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notes.txt
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Structure at $D15E - level attributes
$D15E - Viewport Flags
$D160 - Index of block at left of screen on x-axis (i.e.
camera hpos / 32). See subroutine at $4A9B.
$D161 - Index of block at top of screen on y-axis (i.e.
camera vpos / 32). See subroutine at $4A9B.
$D162 - Number of bank containing 32x32 mappings.
$D163 - Number of bank containing level layout data.
$D164 -> $D165 - Pointer to 32x32 mappings
$D166 -> $D167 - Pointer to level layout data.
$D168 -> $D169 - Pointer to table of multiples of level width.
This is used by collision routines to quickly index
a specific row in the level data. See subroutine at
$7481. Tables are stored in "level_width_multiples.asm".
$D16A -> $D16B - 2's complement of level width in blocks.
$D16C -> $D16D - Width of level (in mapping blocks).
$D16E -> $D16F - Vertical offset into layout data. This should be
set to the row that appears at the bottom of the
screen when the player first starts the level.
This should always be 7 * level width in blocks.
It is used to load tiles into VRAM as the level
scrolls up.
===============================================================================
Miscellaneous Variables
$D12A - Copy of current page number for frame 1.
$D12B - Copy of current page number for frame 2.
$D12C - Level Select flag
$D12D - PAL/NTSC tv flag?
$D12F - Frame counter.
$D132 - Palette reset trigger. When set copies the data at
$65A to VDP colour RAM. See subroutine at $5B2.
$D134 - SAT update trigger.
$D15E -> $D16F - Level attributes (see above).
$D172 - VDP vert. scroll working copy
$D173 - VDP horiz. scroll working copy
$D174 -> $D175 - Horiz cam offset in level?
$D176 -> $D177 - Vert cam offset in level?
$D178 -> $D1B7 - Holds tile data for the next column of mappings
to be copied into VRAM. (see subroutine at $585B).
8 mappings * 4 tiles * 2 bytes-per-tile = 64 bytes.
$D1F8 -> $D23F - Holds tile data for the next row of mappings to be
copied into VRAM (see subroutine at $5966).
$D278 -> $D279 - Address of mapping block for the current
camera position (points into level layout
in RAM - $C001).
$D27C -> $D27D - Minimum camera Y position
$D27E -> $D27F - Maximum camera Y position
$D280 -> $D281 - Minimum camera X position
$D282 -> $D283 - Maximum camera X position
$D284 -> $D285 - Horizontal cam offset in level
$D286 -> $D287 - Vertical cam offset in level
$D288 - Horizontal cam adjust (for looking left/right).
$D289 - Vertical cam adjust (for looking up/down).
$D28A - copy of horizontal cam adjust
$D28B - Right edge of camera (horizontal cam adjust + 8)
$D28C - Left edge of camera (horizontal cam adjust - 8)
$D28D - Copy of Camera Y offset
$D28E - Bottom edge of camera (camera y offset + 16)
$D28F - Top edge of camera (camera y offset - 16
$D292 - Game mode?
$D295 - Current level
$D296 - Current Act
$D297 - Number of lives when starting act
$D298 - Current number of lives
$D299 - Ring counter
$D2B8 - Level timer update trigger.
$D2B9 -> $D2BA - Level timer.
$D2BC -> $D2BD - Level timer frame counter. When the counter reaches
the specified threshold (defaults to $3C) the level
time is incremented.
$D2C7 - Tile attribute byte to use when copying char data
to VRAM. Used by VDP_DrawText.
$D2CA -> $D2CB - Initial screen x pos for current level/act
$D2CC -> $D2CD - Initial screen y pos for current level/act
$D2CE -> $D2CF - Camera horizontal lock position?
$D2D0 -> $D2D1 - Camera vertical lock position?
$D2D2 -> $D2D3 - Address of next controller byte for demo sequences.
$D2D4 -> $D2D5 - Pointer to collision data
$D2D6 - Update object layout trigger. Incremented each frame.
Causes an object layout update to take place every
12th frame. See subroutine at $09E4
$D34E - Player object art flags: (7 = art update required)
$D34F - Player object art frame index (used to determine which tile
patterns top copy to VRAM). Value comes from byte at
$D506 - see $1274.
$D351 - Ring art frame number. Used to animate rotating rings.
$D352 - Used by ring block collision routines ($739A)
to store the 0-based ring block index.
$D353 - Current mapping number. Used by collision routines.
$D354 -> $D355 - Address of current 32x32 block.
$D356 -> $D357 - Adjusted horizontal position in level. Used by
collision routines.
$D358 -> $D359 - Adjusted vertical position in level. Used by
collision routines.
$D35A -> $D35B - X-Coordinate of block involved in collision.
$D35C -> $D35D - Y-Coordinate of block involved in collision.
$D35E - Block surface type.
$D35F -> $D360 - ?
$D361 - Horizontal collision projection value?
$D362 - Mapping block's collision value (see subroutine
at $7481).
$D363 - Metatile speed modifier value.
$D364 - Copy of metatile speed modifier - see $6C3C.
$D365 - Copy of mapping number at collision offset 0x0 - see
subroutine at $6C3C.
$D366 - Copy of metatile surface type - see $6C3C.
$D367 - Copy of mapping number at collision offset 0x0 - see
subroutine at $6C3C.
$D369 -> $D36A - Pointer to working copy of SAT VPOS attributes
$D36B -> $D36C - Pointer to working copy of SAT HPOS/char code attributes
$D36D -> $D36E - Maximum horizontal velocity.
$D36F -> $D370 - Horizontal acceleration. inertia?
$D371 -> $D372 - Gradient horizontal speed adjustment. Used to
alter speed when standing on a slope.
$D39A - Player's position within a loop (GHZ/ALZ)
$D39D - Monitor collision type. See "src\object_logic\logic_monitors.asm".
$D39E -> $D39F - Used by minecart logic to store pointer to minecart
object descriptor.
$D3A8 - "Player Hurt" trigger. Causes player to lose rings
or die.
$D3A9 - Flashing sprite timer.
$D3AA - Jump hold counter.
$D3AB - Pattern load cue trigger.
$D3AC - PLC: source bank. Can be zero if the PLC handler is
designed to use a specific bank (e.g. chaos emeralds).
$D3AD - PLC: number of bytes to copy.
$D3AE -> $D3AF - PLC: source address to copy from.
$D3B0 -> $D3B1 - PLC: destination address in VRAM.
$D3B2 -> $D3B3 - PLC descriptor address.
$D3B4 -> $D3B5 - Idle time counter.
$D3B9 - ??? used by routine at $3B4E
$D3BC -> $D3C4 - 8-byte array used by SHZ3 boss to keep track of
child objects (e.g. small birds & fireballs).
$D400 -> $D418? - Array of active objects. Each element in the array corresponds
to an object descriptor. For example, element 0 represents the
descriptor at $D500 and element 1 represents $D540, etc. The
array holds the object numbers currently occupying each
descriptor slot.
$D465 - Copy of collision value of current block.
$D466 - Collision value of previous block. Written by
routine at $7378.
$D467 - Set to FF when sonic is under water.
$D468 - Under water air timer lo-byte.
$D469 - Under water air timer hi-byte (incremented when lo-byte == $78).
$D46A - SHZ3 Boss "Destroy Children" trigger. Set when the main boss
has been destroyed to tell the logic to clean up any child
objects that were spawned by the main boss object.
$D46D - Counter used by the end credits.
$D46E - Used by ending sequence as index into array of credits.
$D46F -
$D49F - "Lose Life" trigger. Player dies when set (regardless
of ring count).
$D4A2 - Boss flag?
Set by UGZ3 boss cannonball during state 0.
Set by SHZ3 boss (big bird) during state 0.
$D4A3 - "Palette update only" flag. When set, only updates VDP CRAM
during the vblank interrupt.
$D4A4 - Level water-level. When the player Y-pos value is below this
the "UnderWater" flag ($D467) is set and the air timer begins.
$D4E6 - Background Palette control byte. Used to trigger palette fades.
$D4E7 - Background palette number.
$D4E8 - Sprite palette control byte.
$D4E9 - Sprite palette number.
$D4EA - Palette Update control byte. When set triggers copying of
palette to colour RAM.
===============================================================================
Other RAM Values:
$C000 -> $CFFF : Level layout Mappings. Each byte refers to a 32x32 mapping block.
$DB00 -> $DBBF : Working copy of SAT
===============================================================================
Object Descriptor Structure
Each object occupies a 64-byte descriptor structure in memory. The structures
are allocated slots of memory starting from $D500. For example, the first
object will be allocated the slot at $D500, the second will be allocated the
slot at $D540, and so on.
Typically, the high priority objects (player object, invincibility stars, etc)
are placed into slots between $D500 and $D700. Lower priority objects (e.g.
badniks) are placed in slots from $D700 onwards. There are 24 descriptor slots
in this range.
Details for the object descriptors follow (based on sonic object).
Location: $D500 - Player object descriptor
$D500 - Object number
$D501 - Current object state number
$D502 - Object state number (working copy - copied to
"Current object state").
$D503 - Flags?
$D504 - Flags?
$D505 - Number of sprites in the object.
$D506 - Current animation frame number.
$D507 - Frame display time counter.
Load next animation frame when counter == 0.
$D508 - Added to VRAM tile index for right-facing
sprites.
$D509 - Added to VRAM tile index for left-facing
sprites.
$D50C -> $D50D - Pointer to logic subroutine for current state.
$D50E -> $D50F - Pointer to logic sequence data for object's
current state.
$D510 -> $D512 - Horizontal position in level (3-bytes)
$D513 -> $D515 - Vertical position in level (3-bytes)
$D516 -> $D517 - Horizontal speed.
$D518 -> $D519 - Vertical speed.
$D51A -> $D51B - Horizontal offset on screen (calculated by
routine at $3A3B).
$D51C -> $D51D - Vertical offset on screen (calculated by
routine at $3A3B).
$D520 - Index of object that is colliding with
the player (see subroutine at $68E7).
$D521 - Collision flags (object -> background tiles)
$D522 - Collision flags (object -> object).
$D523 - Combination of background & object collision
flags?
$D528 -> $D529 - Pointer to sprite arrangement data for object's
current animation frame.
$D52C - Sprite width in tiles
$D52D - Sprite half-height?
$D52E - Counter used to toggle visibility object's
sprites.
$D532 - Current power up (invincible/speed shoes/etc).
$D53A -> $D53B - Initial horizontal pos (set by object load
routines).
$D53C -> $D53D - Initial vertical pos (set by object load
routines).
$D53E - Index of object in active objects array ($D400).
$D53F - Object param byte (see object load routines).
===============================================================================
Sound Channel Structure
$00 - Channel control.
$01 - Channel number.
$02 - Used as a multiplier value @ $843E
$03 -> $04 - Pointer to next command
$05 - Tone adjustment value.
$06 - Volume adjustment value?
$08 - Volume change trigger.
$09 - Channel stack pointer. Each channel has a
simple empty descending stack starting at
ix + $25.
$0A - Tone duration counter.
$0B -> $0C - Tone data.
$0D - Working copy of tone duration counter.
$25 - Tote adjustment value. Added to any tone data
before writing to the PSG channel.
===============================================================================
Object Animation Sequences
Animation sequence definitions are stored in bank 31 starting at address
$8000.
There are a series of pointers (1 for each object) between $8000 and
$80C0 which point to the animation sequence for the object. Each sequence
has another series of pointers for each frame.
Animation frames are defined by an 11-byte structure:
Byte | Description
--------+--------------------------------------
1 | Number of sprites
2 | ?
3 | ?
4-5 | Pointer to sprite arrangement data.
6-7 | Vertical position offset.
8-9 | Horizontal position offset.
10-11 | Pointer to VRAM char codes.
===============================================================================
Object Sprite Arrangement
Two words at each position represent the vertical and horizontal
offsets, relative to the object's X/Y location in the level, at
which to draw each sprite.
e.g. An object composed of 3x2 sprites might be arranged like
this (see data at Bank 31, $9DBE which is used for sonic's
sprites):
Data at $9DBE:
.dw $FFE0, $FFF4
.dw $FFE0, $FFFC
.dw $FFE0, $0004
.dw $FFF0, $FFF4
.dw $FFF0, $FFFC
.dw $FFF0, $0004
Gives this arrangement:
-12 -4 +4
.----.----.----.
-32 | | | |
| 1 | 2 | 3 |
| | | |
|----+----+----|
-16 | | | |
| 4 | 5 | 6 |
| | | |
0 '----'----'----'
0
You can see from the diagram that the sprite's origin is
bottom-centre.
===============================================================================
Player Object Animations
Tile art for each of the player object's animation frames is copied to
VRAM address $0000 on an as-needed basis (see routine at $10BF). In order
to do this the routine uses the structures in Bank 31 at address $BCCB.
Each 4-byte structure describes a single frame of animation. The first byte
is the number of the bank which contains the tiles. Following word is a
pointer to the tiles for the current frame. The final byte is the sprite
count (i.e. tile_count/2); Sprites are 8x16 so 2 tiles are used for each
sprite.
For example, the idle animation consists of 3 frames:
;frame 1
.db $04 ;bank
.dw $8900 ;address of tile art
.db $06 ;sprite count
;frame 2
.db $04
.dw $AF80
.db $06
;frame 3
.db $04
.dw $B100
.db $06
===============================================================================
Object Logic
Object logic is spread over 3 banks. Logic for object numbers 0 to 31 is
stored in bank 31, logic for objects 32 to 79 is stored in bank 28 and
logic for objects 80 and onwards is stored in bank 30.
Each logic entry consists of either an instruction to change the animation
frame or a command. Frame change instructions consist of 4 bytes:
Byte | Description
------------+-------------------------
1 | Display count. decremented each frame. when count == 0
| next frame is displayed.
2 | Frame number. Index of next frame do display.
3 - 4 | Pointer to input handler for current frame.
Command bytes are variable length and are marked with a preceeding $FF byte.
Command Bytes:
Value | Operands | Description
------------+---------------+------------------------------------------------------------
00 | None | Marks end of animation sequence. Loop back to start of
| | current animation.
01 | None | Deallocate the current object. Always used in place of
| | $FF, $00.
02 | 2 bytes | Operand is a pointer to a subroutine in bank 2.
| | LABEL_6087 is pushed to the stack as the return address
| | for the sub. Animation continues as per LABEL_6087.
03 | None | Do-nothing stub. Exectutes a RET.
04 | 4 bytes | First word is the new horizontal speed.
| | Second word is the new vertical speed.
05 | 1 byte | Change sprite state. Operand gets copied to ix+$02 as
| | the new sprite state.
06 | 6 bytes | Load a new sprite. First byte is the sprite number.
| | Following word is added to horizontal position of current
| | sprite & stored as new sprite's horizontal position. Next
| | word is added to current sprite's vertical position and
| | stored as new sprite's vertical position. Last byte is
| | copied to (iy+$3F). Animation continues as per $6087.
07 | 4 bytes | First word is a function pointer. $60AC is pushed
| | as the return address for the subroutine. Second word is
| | pointer to subroutine to be loaded as the logic for the
| | current frame (copied to (ix+$0C).
08 | 1 byte | Operand is copied to ($D3AB) - triggers loading of chaos
| | emeralds & monitors. Animation continues as per $6087.
09 | 1 byte | If msb of operand is set, triggers playing a sound.
| | If msb is not set sound is stopped.
0A | None | Animation continues as per LABEL_6087.
0B | None | Animation continues as per LABEL_6087.
0C | None | Animation continues as per LABEL_6087.
0D | None | Animation continues as per LABEL_6087.
0E | None | Animation continues as per LABEL_6087.
0F | None | Animation continues as per LABEL_6087.
| |
For example:
Object 01 (Sonic), State 04 (Looking up):
Sonic_State_04: ;$A736
.db $06, $10 ;display animation frame 6 for 16 iterations of the game loop.
.dw VF_Player_HandleCrouched ;logic subroutine for the frame
.db $FF, $00 ;command byte to run this logic sequence again.
===============================================================================
Collision Reaction
Bitmask at $D39D - Collision with monitor - monitor type:
Bit | Type
-----+---------
0 | 10 Rings
1 | Life
2 | Sneakers
3 | Invincibility
4 | Unknown (increments $D2BD)
5 | Unknown (does nothing)
6 | Unknown (does nothing)
===============================================================================
Acceleration Data
Acceleration values are arrays of signed 16-bit values.
struct
{
s16 accelLeft;
s16 accelRight;
};
===============================================================================
VRAM Notes
Sprite tiles written to VRAM between $0000 and $2000.
Level tiles normally written to VRAM between $2000 and $3800. Note that
UGZ writes level tiles between $2100 and $3800 and ALZ writes level tiles
between $20C0 and $3800.
Name table at $3800.
Sprite attribute table at $3F00.
A working copy of the SAT is kept in RAM at $DB00. $DB00->$DB3F = VPOS
attributes. $DB40->$DB7F = HPOS + char codes.
SAT updates are performed in ascending order for even framecounts and
descending order for odd framecounts. This way, overlapping sprites are
drawn in alternating order for each frame. Player sprite attributes are
always written first.
The Off-Screen Tile Buffer:
----------------------------------------------------------
The engine maintains an off-screen tile buffer around 3 edges of the
viewport. The buffer consists of 2 rows of tiles above and below the
camera and one row of tiles to the left. This ensures smooth scrolling
as the player moves through the level.
A lookup table of offset values to this buffer starts at address $59DB.
The offsets are all relative to the nametable address (i.e. $3800) and
are used by tile loading routines to copy the tile values to the
correct address in VRAM (see sub $58BA for loading columns and $5966
for loading rows of tile values).
===============================================================================
Notes on tile loading routines
Sprite tiles are loaded either at the start of the level or by a PLC
that is set during the progression of the level.
At least 3 pieces of information are required to load a tileset into
VRAM: the ROM bank number to swap into frame 2, the source address
in ROM & the destination address in VRAM. For uncompressed tilesets
the number of bytes to copy is also required; For compressed tilesets
this can be inferred.
Note that, for some objects, only the right-facing sprites are stored
in ROM. The left-facing sprites can be generated on the fly by the
engine simply by adding $80 to the bank number (i.e. set the MSB).
Examples of this are the Newtron and Motobug badniks that are both
specified for inclusion in the UGZ tileset (see "zone_tilesets.asm").
For information on how the engine performs the horizontal mirroring
of right-facing sprites see the subroutine "WriteMirroredTileToVRAM"
in "tile_loading_routines.asm" and the comments for
"Data_TileMirroringValues" in "s2.asm".
===============================================================================
Bitfield at $D15E - Level viewport flags
Bit 4 is set by subroutine at $5920 after a row of mapping tile data has
been copied into work RAM. This signales that the row needs to be updated
in VRAM.
Bit 5 is set by subroutine at $585B after a column of mapping tile data
has been copied to work ram. This signals that the column needs to be
updated in VRAM.
Bit | Action when Set | Action when Reset
-----+------------------------------+-------------------------------------
0 | Level scrolling up |
1 | Level scrolling down |
2 | Level scrolling left |
3 | Level scrolling right |
4 | VRAM update required for row | VRAM data is up-to-date.
5 | VRAM update required for col | VRAM data is up-to-date
6 | Camera update required | No camera update required.
7 | Camera locked. | Camera free.
-----+------------------------------+------------------------------------
===============================================================================
Bitfield at $D292
Bit | Action when Set | Action when Reset
-----+----------------------+------------------------
0 | |
1 | |
2 | |
3 | CPU controls sonic. | Player controls sonic.
4 | |
5 | |
6 | |
7 | |
-----+----------------------+-------------------------
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Bitfield at $D293 - Game State
Bit | Action when Set | Action when Reset
-----+-----------------------------+------------------------
0 | |
1 | |
2 | Title card - load level. |
3 | Player dead. |
4 | Score card. Load next zone. |
5 | Score card. Load next act. |
6 | | Game Over/Continue screen.
7 | Ending sequence mode. |
-----+-----------------------------+-------------------------
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BgPaletteControl ($D4E6) and FgPaletteControl ($D4E8) bitfields:
Bit | Action when Set | Action when Reset
-----+------------------+--------------------
0 | No action. |
1 | No action. |
2 | No action. |
3 | No action. |
4 | No action. |
5 | Reset to palette.|
6 | Fade to black. |
7 | Fade to colour. |
-----+------------------+---------------------
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Bitfield at $D503 - Object flags?
Bit | Action when Set | Action when Reset
-----+------------------------------+-------------------------------------
0 | Player in air. | Player on ground.
1 | Destroy enemy on collision. | Lose rings on collision with enemy.
2 | |
3 | |
4 | |
5 | |
6 | |
7 | Player Invnicible? |
-----+------------------------------+------------------------------------
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Bitfield at $D504 - Object sprite flags?
See subroutine at $17DF.
TODO: Check bit 6. Block fragment logic uses this as a deallocation trigger.
Bit | Action when Set | Action when Reset
-----+------------------------------+-------------------------------------
0 | Deallocate object. |
1 | |
2 | |
3 | |
4 | Object moving left. | Object moving right.
5 | Object flashing. | Object solid.
6 | Object's sprites not drawn. | Object's sprites drawn.
7 | Object's sprites not drawn. | Object's sprites drawn.
-----+------------------------------+------------------------------------
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Bitfield at $D521 - Collision flags - Object <> Object
Note: The player object version of this field differs from the regular
object's version; The field is shifted 4 bits to the left. See the sub-
routine at $68E7.
For the player object:
Bit | Action when Set | Action when Reset |
-----+------------------------------+-------------------------------------
0 | |
1 | |
2 | |
3 | |
4 | Collision at top edge | No collision at top edge
5 | Collision at bottom edge | No collision at bottom edge
6 | Collision at right edge | No collision at right edge
7 | Collision at left edge | No collision at left edge
-----+------------------------------+------------------------------------
For other objects:
Bit | Action when Set | Action when Reset |
-----+------------------------------+-------------------------------------
0 | Collision at top edge | No collision at top edge
1 | Collision at bottom edge | No collision at bottom edge
2 | Collision at right edge | No collision at right edge
3 | Collision at left edge | No collision at left edge
4 | |
5 | |
6 | |
7 | |
-----+------------------------------+------------------------------------
===============================================================================
Bitfield at $D522 - Collision flags - Object <> Background tiles
Bit | Action when Set | Action when Reset |
-----+------------------------------+-------------------------------------
0 | Collision at top edge | No collision at top edge
1 | Collision at bottom edge | No collision at bottom edge
2 | Collision at right edge | No collision at right edge
3 | Collision at left edge | No collision at left edge
4 | |
5 | |
6 | |
7 | |
-----+------------------------------+------------------------------------
===============================================================================