Data spoofing

Data spoofing is a common door glitch effect that writes data from one supertile to another.

Layman explanation

Data for rooms is stored in memory as flags that are either on or off. Whenever the room ID changes, this data is written to Save RAM. Under normal conditions, data for the next room is then loaded from Save RAM and overwrites the old data for the previous room. With certain door glitches, the save is performed, but the load is not, because it was interrupted by the transition not happening. When this happens, the old room data is still there but with the current room pointing to the new room. Thus, when the room ID is changed again from any sort of transition or glitch, it will write the original room's data into Save RAM.

Mechanics

Conflict

The main conflict arises when a door glitch triggers the game mode change for supertile transitions. The important information to know is 2 part:

1. These game mode changes are done via incrementation
2. The data loading routine is done immediately, from within the default submodule

The first problem explains why we are able to perform many door glitches in the first place. The second problem explains why data spoofing happens when it does. Let's look at 2 pairs quick examples:

Example 1: YBAs

Using a red YBA on a subtile door, we corrupt the game mode to $0E/$05. Using it on a super tile door, we get $0E/$06. The subtile YBA only corrupted the red potion game mode with a subtile transition; ditto for the supertile YBA. Only the supertile YBA will result in a data spoof.

Example 2: Somaria

When we perform a somaria juke on a subtile door, we corrupt the game mode to $07/$03. This is by having the game run the subtile transition set up (+1) followed by the supertile transition set up (+2). When performing a somaria juke on a supertile door, we get the same game mode, but from a different order. In those cases, we get a supertile set up first (+2), then then subtile set up (+1). In both cases, we get the same corrupted game mode. We also touched the supertile set up for both, so both somaria jukes will result in data spoofs.

Organization of data

Room flags are stored as a bit field in 3 main addresses in WRAM:

Address $0401 Bit Name Description Value 7 D0 Door 0 opened 27 $80 6 D1 Door 1 opened 26 $40 5 D2 Door 2 opened 25 $20 4 D3 Door 3 opened 24 $10 3 u Unused 23 $08 2 u Unused 22 $04 1 u Unused 21 $02 0 u Unused 20 $01

Address $0403 Bit Name Description Value 7 H Heart container obtained/Boss killed 27 $80 6 K Key obtained 26 $40 5 c Unused 6th chest or 2nd key 25 $20 4 C5 Chest 5 opened Rupee tiles grabbed 24 $10 3 C4 Chest 4 opened 23 $08 2 C3 Chest 3 opened 22 $04 1 C2 Chest 2 opened 21 $02 0 C1 Chest 1 opened 20 $01

Address $0408 Bit Name Description Value 7 u Unused 27 $80 6 u Unused 26 $40 5 u Unused 25 $20 4 u Unused 24 $10 3 Q4 North-west quadrant visited 23 $08 2 Q3 North-east quadrant visited 22 $04 1 Q2 South-west quadrant visited 21 $02 0 Q1 South-east quadrant visited 20 $01

When a new room is loaded, these values are transferred to SRAM, creating a new bit field. In the table caption below, X is equal to 2×Room ID.

Address $7EF000, X

Bit	Name	Description	Value
15	D0	Door 0 opened	215	$8000
14	D1	Door 1 opened	214	$4000
13	D2	Door 2 opened	213	$2000
12	D3	Door 3 opened	212	$1000
11	H	Heart container obtained/Boss killed	211	$0800
10	K	Key obtained	210	$0400
9	c	Unused 6th chest or 2nd key	29	$0200
8	C5	Chest 5 opened / Rupee tiles grabbed	28	$0100
7	C4	Chest 4 opened	27	$0080
6	C3	Chest 3 opened	26	$0040
5	C2	Chest 2 opened	25	$0020
4	C1	Chest 1 opened	24	$0010
3	Q4	North-west quadrant visited	23	$0008
2	Q3	North-east quadrant visited	22	$0004
1	Q2	South-west quadrant visited	21	$0002
0	Q1	South-east quadrant visited	20	$0001

Unique uses

Several rooms use the room flags slightly differently:

ID	Room	Bit	Function
$37	Swamp switch 1	C4	Reuses chest 4 as a flag for whether the room has been (un)flooded
$35	Swamp switch 2
$66	Swamp switch 3
$65	Thieves' Town attic	C5	Reuses chest 5 as a flag for whether or not the bombable floor has been broken
$1B	Palace of Darkness moving wall	C4	Reuses chest 4 as a flag for whether or not the wall has been moved
$43	Desert Palace moving wall
$97	Misery Mire torches	C5	Reuses chest 5 as a flag for whether or not the wall has been moved

Dungeon Prizes

Crystals and pendants don't set any bit in room data when collected, but they do specifically check for the Key bit before they spawn. This is likely vestigial behavior from an earlier point in development, but the consequence of it is that spoofing a collected key into a boss room will prevent its prize from being dropped.