MelonDS on_bus_exec hook doesn't respect ARM/THUMB switching bit

[SuuperW]

It should be possible to read a pointer to code from memory and use the address with lua's event.on_bux_exec. ARM processors such as the ARM9 in melonDS use the least-significant bit (in the address used for a jump) for handling switches between ARM mode and THUMB mode. Thus there are pointers to code in a game's memory that are not 2-byte aligned. Giving such a value to on_bus_exec will result in the hook never being triggered.

As a result:

local address = memory.read_u32_le(code_pointer_address)
event.on_bus_exec(function() print("exec") end, address)

may result in no output even when the game loads the value at code_pointer_address then does a branch to that code.

[CasualPokePlayer]

Bit 0 of the PC does not actually exist. The least sigificant bit is cleared out of the PC upon any jump, so any kind of == PC for exec callbacks will fail (and bit 0 is only sometimes used (for bx, blx, and mov pc, instructions), for bl and b instructions, no mode switch would occur and the bit would just be cleared).

The most that could be fixed is just implicitly clearing out bit 0 on any address sent from the API (which would be C# side here).

[SuuperW]

I do also think it's weird to not trigger the callback if an address in the middle of the instruction is given. For example if a 4 byte instruction is executed with the first byte being xx4, I would expect it to trigger a callback for address xx6 or xx7. But I don't know if doing that has much value or if there are any pitfalls if such logic were applied to all systems.

implicitly clearing out bit 0 on any address sent from the API

I think this would be good enough for DS.

[CasualPokePlayer]

or if there are any pitfalls if such logic were applied to all systems.

Most systems don't have this kind of strict alignment. Most systems don't even have 4 byte instructions in the first place. This kind of thing being done would be per system (along with even clearing out bit 0, as many systems BizHawk emulates would have 1 byte instructions)