About the ROMs in the Atari XEGS, could I replace the ROM in an Atari 65XE with that from an XEGS and have Missile Command built-in? Could I put the ROM from any Atari computer onto a cartridge and plug it in, running it that way? What about transplanting a cartridge ROM onto the mainboard?
The 600XL, 800XL, 65XE, 130XE (and 800XE) separated the OS (16KB) and BASIC ROM (8KB) to two separate chips.
The XEGS they combined the two into a single ROM, but since that only occupied 24KB, they still had 8KB left over and put the 8KB missile command in there. Then, the XEGS can swap out the BASIC ROM for the Missile Command ROM at will.
You can do this on the other machines with a simple PCB adapter, you can easily put up to 8 switch selectable replacements for the BASIC ROM, for which many 8KB cartridge images will work, ie Assembler Editor, Altirra BASIC, and many 8KB cartridge games (ie Frogger, Star Raiders (with a fix), MegaMania)
See this thread: http://atariage.com/...ro#entry4059023
This post has a list of ROMs the work and don't work as BASIC replacements in the XL/XE: http://atariage.com/...ited/?p=1707902