mvbg

VM_IDLE

VM_IDLE is a GBVM instruction that signals the thread runner (the GBVM scheduler) that the current script context (thread) is in a waitable state. This means the script can temporarily pause its execution, allowing the Game Boy’s CPU to process other tasks, render frames, or allow other threads to run.

Purpose: In game development, it’s often necessary to introduce delays or to yield control to the game engine for a short period. VM_IDLE is crucial for:

Pausing script execution: Creating delays for animations, dialogue pacing, or timed events.
Preventing busy-waiting: Instead of continuously checking for a condition (which consumes CPU cycles), VM_IDLE allows the script to pause and wait efficiently.
Cooperative multitasking: In a multi-threaded GBVM environment, VM_IDLE allows other threads to get CPU time, ensuring smooth execution of all concurrent processes.
Frame-based timing: Since the Game Boy runs at a fixed frame rate (approx. 60 FPS), VM_IDLE is often used in conjunction with frame counts to create precise delays.

Syntax:

VM_IDLE

VM_IDLE does not take any arguments. It simply tells the scheduler to put the current thread into an idle state until the next frame or until another event wakes it up.

Usage Example: Simple Delay for Animation or Dialogue

Imagine you want to display a piece of dialogue, then wait for a short period before displaying the next line, or you want to pause an animation for a few frames.

; Example: Display dialogue with a pause

VM_LOAD_TEXT DIALOGUE_LINE_1
VM_DISPLAY_TEXT
VM_IDLE 60 ; Wait for 60 frames (approx. 1 second at 60 FPS)

VM_LOAD_TEXT DIALOGUE_LINE_2
VM_DISPLAY_TEXT
VM_IDLE 60 ; Wait for another 60 frames

; ... rest of script ...

DIALOGUE_LINE_1:
  .TEXT "Welcome to the village!"
  .TEXT_END

DIALOGUE_LINE_2:
  .TEXT "I hope you enjoy your stay."
  .TEXT_END

In this example, after displaying DIALOGUE_LINE_1, VM_IDLE 60 (assuming a version of VM_IDLE that takes a frame count, or a loop around VM_IDLE and a counter) pauses the script for approximately one second. During this pause, the game continues to render, and other background processes can run. This creates a natural pacing for the dialogue.

Note: While the basic VM_IDLE might just yield for one frame, often in practical GBVM implementations, it’s used in a loop with a counter (e.g., VM_IDLE_N_FRAMES or a custom loop using VM_IDLE and a variable) to achieve specific delays in frames.

Analogy to other programming languages: This is similar to Thread.sleep() in Java, time.sleep() in Python, or setTimeout(..., 0) in JavaScript (though setTimeout is non-blocking). It’s a way to voluntarily give up CPU time, allowing the operating system or scheduler to run other tasks. In game development, it’s often tied to the game loop or frame rate to ensure consistent timing.