INES Mapper 219

"A9746"

Banking

PRG: 4x8KiB (none fixed), 4 bits = 64KiB max CHR: 8x1KiB, 12 bits = 4MiB max

Registers

Mask: $E003
$8000 Select Register 1
$8002 Select Register 2
$8001 Control Register
Whichever of $8000, $8002 is last written decides what $8001 writes do.

It appears to have MMC3-like IRQ registers, as the implementation just falls through and uses those directly.

Mask $E001 
$C000 IRQ latch
$C001 IRQ reload
$E000 IRQ disable
$E001 IRQ enable

CHR banking is very odd. The following bits are ORed together:
Latch: CBA98765.... [write << 4]
Inner: .....7654321 [write >> 1]
and, if the bank is $0400 or $0C00, the least significant bit is further ORed with 1.
This has the effect of allowing consecutive writes to adjacent bank regs to get "sort of" a 2k bank.

$8000 write	$8002 write	bits	$8000 effect
$26	-	..PPPP..	(reversed) bits of PRG bank $8000-$9FFF, central nibble only
$25	-	..PPPP..	(reversed) bits of PRG bank $A000-$BFFF, central nibble only
$24	-	..PPPP..	(reversed) bits of PRG bank $C000-$DFFF, central nibble only
$23	-	..PPPP..	(reversed) bits of PRG bank $E000-$FFFF, central nibble only
-	$08-$1e even	CBA98765	Outer CHR bit latch for any 1k bank, 8 bits
-	$09	7654321.	CHR bits for $0000-$03FF
-	$0B	765432*.	Inner 7 CHR bits for $0400-$07FF (*least bit ORed to 1 regardless)
-	$0D	7654321.	Inner 7 CHR bits for $0800-$0BFF
-	$0F	765432*.	Inner 7 CHR bits for $0C00-$0FFF (*least bit ORed to 1 regardless)
-	$11	7654321.	Inner 7 CHR bits for $1000-$13FF
-	$15	7654321.	Inner 7 CHR bits for $1400-$17FF
-	$19	7654321.	Inner 7 CHR bits for $1800-$1BFF
-	$1D	7654321.	Inner 7 CHR bits for $1C00-$1FFF

Sources

FCEUX implementation Nestopia's source does not appear to differ significantly.