Trials of Mana (SNES)/Threading System

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

• Object Script Thread
• Graphics Loader

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