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| [[iNES Mapper 004]] is a wide abstraction that can represent boards using the Nintendo [[MMC3]], Nintendo [[MMC6]], or exact clones of any of the above. Most games utilizing [[TxROM]], [[DxROM]], and [[HKROM]] boards use this designation.
| | #REDIRECT [[MMC3]] |
| | | {{DEFAULTSORT:004}}[[Category:iNES Mappers]][[Category:MMC3-like mappers]][[Category:in NesCartDB]][[Category:Nintendo licensed mappers]][[Category:NES 2.0 mappers with submappers]][[Category:Mappers with scanline IRQs]] |
| A few specific TxROM boards handle the PPU bus differently and thus have separate mapper numbers:
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| *[[iNES Mapper 118]] covers TKSROM and TLSROM, which use CHR bank bits to control VRAM mirroring
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| *[[iNES Mapper 119]] covers TQROM, which has both CHR RAM and CHR ROM
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| Some [[:Category:MMC3-like mappers|MMC3-like mappers]] have different behaviors and thus their own mapper numbers:
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| *[[iNES Mapper 204]] covers Tengen MIMIC-1 or Namco 109 chip, the MMC3's predecessor with no mirroring control or IRQ
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| *[[iNES Mapper 64]] covers Tengen RAMBO-1, an MMC3 with additional PRG and CHR bankswitching and IRQ functionality
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| *Several MMC3-based multicarts
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| Here are Disch's original notes:
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| ========================
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| = Mapper 004 =
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| ========================
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|
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| aka
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| --------------------------
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| MMC3
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| TxROM
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| (MMC6)
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| (HxROM)
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|
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|
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| Example Games:
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| --------------------------
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| Mega Man 3, 4, 5, 6
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| Kirby's Adventure
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| Gauntlet
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| Rad Racer 2
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| Startropics 1, 2 (MMC6)
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| Super Mario Bros. 2, 3
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| ... a zillion other games (most common mapper)
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|
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|
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| 4-Screen Notes:
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| ---------------------------
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| TR1ROM and TVROM are two of the *very few* boards to use 4-screen mirroring. The only mapper 004 games which
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| use 4-screen mirroring I know of are Rad Racer 2 and Gauntlet. Several other games are incorrectly labelled
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| as being 4-screen when they are in fact, not (ex: Gauntlet 2).
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|
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| TR1ROM and TVROM are both configured in a way which uses on-cart WRAM as VRAM for the nametables. However
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| this means the WRAM is not tied to the CPU, therefore they do not have any SRAM/WRAM!
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|
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| So to be "safe":
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| - when homebrewing: choose either 4-screen or WRAM. You can't have both
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| - when emudeving: permanently disable WRAM when 4-screen. This does not break any games.
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|
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| 4-screen mirroring for these boards is hardwired! When in 4-screen mode, your emu must ignore writes to the
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| mirroring reg.
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|
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| Also note that many Rad Racer 2 dumps are floating around which do not indicate it is 4-screen. So if you
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| try that game in your emu and the graphics are screwed, that's the first thing to check.
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|
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|
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| IRQ Notes:
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| ---------------------------
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| IRQ Operation on this mapper is simple at first glance, however its precise operation gets very complex.
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| This mapper is infamously one of the hardest (if not the very hardest) mapper to emulate accurately -- a
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| double-whammy since it's also hands down the most common mapper around.
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|
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| Be sure to read 'Basic IRQ operation' below, and I recommend you seriously consider skimming 'Detailed IRQ
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| operation' as well -- especially if you're emudeving.
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|
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|
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|
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| Other notes:
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| ---------------------------
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| Low G Man will actually confirm that WRAM disabling works properly, and will break if it isn't. So if your
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| emu ignores WRAM disabling and always has it enabled, Low G Man will break (specifically, during the
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| level 1 boss fight)
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|
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|
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|
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| Registers:
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| ---------------------------
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|
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| Range,Mask: $8000-FFFF, $E001
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|
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|
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| $8000: [CP.. .AAA]
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| C = CHR mode select (see CHR setup)
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| P = PRG mode select (see PRG setup)
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| A = Address for use with $8001
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|
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|
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| $8001: [DDDD DDDD] -- data port
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| R:0 -> CHR reg 0
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| R:1 -> CHR reg 1
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| R:2 -> CHR reg 2
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| R:3 -> CHR reg 3
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| R:4 -> CHR reg 4
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| R:5 -> CHR reg 5
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| R:6 -> PRG reg 0
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| R:7 -> PRG reg 1
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|
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|
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| $A000: [.... ...M]
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| Mirroring: 0=Vert
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| 1=Horz
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|
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| Ignore when 4-screen
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|
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|
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| $A001: [EW.. ....]
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| E = Enable WRAM (0=disabled, 1=enabled)
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| W = WRAM write protect (0=writable, 1=not writable)
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|
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|
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| $C000: [IIII IIII] IRQ Reload value
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| $C001: [.... ....] IRQ Clear
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| $E000: [.... ....] IRQ Acknowledge / Disable
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| $E001: [.... ....] IRQ Enable
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|
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|
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| CHR Setup:
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| ---------------------------
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|
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| $0000 $0400 $0800 $0C00 $1000 $1400 $1800 $1C00
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| +---------------+---------------+-------+-------+-------+-------+
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| CHR Mode 0: | <R:0> | <R:1> | R:2 | R:3 | R:4 | R:5 |
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| +---------------+---------------+---------------+---------------+
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| CHR Mode 1: | R:2 | R:3 | R:4 | R:5 | <R:0> | <R:1> |
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| +-------+-------+-------+-------+---------------+---------------+
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|
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| PRG Setup:
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| ---------------------------
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|
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| $8000 $A000 $C000 $E000
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| +-------+-------+-------+-------+
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| PRG Mode 0: | R:6 | R:7 | { -2} | { -1} |
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| +-------+-------+-------+-------+
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| PRG Mode 1: | { -2} | R:7 | R:6 | { -1} |
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| +-------+-------+-------+-------+
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|
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|
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| Basic IRQ Operation
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| ---------------------------
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|
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| MMC3 IRQs utilize a scanline counter. The basic steps to making it work are as follows:
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|
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| 1) Set the desired number of scanlines you want to wait by writing N to $C000
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| 2) Reset the internal IRQ counter by writing any value to $C001
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| 3) Enable IRQs by writing any value to $E001
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|
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| An IRQ will then fire after N+1 rendered scanlines, at which point, you would write any value to $E000 to
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| acknowledge the IRQ, and disable further IRQs (and possibly repeat the above 3 steps if you want to fire
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| another IRQ this frame).
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|
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| The MMC3 counts scanlines by watching the CHR accesses the NES makes. Therefore in order for this IRQ
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| counter to work properly, the NES must be making the accesses that the MMC3 is expecting. If you have
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| abnormal settings, you will confuse the MMC3 and IRQs will not work properly. Therefore, you must follow
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| these rules:
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|
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| 1) The IRQ counter will only count when the PPU is on (sprites and/or BG enabled).
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| 2) Do not manipulate $2006 or $2007 while using IRQ counter
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| 3) Do not set $C000 to $00
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| 4) BG and Sprites must use opposing pattern tables for CHR. EG:
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| a) if 8x16 sprites, BG must use $0xxx, *ALL* sprites must use $1xxx
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| b) if 8x8 sprites, if BG is using $0xxx, sprites must use $1xxx
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| c) if 8x8 sprites, if BG is using $1xxx, sprites must use $0xxx (slightly abnormal)
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|
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| With settings 'a' and 'b', the IRQ will occur after dot 260. With setting 'c', it will occur after dot 324
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| of the *previous* scanline.
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|
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|
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| The IRQ Counter consists of several parts:
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| 1) Actual 8-bit IRQ counter (not directly accessable)
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| 2) 8-bit latch, or reload value (reg $C000)
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| 3) IRQ Enable flag
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| 4) IRQ Pending flag
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|
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| IRQ Registers interact with the above parts as follows:
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| reg $C000 - sets IRQ Reload value
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| reg $C001 - sets the actual IRQ counter to 0 (regardless of what value is written)
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| reg $E000 - clears both IRQ Enable flag and IRQ Pending flag
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| reg $E001 - sets IRQ Enable flag
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|
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| Every time the MMC3 detects a scanline, the following IRQ Counter logic is executed. Note this occurs EVEN
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| IF IRQs are disabled (the IRQ counter is always counting):
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|
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| - If IRQ Counter is 0...
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| a) reload IRQ counter with IRQ Reload value
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|
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| - Otherwise...
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| a) Decrement IRQ counter by 1
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| b) If IRQ counter is now 0 and IRQs are enabled, trigger IRQ
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|
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| Note that 241 scanlines are counted per frame (the 240 rendered scanlines, and the "prerender" scanline).
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|
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|
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|
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|
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|
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| Detailed IRQ Operation
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| ---------------------------
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|
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| MMC3 detects scanlines by watching A12 ($1000) on the PPU bus. Every time a rising edge occurs (transitions
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| from 0->1), and it hasn't been too close to the previous rising edge, the IRQ counter gets clocked.
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|
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| Under *normal* conditions (BG using $0xxx, sprites using $1xxx), A12 will rise exactly 8 times every scanline
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| (once for each sprite CHR fetch). However the 8 rises are so close together that only the first is 'seen'.
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|
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| During rendering and pre-render scanlines the PPU is fetching NT and CHR data from the cart through a series
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| of reads. Each read updates the PPU Address lines (including A12), and each read takes 2 PPU cycles (2
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| dots). There are 4 reads per tile, and 42 tiles per scanline:
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|
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| - 32 BG tiles
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| - 8 Sprite tiles (for the next scanline)
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| - 2 BG tiles (for the next scanline)
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|
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|
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| Each tile requires 4 reads, each read is 2 dots:
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| dot 0: Name table fetch ($2xxx -- A12 is low)
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| dot 2: Attribute fetch ($2xxx -- A12 is low)
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| dot 4: Low CHR fetch ($0xxx or $1xxx -- A12 is low or high)
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| dot 6: High CHR fetch ($0xxx or $1xxx -- A12 is low or high)
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|
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| If the tile being fetched is using the right-hand pattern table ($1xxx), then A12 goes high on dot 4 of that
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| 8-dot sequence. Otherwise, A12 stays low throughout.
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|
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| This 8-dot sequence is repeated for each tile.. meaning there are 42 opportunities for A12 to rise. These
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| opportunities occur on the following dots:
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|
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| 4, 12, 20, ..., 244, 252 (32 BG tiles)
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| 260, 268, 276, 284, 292, 300, 308, 316 (8 Spr tiles)
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| 324, 332 (2 BG tiles)
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|
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| (You might be able to see now how I came up with those 260, 324 numbers I threw at you earlier)
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|
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| MMC3 seems to ignore rises that are too close together. This is why the 8 sprite fetches will only clock
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| the counter once. Exactly how far apart the rising edges have to be is unknown, but it is somewhere between
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| 14 and 16 dots. So any two consecutive opportunities are too close together (including the most distant
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| 332->4), but any two non-consecutive opportunities will both be acknowledged.
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|
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| Figuring whether the tile is being fetched from $0xxx or $1xxx is usually easy. BG and 8x8 sprites are
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| always fetched from an assigned pattern table (configurable by PPU reg $2000). However, 8x16 sprites can
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| come from either pattern table. So which tile is begin fetched depends on which sprite is being fetched....
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| which depends on what scanline you're on, and what sprites are found to be in-range on that scanline. For
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| scanlines which contain less than 8 sprites, tile $FF is fetched as a dummy (in 8x16 sprites, this would be
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| from the $1xxx pattern table).
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|
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| This is why, when you have 8x16 sprites, ALL sprites must use the right-hand pattern table. If you have
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| sprites using the left and the right, you'll probably end up having some scanlines where the IRQ counter
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| counts the same scanline multiple times! All depending on which sprites are in-range and when. For example,
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| if there are 4 sprites on the scanline using $0xxx, and 4 using $1xxx, the IRQ counter might count the
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| scanline anywhere from 1 to 4 times!
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|
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| 0,0,0,0,1,1,1,1 <--- all 4 rises consecutive, will only clock once
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| 0,1,0,1,0,1,0,1 <---- all 4 rises nonconsecutive, counter clocked each time!
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|
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| This is also why the IRQ counter isn't clocked when both BG and sprites use the left pattern table (since
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| there is never any rising edge, the MMC3 never detects any scanlines).
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|
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|
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|
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| $2006 and $2007
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| ---------------------------
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| A game can manually clock the IRQ counter (either on accident, or by design) by manipulating $2006 and $2007.
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| A12 is updated (potentially triggering a rising edge) when the PPU address is updated by these registers.
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| On $2007 reads/writes, and on the second $2006 write. This is why messing with $2006 and $2007 after you
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| prep your IRQ stuff may screw up your IRQs unless you're careful.
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|
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|
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|
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|
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| IRQ Counter priming
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| ---------------------------
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| Some games seem to prime the IRQ counter by repeatedly writing $0000 and $1010 to $2006. This toggles A12,
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| clocking the IRQ counter. It is unknown whether or not this is actually required.
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|
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|
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|
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| Reload value of $00
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| ---------------------------
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| Different MMC3 versions behave differently when you set $C000 to $00. There are at least two (possibly more)
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| behaviors. These behaviors are mentioned in the readme accompanied with blargg's MMC3 test ROMs, which, if
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| you're emudeving, I highly recommend you pick up. Otherwise, the behavior of having a reload value of $00
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| is unreliable and/or undesirable, and should be avoided at all costs when homebrewing/hacking.
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|
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|
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|
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|
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| Special Variant -- MMC6:
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| --------------------------
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|
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| Startropics 1 and 2 are both MMC6 games (not MMC3). However, they are unfortunately assigned the same mapper
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| number, despite being slightly incompatible. There is no simple way to determine MMC3 from MMC6. You'll
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| probably have to use a CRC or hash check or something.
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|
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| For the most part they are the same -- but the big difference is the WRAM. MMC6 has only 1k of WRAM,
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| whereas MMC3 games have 8k. It is also mapped a bit differently, and is enabled/disabled differently from
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| MMC3.
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|
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| MMC6 registers are as follows. All other registers behave just as they do on MMC3:
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|
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|
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| $8000: [CPW. .AAA]
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| C,P,A = Same as on MMC3
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| W = WRAM Enable (0=disabled, 1=enabled)
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|
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| $A001: [HhLl ....]
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| H,L = Enable WRAM block (0=disabled, 1=enabled)
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| h,l = WRAM block write protect (0=writes disabled, 1=writes enabled)
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|
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|
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| The 1k of WRAM is split into 2 512 byte blocks... one at $7000-71FF and another at $7200-73FF. Each block
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| can be controlled independently through $A001. H,h bits deal with the high block ($7200), and L,l bits deal
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| with the low block ($7000).
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|
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| If only one block is enabled, the disabled block will read back as $00. However if BOTH blocks are disabled,
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| reading either will return open bus.
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|
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| $7000-73FF is mirrored throughout $7400-7FFF. However, $6000-6FFF is always open bus (unmapped).
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|
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| $8000.5, when clear (to disable WRAM), simply sets $A001 to $00 and keeps it there. Writing to $A001 when
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| $8000.5 is clear will have no effect.
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| == See also ==
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| *[[iNES Mapper 076]]: more specific Namco 109
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| *[[iNES Mapper 064]]: [[Tengen RAMBO-1]], successor to MIMIC-1
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| [[Category:iNES Mappers]][[Category:MMC3-like mappers]] | |
