memory in Little-Endian rather than big-endian. This affects the stored order of multiple-byte values in the processor. The difference between the two can be seen in the table below:
|Hex Value||2-Byte Value|
The 6502 has no native support for 2-byte integers, so its endianness doesn't apply to them, but memory addresses are often stored as 2-bytes. Thankfully, compilers of 6502 assembly don't require you to input memory addresses in little-endian. Instead, they swap the two bytes for you when a program is compiled.
However, if you're reading 6502 machine code from a debugger or from the ROM, the memory addresses will be stored in little-endian. For example, if you wanted to load memory from address $1000 using assembly, the code would look like this:
LDA $1000 ; Loads A with whatever value is in $1000.
However, if you open the compiled machine code in a hex editor, it will look like this:
AD 00 10
Notice how the low and high bytes of the memory address are swapped by the compiler due to the 6502 using little-endian.