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nes-contra-us/docs/Level Headers.md

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Overview

Contra loads various level-specific configuration data from in bank 2. These are called "level headers". These headers are #$20 (32) bytes for each level and contain information about the level, for example whether the level is and indoor level, or an outdoor level, vertical level or horizontal level, etc.

In the code, the level headers are labeled level_headers and each level is labeled with an appropriate level_x_header label.

Byte Offset 0 - Location Type

  • Offset: 0
  • Memory Location: $40
  • Variable Name: LEVEL_LOCATION_TYPE

The first byte is the level's "location type". There are 3 location types

  • Outdoor - #$00
  • Indoor/Base - #$01
  • Indoor Boss - #$80

Outdoor levels are typical platformer side-scrolling levels and scrolls horizontally or vertically. Indoor levels are also known as base levels and are the levels where Bill and Lance are facing away from the player in a pseudo 3D experience (Levels 2 and 4).

Although the value is never #$80 in the level headers table, for the boss fights on the indoor levels (2 and 4), the location type will be set to $80. It is similar to the indoor location type (#$01), except the player cannot go prone and shooting up looks like shooting forward (z-plane). When animating the players walking into the screen between 3D screens, the value is also set to #$80.

Byte Offset 1 - Outdoor Scrolling Type

  • Offset: 1
  • Memory Location: $41
  • Variable Name: LEVEL_SCROLLING_TYPE

The second byte is the level's scrolling type. It is only used in outdoor levels.

  • Horizontal - #$00
  • Vertical - #$01

Byte Offsets 2 and 3 - Screen Super-Tile Pointer

  • Offset: 2 and 3
  • Memory Location: 2-byte address $42 and $43
  • Variable Name: LEVEL_SCREEN_SUPERTILES_PTR

The 3rd and 4th bytes make up the pointer that point to the data that specifies which super-tiles are on each screen. This is an address into Bank 2. The address is 2 bytes and stored in little-endian, i.e. the least significant byte first.

For example, on level 3, the screen super tile pointer points to level_3_supertiles_screen_ptr_table (ROM: $84ce, MEM: Bank 2 $84ce). This address contains a list of pointers for each "screen" of the level. A screen is a set of super-tiles for a level that is 256 pixels wide. Since level 3 is a vertical level, each screen is 64 super-tiles (8x8). Looking at the data specified at the address of a specific screen pointer, e.g. level_3_supertiles_screen_01 (ROM: $8635, MEM: Bank 2 $8635), you will find an RLE-encoded set of bytes that specify the super-tile indexes used. After decompressing, each byte is the offset into the level_3_supertile_data (ROM: $cef8, MEM: Bank 3 $8ef8).

Byte Offsets 4 and 5 - Super-Tile Data

  • Offset: 4 and 5
  • Memory Location: 2-byte address $44 and $45
  • Variable Name: LEVEL_SUPERTILE_DATA_PTR

The 5th and 6th bytes of the level headers are a pointer to the level's super-tiles. Each entry in table at the specified address is #$10 bytes long and is a single super-tile. This data is not RLE-encoded. Each byte in each super-tile represent a pattern table tile. An example pointer is level_3_supertile_data (ROM: $cef8, MEM: Bank 3 $8ef8)

Byte Offsets 6 and 7 - Super-Tile Palette Data

  • Offset: 6 and 7
  • Memory Location: 2-byte address $46 and $47
  • Variable Name: LEVEL_SUPERTILE_PALETTE_DATA

Address to byte table that specifies the palette used for each super-tile in the level. Each byte describes the 4 palettes for a single super-tile. An example is level_2_palette_data

Byte Offset 8 - Alternate Graphics Loading Location

  • Offset: 8
  • Location: $48
  • Variable Name: LEVEL_ALT_GRAPHICS_POS

This byte specifies the alternate graphics loading location. This is the screen where the game has the ability to change out the pattern table tiles for a level.

Byte Offsets 9, 10, and 11 - Tile Collision Limits

  • Offset: 9, 10, and 11
  • Location: $49, $4a, $4b
  • Variable Name: COLLISION_CODE_1_TILE_INDEX, COLLISION_CODE_0_TILE_INDEX, and COLLISION_CODE_2_TILE_INDEX (in that order)

These three bytes are used to specify which pattern table tiles are which collision codes.

  • Pattern table tiles below the value in $49 (excluding #$00) are considered Collision Code 1 (floor).
  • Pattern table tiles >= the value in $49, but less than the value in $4a are considered Collision Code 0 (empty)
  • Pattern table tiles >= $4a but less than this tile index are considered Collision Code 2 (water)

Pattern table tiles above the last entry, are considered Collision Code 3 (Solid).

Byte Offset 12, 13, 14, and 15 - Cycling Palette Colors

  • Offset: 12, 13, 14, and 15
  • Location: $4c, $4d, $4e, and $4f
  • Variable Name: LEVEL_PALETTE_CYCLE_INDEXES

Each level natively supports having its 4th nametable (background) palette cycle through up to 4 different sets of colors, where each set of colors is represented by an index into the game_palettes table. This header section contains those specific indexes to cycle through.

For more details see Graphics Documentation.md. There is a section titled Palette -> Cycling.

Byte Offset 16, 17, 18, and 19 - Nametable Palette Initial Colors

  • Offset: 16, 17, 18, and 19
  • Location: $50, $51, $52, and $53
  • Variable Name: LEVEL_PALETTE_INDEX

Initial background tile palette colors for the level. Indexes into game_palettes.

Byte Offset 20, 21, 22, and 23 - Nametable Palette Initial Colors

  • Offset: 20, 21, 22, and 23
  • Location: $54, $55, $56, and $57
  • Variable Name: LEVEL_PALETTE_INDEX

Initial sprite tile palette colors for the level. Indexes into game_palettes.

Byte Offset 24 - Scroll Stop Screen

  • Offset: 24
  • Location: $58
  • Variable Name: LEVEL_STOP_SCROLL

The screen of the level to stop scrolling. The value in memory is set to #$ff when boss auto scroll starts

Byte Offset 25 - Solid Background Collision

  • Offset: 25
  • Location: $59
  • Variable Name: LEVEL_SOLID_BG_COLLISION_CHECK

Specifies whether to check for bullet and weapon item solid bg collisions.

  1. When non-zero, specifies that the weapon item should check for solid bg collisions (see weapon_item_check_bg_collision)
  2. When negative (bit 8 set), specifies to let bullets for both players and enemies to check for bullet-solid background collisions. This is used on levels 6 - energy zone and 7 - hangar. (see check_bullet_solid_bg_collision and enemy_bullet_routine_01)

Byte Offset 26 through 32 - Unused

The last 6 bytes in each level's level header are unused and set to #$00.