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Revision as of 02:42, 24 January 2024
Most of the game's code is executed inside its threading system. A thread is basically an execution context. Each thread has its own direct page memory and stack memory (between 7e:0000 and 7e:1bff). Threads can send messages to each other, and when a thread wants to receive a message, the system takes that opportunity to switch threads. Since return addresses are part of the stack, the system switches to the proper stack pointer and returns to where that thread left off.
The thread system code is in bank C0, between 495b and roughly 62ed. When looking at game ASM you'll often see variations on the following pattern:
C1/2A44: lda #$80 ; Message Type C1/2A46: jsr $C00520 ; Create new message of Type A C1/2A4A: lda #$8E C1/2A4C: sta $0004,X ; First param. Often an opcode. C1/2A4F: pla C1/2A50: sta $0005,X ; Additional param C1/2A53: lda $22 C1/2A55: sta $0006,X ; Additional param C1/2A58: lda $7E2008 ; A specific Thread ID C1/2A5C: jsr $C00528 ; Send message X to Thread A
Thread IDs for permanent threads are stored at 7e:2000-7e:2008 (one byte each). Some threads live forever during gameplay, and other threads are temporary and complete an operation and are then recycled.
Looking at this code, it's triggering something, but there's no way to determine what it's actually doing without just observing the game behavior or finding the code that the other thread executes and seeing how it responds to a message with a code of #8e. The documentation for each thread will make that easier.
Threads are created by calling c00440 (c04d70). The long address to the thread params are passed in via A and X. These parameters specify the thread's start location, type, and priority. Type is used when code sends a message to all threads matching a certain type. Priority determines how quickly the thread gets switched to, with lower values being faster.
Permanent Threads
| Thread ID Memory Location | Thread ID | Name |
|---|---|---|
| n/a | 2 | Input/Monitor Thread |
| 7e2003 | 3 | Audio/Clock Thread |
| 7e2001 | 4 | Thread 4 |
| 7e2000 | 5 | Thread 5 |
| 7e2002 | 6 | Thread 6 |
| 7e2007 | 7 | Thread 7 |
| 7e2008 | 8 | Thread 8 |
| 7e2006 | 9 | Thread 9 |
| 7f0386 | a | Thread 0A |
| 7e33f1 | b | Character Overlay UI Thread |
Audio/Clock Thread
All audio-related code as well as incrementing the game clock (day/night cycle). Operations 00-2c are audio commands.
| Thread ID | 3 |
|---|---|
| DP/Stack Memory | 0100 |
| Code Bank | C0 |
| Parent Thread | 1 |
| Priority | 80 |
| Type | 10 |
| Thread Params Location | c0ff80 |
| Start Location | c0b84d |
Direct Page:
| Offset | Use |
|---|---|
| $00-01 | Current message |
| $02 | A thread ID |
Operations:
| Op Code | Routine Address | Description | Parameters |
|---|---|---|---|
| 00 | bc23 | Play sound effect | [5]: Effect id, [6]: Pan |
| 80 | b953 | Pause game clock (ref counted) | None |
| 81 | b96a | Resume game clock (ref counted) | None |
| c9 | b97d | Respond with 8:ca or 8:cb depending on $02 | None |
| cc | b9ac | If sender == $02, clear $02 | None |
| c6 | ba08 | If $06[msg[5][3]] == 0, $04++. $06[msg[5][3]] = sender. | [5-6]: Unknown pointer |
| c7 | ba25 | If $06[msg[5][3]] != 0, $04-- and $06[msg[5][3]] = 0. | [5-6]: Unknown pointer |
| c8 | ba3f | Related to $04 | None |
| e3 | b9c6 | Allocate buffer. Respond with 8:0:bufferAddress | [5-6]: Requested buffer size |
| f8 | b9ef | Get 1c06, returned as 8:1c06 | None |
Character Overlay UI Thread
Draws the UI for each character that is displayed during gameplay.
| Thread ID | B |
|---|---|
| DP/Stack Memory | 0900 |
| Code Bank | C4 |
| Parent Thread | 6 |
| Priority | 70 |
| Type | 0 |
| Thread Params Location | c462db |
| Start Location | c461a2 |
Direct Page:
| Offset | Use |
|---|---|
| $04-05 | (character selector? 0-2) |
| $20-21 | Draw location 1 |
| $22-23 | Draw location 2 |
| $24-25 | Draw location 3 |
| $26-27 | Character addr 1 |
| $28-29 | Character addr 2 |
| $2a-2b | Character addr 3 |
| $2e | Bitwise (bit 1 checked at 5a09) |
| $42 | (Counter) |
| $44 | Character count |
| $46 | Current sender |
| $48 | AT mode for character 1 |
| $49 | AT mode for character 2 |
| $4a | AT mode for character 3 |
Operations:
| Op Code | Routine Address | Description | Parameters |
|---|---|---|---|
| 1 | 5a3d | Clear, calculate, redraw character UIs | None |
| 2 | 614c | Clear all UIs, respond with Type8 msg | None |
| 3 | 5a4b | Prep and draw Char UIs if 2e bit 1 is set | None |
| 4 | 5a09 | ? | |
| 5-8 | 5a5b | Do nothing | None |
| 9 | 60ee | Set character address and redraw all | [5]: Character index |
| a | 612b | Remove character 3 from UI | None |
| b | 60af | Set AT mode for character, redraw if necessary | [5]: Character thread ID |
| c | 60b4 | Clear AT mode for character, redraw if necessary | [5]: Character thread ID |
Temporary/Transient Threads
Graphics Loader
Loads various graphics into VRAM.
| Code Bank | C4 |
|---|---|
| Priority | 50 |
| Type | 0 |
| Thread Params Location | c4681b |
| Start Location | c4679f |
Operations:
| Op Code | Routine Address | Description | Parameters |
|---|---|---|---|
| 0 | 63be | Load overlay UI and battle-related graphics, respond with Type8 message | None |
| 1 | 631e | Load fullscreen menu background graphics | None |
| 2 | 62e9 | Load window background graphics | [5]: Window style, [6-7]: Buffer to use |
| 3 | 65d3 | ? | None |