Corruption when viewing the System Bus for Mountain King A2600

[Bloopiero]

Summary

In the game Mountain King for the A2600, on the Stella core, trying to view memory addresses in the System Bus domain causes the game to corrupt and become unplayable. Not even touching the addresses, just trying to look at them at all.

Loaded core: Stella (does not happen on Atari2600Hawk)

Loaded rom:
Mountain King (1983) (CBS Electronics) [!]
SHA1:0A84B0A6BD0E79F5FA0B1BB9112160CB564AB836
MD5:DB4EB44BC5D652D9192451383D3249FC
Mapper Impl "FA"

Open tools: Ram search

Repro

1. Set the preferred A2600 core to Stella
2. Open the Mountain King rom
3. Open Tools -> Ram Search
4. Switch to Options -> Memory Domains -> System Bus
5. Refresh search or press the New button

Host env.

Windows 11, HP Laptop 17-cn4xxx, Intel Core 5 120U processor
-Clean install of the BizHawk dev build as of 4/25/2026 5 PM PT
-BizHawk 2.11
-BizHawk 2.10

I didn't extensively test whether this happens for other games, but I can at least say that Pac-Man A2600 works fine when following these steps.

If you do this while the game is running normally, it'll usually just freeze on a screen suddenly filled with ladder sprites. Closing the Ram Search window may even allow it to resume.

However, if you do it on one of the first two frames that the game is loading, you'll get some pretty colors! And incredibly loud noises. I've gotten different results by messing around with soft resets as well, but I'm not sure what triggered them, and I can't replicate anything specific for this issue report.

[CasualPokePlayer]

This isn't too surprising. On most systems, there are side effects in certain places (typically I/O) when reading with the native address space. Core ports that aren't careful in blacklisting these areas will end up having this kind of issue.

RAM Search also ends up reading the entire domain, so if any area of the System Bus is problematic you will get this kind of issue. Hex Editor / RAM Watch / Lua typically won't have issues unless you end up actually looking at the problematic addresses (it's likely just a few addresses are actually problematic here and need to be blacklisted in the System Bus domain impl)