Atari 800XL Memory Expansion
Can you use the memory expansion intended for the 600XL to expand the memory of the 800XL?
The 600XL memory expansion brings the total memory of the 600XL up to the maximum of 64K. Although it would plug into the 800XL, this would be futile (and would confuse the computer), since the 800XL already has 64K. Memory is not merely an add-on item—it has to fit correctly into the computer's memory map. Since the memory map is full on the 800XL, you would need some other kind of expansion memory that uses bank switching or windowing to get more than 64K of system memory. You could never have more than 64K of memory at one time, but you could swap out (bank switch) portions of the expanded memory in a cartridge that allows this. Don't expect any commercial software to take advantage of such an unusual memory configuration, though.