Atari 800XL with mystery toggle switch

[Yannil]

I recently bought an Atari 800xl. It works fine but it has a little 2-way toggle switch fitted on the back of the machine? 
I opened it up to have a look inside, and the wires go to a piggy backed chip in U5.

The machine loads games whether the switch is up or down. 
Any ideas as to what it is. 

[+selgus]

I would assume it was some sort of toggle to switch between different OS's stored in the OS EPROM. I don't know which ones were burned into that EPROM, but that would be my guess.

[+x=usr(1536)]

10 minutes ago, selgus said:

I would assume it was some sort of toggle to switch between different OS's stored in the OS EPROM. I don't know which ones were burned into that EPROM, but that would be my guess.

Could have been OSB in order to eliminate the need for a translator disk, or possibly OMNIMON.  Either one is a good bet.

 

What happens with the switch in different positions when the machine is powered-up?

[SoundGammon]

I made some like those for our old Atari computer club members.

 

Had the XL OS on one and the 400/800 OS Rev A with the Fast Math & custom computer font/character set on the other!

[Yannil]

Tried a few different things. Mostly the game froze. If the switch was up or down and I switched it down or up. 
With the switch down and I pressed Select and then reset I got this screen

 

[SlagOMatic]

Quote

OMNIMONXL is a resident machine language monitor which, once 
installed, is always available to you. What that means is that you never 
have to load it and you can call it up no matter what program happens to 
be running at the time. Once running, OMNIMONXL gives you complete 
control over your computer. This includes the ability to easily examine 
and modify memory or the 6502's registers, to dump data to a printer, 
and to read and write to the disk drive(s) without DOS. It also has a 
complete set of debugging tools including a disassembler, single step, 
and a unique JSR function for testing out subroutines. And all of these 
features are available to you at any time, no matter what program is 
running, simply by pressing SYSTEM RESET along with either the 
OPTION/SELECT or SELECT button! 

https://archive.org/stream/Atari_Omnimon_XL_users_guide/Atari_Omnimon_XL_users_guide_djvu.txt

[Yannil]

Thanks very much for the info and the links. I will read through the users guide, it will be most helpful.