Action Replay 1MB/4MB "automatic" switch


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Some earlier Action Replay cartridge models had a physical switch to select RAM mode, but recent ones are advertised as automatically selecting it, which is not technically documented. So does anyone have some technical informations (which register to write, etc) about how to configure expansion RAM mode ?

AFAIK, there's no way to guess which mode to use from cartridge hardware point of view, so I guess that when loading a game, Action Replay firmware verify its IP header contents, and select 1MB mode for the few games that don't support 4MB one ... but that's just a guess, so maybe it is working in a more clever way :)

I would like to use this automatic configuration feature in order to test 1MB mode in Pseudo Saturn Kai's cartridge self test feature, and indicate if cartridge have automatic RAM mode switch feature available.
Basically, main goal is to double-check the RAM, and verify if cartridge have automatic switch feature available or not.
The fact that 1MB cart games work with Sega's 4MB cart should tell you all there is to know about this supposed auto-switching. I believe the AR uses a set of 4 reverse-biased diodes directly on the data bus to generate the cartridge id value, so it's not even possible to change it.

The RAM cartridge tech note points out that 1MB games should also check for the 4MB cart id, so there shouldn't even be a need for switching to begin with.
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Ah OK, so it's just advertising SEGA technical specifications and game coding guidelines :D

I initially thought that some games would make some weird usage of mirrors in 1MB mode, for example with a module writing in a mirror, and another module reading from another mirror, but accessing to same memory area, hence making game only compatible with 1MB carts. Maybe it was over-thinking.
Some games are actually supporting only 1MB RAM cartridge (list here, listed as corrupting graphics), but as you mentioned, theses are probably caused only by tight access timings.

Anyway, I'm relieved about that "automatic by design" feature, because that reduces the number of features to add in Pseudo Saturn Kai :)
Let's revive a year old thread does anyone know of a way I can add a usb or a sd card reader to the action replay plus 4m without removing anything? Maybe use wires to connect a PCB board with the SD card reader on it. If there is a way I can connect to one of the logic controllers I can do it from the rear of the board and fit it back inside the cart case .