psxrecomp/docs/MOD_PACKAGES.md

PSXRecomp mod packages and features

A .psxmod is a versioned installation, provenance, and trust boundary. A package may contribute any number of independently configurable features. The launcher presents those features as the primary Mods list; package installation, version selection, and removal are a secondary management view.

Feature identity is always (package_id, feature_id). Enabling one feature never enables, disables, or reconfigures another feature.

The player selects a verified stock BIN/CUE. Resolution produces guarded native operations and sparse disc overlays without rewriting or replacing that stock image.

Feature manifest

format_version = 1
id = "example.localization"
version = "1.2.0"
name = "Example Localization Pack"
author = "Example Author"
description = "Independent title and script features."
license = "MIT"
resolver = "declarative"

[[target]]
game_id = "SLUS-00000"
# Required for disc overlays. Use the digest of the supported stock image.
disc_sha256 = "0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef"

[[feature]]
id = "title-screen"
name = "Title Screen"
description = "Selects the title-screen artwork."
group = "Localization"
default_enabled = false

[[feature]]
id = "retranslation"
name = "Retranslation"
description = "Uses the revised English script."
group = "Localization"

[[option]]
feature = "title-screen"
id = "variant"
label = "Title artwork"
type = "choice"
default = "rockman"

[[option.choice]]
value = "mega-man"
label = "Mega Man X6 (USA)"

[[option.choice]]
value = "rockman"
label = "Rockman X6 (Japan)"

[[patch]]
feature = "title-screen"
target = "main_exe"
address = 0x80041234
expected = "2a 00 02 24"
replace = "0e 00 02 24"
when = { variant = "rockman" }

[[overlay]]
feature = "retranslation"
target = "disc_raw"
offset = 123456
file = "assets/retranslated-script.bin"
sha256 = "..."
# Optional additional guard over the same range in the stock image.
expected_sha256 = "..."

Every [[option]], [[patch]], and [[overlay]] in a feature-style manifest must name its owning feature. Ambiguous operations are rejected.

Option types are boolean, choice, and bounded integer. Conditions are feature-local: when = { option = "value", ... } requires every listed option to match. The legacy when_option/when_value pair remains accepted for a single condition.

Bounded integer patches

Package format 2 can encode a bounded integer option directly into a guarded write:

format_version = 2

[[feature]]
id = "starting-lives"
name = "Starting Lives"

[[option]]
feature = "starting-lives"
id = "count"
label = "Lives"
type = "integer"
min = 0
max = 99
step = 1
default = 2

[[patch]]
feature = "starting-lives"
target = "main_exe"
address = 0x8001DE64
expected = "02 00 02 24"
replace_from = { option = "count", encoding = "u16le", offset = 0 }

[[patch]]
feature = "starting-lives"
target = "main_exe"
address = 0x8001DE70
expected = "03 00 02 24"
replace_from = { option = "count", encoding = "u16le", offset = 0, addend = 1 }

replace_from and literal replace are mutually exclusive. The referenced option must be a bounded integer owned by the same feature. The initial encodings are u8, u16le, and u32le. offset selects a byte field inside the expected guard and defaults to zero. Generated replacement bytes begin as an exact copy of the expected bytes, then the encoded value replaces only that field. This lets a MIPS immediate, for example, retain a guard and collision claim over its complete instruction. addend is the only supported transform, and the complete declared option range after that addend must fit the unsigned encoding.

There is deliberately no host-endian encoding, signed inference, mask, shift, scale, expression language, or partial-field merge. A package uses multiple guarded [[patch]] entries when the same value has multiple destinations. Generated bytes enter the ordinary pre-boot write plan, collision checks, and fingerprint. A generated value identical to the stock guard is omitted as a no-op, so an enabled stock-valued option does not claim or conflict on bytes it does not change.

Integer values use canonical decimal text. Leading plus signs, redundant leading zeroes, values outside the bounds, and values not aligned to step are rejected.

Ordered values and split MIPS immediates

Package format 3 adds feature-local ordering constraints for related integer fields:

format_version = 3

[[constraint]]
feature = "rank-thresholds"
kind = "ordered_integer"
direction = "nondecreasing"
options = ["rank-c", "rank-b", "rank-a"]

All listed options must be integer options on that feature. Defaults must satisfy the constraint. While a feature is enabled, an edit that would invert the order is rejected with the neighboring option labels. Disabled features may retain an incomplete or invalid draft, but cannot be enabled until it is valid. nonincreasing is also supported.

Format 3 also provides a narrow, typed transform for constants constructed by a linked MIPS LUI/ORI pair:

replace_from = {
  option = "speed",
  encoding = "mips_lui_ori_u32",
  omit_when_default = true
}

The patch must target one aligned, fully guarded eight-byte main_exe instruction pair. The loader verifies the opcodes and register linkage, then places the raw high and low 16-bit halves into the two immediates. It does not apply signed-ADDIU carry adjustment. offset and addend are not accepted for this encoding.

omit_when_default suppresses the entire patch when the selected value equals the option default. This models source tools whose declared default means "make no writes," including cases where multiple guarded sites contain different stock values. For any nondefault selection, every declared site retains its collision claim even if one generated replacement happens to equal its stock guard.

Sparse fields and integer predicates

Package format 4 separates a patch's complete expected-byte guard from the fields it owns and writes. This is for semantic records whose independently configurable fields share one useful guard:

format_version = 4

[[option]]
feature = "saber-timing"
id = "frames"
label = "Frames"
type = "integer"
min = 0
max = 99
default = 2

# Positive values change only byte 0.
[[patch]]
feature = "saber-timing"
target = "main_exe"
address = 0x80077640
expected = "02 42 01 02"
fields = [
  { offset = 0, option = "frames", encoding = "u8" },
]
when_integer = { option = "frames", op = "gt", value = 0 }

# Zero has a narrow, explicitly declared compound representation.
[[patch]]
feature = "saber-timing"
target = "main_exe"
address = 0x80077640
expected = "02 42 01 02"
fields = [
  { offset = 0, replace = "01" },
  { offset = 2, replace = "00" },
]
when_integer = { option = "frames", op = "eq", value = 0 }

fields is mutually exclusive with replace and replace_from. Every field has a nonnegative offset and exactly one payload form:

• replace = "..." supplies non-empty literal bytes; or
• option plus encoding uses a bounded integer option on the same feature, with the optional checked addend.

Fields must fit inside expected and their owned byte ranges may not overlap. The supported dynamic encodings are the existing u8, u16le, u32le, and linked mips_lui_ori_u32 forms. A sparse MIPS field owns only the two immediate halfwords while the complete linked instruction pair remains guarded; as with format 3, that encoding does not accept an addend.

Resolution omits individual fields whose generated payload already equals the guard. If no field changes, the patch is a no-op. Otherwise the plan retains the complete guard and the exact remaining owned ranges. Runtime guard validation checks every byte of expected before applying any writes, while collision detection and writing use only the owned fields. Thus two features can safely own adjacent bytes in one record without either overwriting the other. Overlapping guards must still agree on their expected bytes; conflicting guards make the plan unsatisfiable and are rejected.

when_integer is a patch-level, feature-local predicate over one bounded integer option. op is exactly one of eq, ne, lt, le, gt, or ge, and value is an integer constant inside the option bounds. Equality constants must also be selectable under the option's step. String-valued when conditions and one when_integer predicate may coexist and are ANDed.

Sparse fields and integer predicates are still pre-boot plan construction. They do not provide a general expression evaluator, masks, arithmetic beyond the checked field addend, package code execution, or per-frame dispatch.

Trusted static plugins

Package format 5 can activate a game-owned plugin that is already statically linked into the executable:

format_version = 5

[[feature]]
id = "warp-debug-menu"
name = "Warp Debug Menu"

[[plugin]]
feature = "warp-debug-menu"
id = "example.warp-debug"

The plugin id is a stable registry key, not a library path or symbol name. The package archive supplies no native code. Resolution fails before launch when an enabled plugin has no registered implementation or when two features claim the same plugin id. Active plugin identities and owners participate in the canonical plan fingerprint.

An implementation may register an activation callback, a deterministic guest-VBlank callback, or both under the same id. Activation runs after the launcher's final mod-plan commit and before renderer/window initialization; it is appropriate for a game-owned mod that selects a fixed display aspect or another pre-boot host feature. VBlank callbacks run from the emulated GPU VBlank event, independent of host presentation, pacing, turbo, or skipped frames. Trusted callbacks receive only the narrow C services exposed by runtime/include/mod_plugins.h. Games should continue to use declarative patches and overlays when those operations are sufficient.

Native operations

main_exe writes use PSX guest virtual addresses. Expected bytes are checked after the BIOS loads the executable, then the complete write plan is applied before its entry point. Changed executable ranges use the existing dirty-RAM interpreter/native-overlay machinery; untouched functions remain on the static native path.

Small disc_raw and disc_user patches are equal-length guarded writes and may not cross a sector boundary:

• disc_raw offsets use lba * 2352 + byte_in_sector.
• disc_user offsets use lba * 2048 + byte_in_sector.

File-backed [[overlay]] operations are intended for large assets and may span any number of sectors. Their paths must remain inside the archive. Payload size and SHA-256 are verified while scanning, but disabled payloads are not retained in memory. Enabled payloads are loaded and reverified during resolution, then indexed by target and LBA before boot. A CD read performs a direct indexed lookup rather than scanning every installed mod.

Feature disc overlays require an exact disc_sha256 on every target entry. expected_sha256 can additionally guard the replaced stock range.

State and migration

mods/state.toml format 2 stores selected package versions separately from per-feature enabled states and values:

format_version = 2

[[package]]
id = "example.localization"
version = "1.2.0"

[[feature]]
package_id = "example.localization"
id = "title-screen"
enabled = true

[feature.values]
variant = "rockman"

State format 1 and package-only manifests remain readable as a migration aid. They appear through one synthetic legacy feature. New packages should use explicit features.

The old derived_disc VCDIFF mechanism is legacy conversion scaffolding only. Feature-style manifests reject it. It is not a product mod primitive, fallback, or image-selection workflow; patched discs may be used offline as parity oracles while converting known mods to native operations.

Resolution and diagnostics

Before boot, the manager:

1. verifies the selected stock game and revision;
2. expands only enabled features and their selected options;
3. orders active packages deterministically by dependencies;
4. verifies enabled payloads and operation bounds;
5. collision-checks the complete owned byte-range plan and guard compatibility;
6. coalesces only truly identical target/range/expected/replacement writes or identical overlays; and
7. produces a canonical SHA-256 plan fingerprint.

Incompatible overlaps fail before launch. Structured diagnostics identify both (package, feature) owners and the exact contested target range. The launcher marks both feature rows and lets the user decide what to disable. It never silently chooses a winner.

Operation boundaries are not semantic boundaries. Legacy full-record writes compose when both their expected and replacement bytes agree throughout the owned intersection. Format-4 sparse patches collide only on declared owned fields, while their complete guards must remain mutually compatible. Partially overlapping overlays compose when their replacement payload bytes agree. A differing owned byte or incompatible guard produces a diagnostic at that exact location. Exact duplicate operations may be coalesced.

Package-level dependencies and conflicts are reserved for actual implementation relationships. Mutually exclusive choices such as US versus Japanese artwork belong inside one feature as option values.

Trusted adapters and archive safety

resolver = "builtin:<id>" selects a resolver statically registered by the game. Format-5 plugin ids likewise select only statically registered implementations. Packages cannot load arbitrary native code or select arbitrary symbols.

The installer accepts stored or DEFLATE-compressed ZIP entries, validates CRCs, rejects encrypted entries and unsafe or absolute paths, limits archives to 4096 files and 256 MiB expanded size, stages extraction, validates the manifest, and publishes the version atomically.