NES 2.0 Mapper 518

NES 2.0 Mapper 518 is used for several games and educational computers from Subor:

• 小霸王 Subor 999
• 小霸王 Subor V
• 跳舞天使: 動感 2000 (also known as Dance 2000 12-in-1)

Its UNIF board name is UNL-DANCE2000. It banks PRG-ROM in 16 or 32 KiB amounts, has 8 KiB WRAM, and 2x4 KiB of CHR-RAM that can be bankswitched according to the current nametable offset during rendering.

PRG-ROM Bank Register ($5000)

Bit 7654 3210
    ..PP PPPP
      ++-++++- Select 16 or 32 KiB PRG-ROM bank at CPU $8000

Mode Register ($5200)

Bit 7654 3210
    .... .SBM
          ||+- Select nametable mirroring
          ||   0: Vertical
          ||   1: Horizontal
          |+-- Select CHR-RAM banking mode (during PPU read accesses only)
          |     0: 8 KiB CHR-RAM at PPU $0000-$1FFF
          |     1: 4 KiB CHR-RAM at PPU $0000-$0FFF automatically switched, 4 KiB CHR-RAM at PPU $1000-$1FFF fixed to second half of CHR-RAM
          +--- Select PRG-ROM bank size
                0: 16 KiB PRG-ROM bank at CPU $8000-$BFFF, $C000-$FFFF fixed to first PRG-ROM bank
                1: 32 KiB PRG-ROM bank at CPU $8000-$FFFF

Notes

• When Bit 1 of the Mode register is set, PPU $0000-$0FFF will point to the first half of CHR-RAM while the PPU renders from the first CIRAM nametable, and the second half of CHR-RAM while the PPU renders from the second CIRAM nametable (offset $2400/$2C00 with vertical mirroring, or offset $2800/$2C00 with horizontal mirroring). The game uses this feature to have a CHR bankswitch in the middle of the screen by setting the scroll value such that the position at which the CHR bank needs to be switched is the seams between the two CIRAM nametables. This works even in the horizontal direction, providing a simple way for a mid-scanline bankswitch. Note that write accesses to CHR-RAM are never bankswitched.

• Note that these games require Dendy video timing and will produce graphical glitches with NTSC or regular PAL timing; Subor V will freeze with NTSC timing.

• NES 2.0 Mapper 514 is a simpler form of this mapper with only 32 KiB PRG-ROM banking.