mvbg

VM_BEGINTHREAD

VM_BEGINTHREAD is a powerful GBVM instruction that allows you to spawn a new execution thread, enabling concurrent operations within your Game Boy game. This means you can have multiple pieces of code running seemingly at the same time, without one blocking the other.

Purpose: In game development, it’s often necessary to perform tasks in the background or to manage independent behaviors for different entities. For example:

An NPC (Non-Player Character) moving along a path while the player is free to move.
A timer counting down in the background.
An animation playing on a specific object without interrupting the main game logic.

VM_BEGINTHREAD facilitates this by creating a new, independent execution context (a “thread”) that runs a specified function (THREADPROC). This new thread has its own stack and can execute code concurrently with the main game loop or other threads.

Syntax:

VM_BEGINTHREAD BANK, THREADPROC, HTHREAD, NARGS

BANK: The memory bank number where the THREADPROC function resides. This is important for memory management on the Game Boy.
THREADPROC: The address (label) of the function that the new thread will execute. This function will run independently.
HTHREAD: A variable that will receive the “handle” (an identifier) of the newly created thread. This handle can be used later with instructions like VM_JOIN or VM_TERMINATE to manage the thread.
NARGS: The number of values from the current stack that should be copied onto the stack of the new thread. This allows you to pass arguments to the THREADPROC function.

Usage Example: Independent NPC Movement

Let’s imagine you have an NPC that you want to patrol back and forth along a specific path, independently of the player’s actions. You can define a NPC_PATROL_SCRIPT function and spawn a thread for it.

; Define the NPC patrol script
NPC_PATROL_SCRIPT:
  ; Assume ACTOR_NPC_ID is a variable holding the ID of the NPC actor
  ; Assume PATROL_TARGET_X and PATROL_TARGET_Y are coordinates

  VM_ACTOR_MOVE_TO ACTOR_NPC_ID, PATROL_TARGET_X, PATROL_TARGET_Y, 16 ; Move to first point
  VM_IDLE 60 ; Wait for a second (60 frames)
  VM_ACTOR_MOVE_TO ACTOR_NPC_ID, START_X, START_Y, 16 ; Move back to start
  VM_IDLE 60 ; Wait for a second
  VM_LOOP ; Loop indefinitely

; In your main game script or scene initialization:

; Declare a variable to hold the thread handle
VAR_NPC_THREAD_HANDLE:
  .R_INT16 0 ; Initialize to 0

; Spawn the NPC patrol thread
VM_BEGINTHREAD BANK(NPC_PATROL_SCRIPT), NPC_PATROL_SCRIPT, VAR_NPC_THREAD_HANDLE, 0
; (NARGS is 0 because we're not passing any arguments from the stack to NPC_PATROL_SCRIPT)

; ... rest of your main game logic ...

In this example, VM_BEGINTHREAD starts NPC_PATROL_SCRIPT in its own thread. This script will continuously make the NPC move between two points without pausing the main game execution. The VAR_NPC_THREAD_HANDLE can later be used to stop or pause this specific NPC’s behavior if needed (e.g., if the player interacts with them).

Analogy to other programming languages: This concept is directly analogous to creating a new thread in languages like C++ (std::thread), Java (new Thread().start()), or Python (threading.Thread). It allows for parallelism, where different parts of your program can execute concurrently, improving responsiveness and enabling complex behaviors that don’t block the main program flow. In simpler game engines, this might be achieved with coroutines or state machines, but VM_BEGINTHREAD provides a more direct threading model.