User:NovaSquirrel/Nova-7
This proposal for an NES mapper combines the runtime flexibility of CHR RAM with the tile animation capability of CHR ROM.
Overview
- PRG ROM size: Up to 1024 KB
- PRG ROM bank size: 8 KB
- PRG RAM: Up to 1024 KB
- CHR capacity: Up to 256 KB RAM or ROM
- CHR bank size: .5 KB to 4 KB
- Nametable mirroring: Controlled by mapper; horizontal, vertical, single screen or four screen
- Subject to bus conflicts: No
Registers
Configuration is accomplished by first writing the command number to the Command Register at $5000-57FF, then writing the command's parameter byte to the Parameter Register at $5800-5FFF. If $8000-$FFFF is write-protected, $8000-9FFF and $A000-BFFF will also work for the command and parameter registers, respectively.
There are 15 commands:
- $0-7 control CHR banking
- $8-B, E control PRG banking
- $C controls nametable mirroring and CHR mode and write protect
- $F controls IRQ
$D is reserved
Command Register ($5000-$57FF)
7 bit 0
---- ----
.... CCCC
||||
++++- The command number to invoke when writing to the Parameter Register
Parameter Register ($5800-$5FFF)
7 bit 0 ---- ---- PPPP PPPP |||| |||| ++++-++++- The parameter to use for this command. Writing to this register invokes the command in the Command Register.
IRQ register ($4800-4FFF)
Writes go to the IRQ register at $0F
Commands
CHR Bank 0-7 ($0-7)
7 bit 0 ---- ---- BBBB BBBB |||| |||| ++++-++++- The bank number to select for the specified bank.
| Register | CHR mode 0 | CHR mode 1 | CHR mode 2 | CHR mode 3 |
|---|---|---|---|---|
| $0 | $0000-$03FF | $0000-$07FF | $0000-$07FF | $0000-$0FFF |
| $1 | $0400-$07FF | $0800-$0BFF | $0800-$0FFF | $1000-$13FF |
| $2 | $0800-$0BFF | $0C00-$0FFF | $1000-$13FF | $1400-$05FF |
| $3 | $0C00-$0FFF | $1000-$07FF | $1400-$07FF | $1600-$07FF |
| $4 | $1000-$13FF | $1800-$19FF | $1800-$19FF | $1800-$19FF |
| $5 | $1400-$17FF | $1A00-$1BFF | $1A00-$1BFF | $1A00-$1BFF |
| $6 | $1800-$1BFF | $1C00-$1DFF | $1C00-$1DFF | $1C00-$1DFF |
| $7 | $1C00-$1FFF | $1E00-$1FFF | $1E00-$1FFF | $1E00-$1FFF |
PRG Bank 1-3 ($9-B, $E)
7 bit 0 ---- ---- RBBB BBBB |||| |||| |+++-++++- The bank number to select for the specified bank. +--------- If 1, bank is RAM instead of ROM
Bank $8 - CPU $6000-$7FFF Bank $9 - CPU $8000-$9FFF Bank $A - CPU $A000-$BFFF Bank $B - CPU $C000-$DFFF Bank $E - CPU $E000-$FFFF
Loss of M2 oscillation (caused by resets) causes register $0E to revert to a value of $7F, mapping the last ROM bank in the cart into $E000-$FFFF.
Nametable/CHR control ($C)
7 bit 0
---- ----
.WCC SFMM
||| ||||
||| ||++- Mirroring Mode
||| || 0 = Vertical
||| || 1 = Horizontal
||| || 2 = One Screen Mirroring from $2000 ("1ScA")
||| || 3 = One Screen Mirroring from $2400 ("1ScB")
||| |+--- If 1, four screen mirroring in last 4KB of CHR RAM, ignores mm if on
||| +---- If 1, swap PPU $0xxx with $1xxx
|++------ CHR bank switching mode
+-------- If 0, any RAM at $8000-FFFF is write protected
When combined with sprite 0 or the scanline IRQ feature, this allows four-screen mirroring and a status bar at the same time, as the playfield is in CHR RAM and the status bar is in CIRAM.
IRQ Control ($F)
7 bit 0 ---- ---- NNNN NNNN ++++-++++- Scanlines until IRQ
At the start of each scanline, the PPU freezes for a few cycles, and PPU A13 stays high for at least three consecutive cycles of PPU /RD. The mapper detects this and subtracts 1 from the value in $0F unless the value is $F0-$FF. While the value is 0, /IRQ is pulled low. Programming tip: Reading from the nametables or palette during vertical or forced blanking will cause counts unless you write $FF to port $0F.
