skids/docs/FORMATS.md

Super Skidmarks data formats

Notes from reverse engineering the AGA/CD32 release of Super Skidmarks (Acid Software, 1995), worked out from all six disks. The one thing not yet decoded is the executable's game logic. See Gaps.

All multi-byte values are big-endian unless noted.

Disks

Each disk is a standard 880 KB double-density ADF (1760 blocks of 512 bytes). Disk 1 is FFS (DOS\1); disks 2–6 are OFS (DOS\0). The [HD] in the file names refers to the hard-drive-installable release.

Disk	Archive	Game files
1	smcd32a.lha	Executables, sfx/, gfx/, the tune, vehicles/*.set, Camaro.aga, caravan.aga
2	smcd32b.lha	vehicles/cow.aga, track/TRACK1
3	smcd32c.lha	track/TRACK2, vehicles/TRUCK.aga
4	smcd32d.lha	track/TRACK3, vehicles/PORSCHE.aga
5	smcd32e.lha	track/track4, vehicles/vw.aga
6	smcd32f.lha	vehicles/F1.aga, Midget.aga, mini.aga, Police.aga

Disk 1 also carries an Installer script that installs from all six disks, and a few AmigaDOS commands.

Each archive stands on its own; the set isn't one archive split across disks. They use level-0 headers under a top-level super_skidmarks_cd³²\ directory.

• Compression: disks 2–6 store their members (-lh0-), which are themselves CrunchMania-packed. Disk 1 mostly uses -lh5- (LZSS with static Huffman blocks, decoded by src/formats/lzh.rs).
• Integrity: every member is checked against its header CRC-16. One file, c/iconx (an AmigaDOS utility the game doesn't use), fails its CRC in this dump. The reference lha tool reports the same failure.

CrunchMania

0   4   id: "CrM!" (LZ + fixed codes) or "CrM2" (LZ + Huffman)
        lower-case "Crm" = delta-encoded samples (running byte sum)
4   2   unused
6   4   unpacked size
10  4   packed size (excludes this 14-byte header)
14  …   packed stream

The stream is read backwards from 14 + packed - 6, a byte at a time, LSB first. The last six bytes seed the bit buffer: a 32-bit word shifted right by 16 - shift with 16 + shift valid bits, where shift is the final 16-bit word. Output is also written from the end towards the start. src/formats/crm.rs implements both variants. Every packed file on the disks decodes exactly: the output buffer fills completely and the input is consumed to its first byte.

Tracks (track/TRACKn)

0x00  7 × u32   end offset of sections 0..6
0x1C  …         section 0: thumbnails and palette for the whole file (20622 bytes, raw)
…               sections 1..6: one track each, CrunchMania-packed

So the four files hold 24 tracks.

The last 142 bytes of section 0 hold:

Offset	Size	Contents
0	2	File number (1–4)
2	96	The palette shared by the file's six tracks: 16 colours × R, G, B, each a u16 from 0 to 15 (Amiga 12-bit colour)
98	12	Unknown
110	…	The name of the set of tracks, NUL-terminated

The first 20480 bytes of section 0 are the track-selector thumbnails:

• Picture: one 640 × 64, 16-colour picture in the file's palette, stored as four bitplanes one after another (80 bytes per row, 5120 bytes per plane).
• Layout: the six tracks' thumbnails, 123 × 32 each, sit in a 3 × 2 grid at its top left, in track order: top row tracks 1–3, bottom row 4–6.
• Display: the picture is Amiga hi-res, whose pixels are half as wide as low-res ones. So a thumbnail shows at 123 × 64 next to the low-res track pictures, the same shape as the track.

Each track section unpacks to 281240 (0x44A98) bytes:

Offset	Size	Contents
0x0000	8192	Height map: 64 × 64 u16, 8.8 fixed point, row-major. Each cell covers 16 × 8 screen pixels.
0x2000	8192	Unused, zero on every track.
0x4000	8	u16 gate count, u16 start x, u16 start y, u16 flags.
0x4008	8 × n	Gates: x1 y1 x2 y2, world coordinates × 32. The list repeats after the last gate.
…		Unknown data, mostly a bit pattern, up to 0x4A98.
0x4A98	4 × 65536	Four non-interleaved 1024 × 512 bitplanes, giving 16 colours.

Track geometry

Gates are line segments across the road and run in race order; gate 0 is the start/finish line. They're defined on an isometric ground plane rotated 45° to the picture. The projection that best fits the data is:

screen x = wx - wy + 512
screen y = (wx + wy) × 31/64 - 252

With these constants, 96% of points sampled along the gates land on road pixels across all 24 tracks (92% minimum). A plain screen-aligned 2:1 squash gives 40–70%. Adding a height term doesn't improve the fit, so the height map is used only for slope physics and body tilt.

The stored start point is on the road for 14 of the 24 tracks read as screen coordinates, and 10 read as world coordinates. It isn't used; the grid forms up behind gate 0.

Road and scenery

Colours 8–15 are the driving surface on every track, so the fourth bitplane is effectively a road mask. Off the road means less grip and lower speed.

Vehicles

Sprite banks (vehicles/*.aga)

CrunchMania-packed, 409600 bytes unpacked: 25 tilt blocks × 32 headings × 512-byte frames.

A frame is a 64 × 32 AGA hardware sprite. Each line holds 8 bytes of plane 0, then 8 bytes of plane 1, giving 3 colours plus transparent:

Index	Use
0	transparent
1	tyres, shadow, dark
2	trim / accent
3	bodywork

• Heading: frame 12 faces screen-right, and frames advance clockwise in 11.25° steps. In world terms, frame = 12 + (angle + 45°) / 11.25°.
• Tilt: block = roll × 5 + pitch, each in 0..4, with 2 = level. Block 12 is the car on flat ground.

The caravan (caravan.aga) uses the same layout. It's a trailer with a tow bar, not a car.

Police.aga reads as all zeros with one third-party ADF tool, but it's intact when read directly from the disk image.

Settings (vehicles/*.set)

• Header: the car's name and its driver's name, each followed by a newline.
• Settings: 16 u16 values. Eleven are used by the cars; the rest are zero. The caravan's values differ in kind; two of them are −4.
• Menu picture: 512 bytes. A 64 × 32 picture in two bitplanes, plane 0 (256 bytes) then plane 1.

The settings are clearly handling values. The first is smallest for the Cow, the second largest for the F1, and the Police car and Porsche are nearly identical. What each one means lives in the executable and isn't decoded yet.

Sounds (sfx/*.iff, gfx/*.iff)

Uncompressed IFF 8SVX, 8-bit signed, mostly at Amiga rates (8363 Hz and others). Looping sounds use the repeat part of VHDR. gfx/ holds a few extra and alternative sounds (crash1, crash2, engine, motor, screech); gfx/crash4.iff isn't IFF.

Pictures (gfx/*.iff)

IFF ILBM with ByteRun1 compression. skidtoon.iff is the 608 × 180, 16-colour title picture; chat2, mouse and sign are small odds and ends.

ball.spr is a bank of 64 saved hardware sprites: a spinning ball with the Acid logo. Each 544-byte frame is:

• Header (16 bytes): u32 saved memory pointer, u16 lines (33), u16 strips (4), u16 flags (0x8020), u16 bytes per strip (132), 4 unused bytes.
• Strips: four 16-pixel sprites, each line two bitplane words.

Flag bit 15 marks attached sprites. The strips pair up into two 16-colour columns, so a frame is 32 × 33 pixels. The bank carries no colours.

Music (sfx/tune.crn)

A CrunchMania-packed OctaMED MMD1 module: 4 tracks, 21 plain 8-bit sample instruments, 70 blocks played in order. The main song is positions 0–65, ending with a jump back to the start. Positions 66–69 hold separate short pieces that end with "stop".

The song uses these commands:

• 1, 2 (slide up, slide down)
• 3 (portamento)
• 9 (speed)
• B (jump)
• C (volume, decimal)
• D (volume slide)
• F00 (break), F1 (play twice), F2 (delay), FD (set pitch), FE (stop)

src/music.rs plays these. Its loudness tracks OpenMPT's rendering of the module second by second through the whole song (correlation 0.996 over the first minute), and it loops at the same point.

Executables

• SKIDMARKS_CD³² (237 KB): a single-hunk compiled Blitz Basic 2 program. It includes IFF ILBM/8SVX/ANIM loaders and a tracker replay routine with note-period tables.
• skidmarks2 (90 KB): a three-hunk program whose code is packed.

Gaps

• Game logic. The handling, AI and rules in the executable aren't decoded; this port's versions are reconstructions. Decoding the .set values would be the first step towards the original handling.
• Unknown data: the 12 bytes after each track file's palette, the bit pattern after the gates in each track section, and the colours of ball.spr (probably set by the executable).

The engine's own editable formats, which skids extract converts all of the above into, are described in assets/README.md.