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8-bit Normal Serial IO mode not working #2172

felixjones opened this issue May 13, 2021 · 0 comments

8-bit Normal Serial IO mode not working #2172

felixjones opened this issue May 13, 2021 · 0 comments


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@felixjones felixjones commented May 13, 2021

ELF & ROM download (
((please excuse the use of "master/slave" terminology))

On mGBA the master player must await for the ready state of the slave player. This is done by reading bit 2 of REG_SIOCNT (AKA "SI State (opponents SO)") until the value is "low" (0, ready state).

This status never changes in mGBA. It is always "high" (1, not ready state).

Video of the above ROM on hardware:

In this demo ROM player 1 can send buttons A and/or B to player 2 (regardless of master/slave setting, however P2 cannot send to P1 - due to normal mode being one directional with AGB cables. CGB cables are two directional).

Tested on latest mGBA as of this writing (0.10-6995-96988133b)
Recreation steps:

  • Load up ROM
  • Open multiplayer window
  • Load up same ROM
  • Select Master on Player 1
  • Select Slave on Player 2
  • Press A and/or B on Player 1
  • Expected: Player 2 receives value. Actual: Master "times out" (bit 2 of REG_SIOCNT never changes).

EDIT: Tested with Normal32, which seems to work fine. I suppose this issue is Normal8 only.

@felixjones felixjones changed the title Normal Serial IO mode not working 8-bit Normal Serial IO mode not working May 13, 2021
@endrift endrift added this to the mGBA 0.9.2 milestone May 15, 2021
@endrift endrift closed this in 0865b89 May 15, 2021
endrift added a commit that referenced this issue May 15, 2021
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