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.