Kega Fusion Inaccurate VRAM Copy Emulation

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    • #629
      Random64
      Participant

      I found an inaccuracy with Kega Fusion’s Genesis/Mega Drive emulation. On real hardware, during VRAM to VRAM copies, the address’s least significant bit is effectively inverted. Kega Fusion emulates VRAM to VRAM copies without inverting this bit. For copies starting from an even address, with an even length, and the auto increment register set to 1, it makes no difference. For transfers that don’t meet these criteria, it produces incorrect results.

      Example: 4 byte copy from 0x2000 to 0x4000 with the auto increment register set to 1
      Here, bytes are copied in a different order, but the results are effectively the same.

      Kega Fusion:
      (source -> destination)
      0x2000 -> 0x4000
      0x2001 -> 0x4001
      0x2002 -> 0x4002
      0x2003 -> 0x4003

      Real hardware (least significant bit is inverted):
      0x2001 -> 0x4001
      0x2000 -> 0x4000
      0x2003 -> 0x4003
      0x2002 -> 0x4002

      Example: 4 byte copy from 0x2000 to 0x4000 with the auto increment register set to 2
      Here, bytes are copied to incorrect addresses.

      Kega Fusion:
      0x2000 -> 0x4000
      0x2001 -> 0x4002
      0x2002 -> 0x4004
      0x2003 -> 0x4006

      Real hardware (least significant bit is inverted):
      0x2001 -> 0x4001
      0x2000 -> 0x4003
      0x2003 -> 0x4005
      0x2002 -> 0x4007

      I made a ROM (linked here) to test this. It displays two copies of the same image. The image on the left is transferred to VRAM and displayed as is. The image on the right is transferred to VRAM and then rearranged using 4 separate VRAM copies with the auto increment register set to 4. After rearranging the image, it should look identical to the image on the left.

      VRAM Copy Test in Gens
      This screenshot from Gens matches what I saw running this ROM on real hardware.

      VRAM Copy Test in Kega Fusion
      This screenshot shows the result of not inverting the least significant bit.

    • #632
      King
      Participant

      Kega hasn’t been updated in 6 years now, so this isn’t surprising.

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