mvbg

VM_MUSIC_ROUTINE

VM_MUSIC_ROUTINE is a GBVM instruction used to attach a script to a specific music event routine, allowing your game to trigger custom logic in sync with the music playback.

Purpose: Music in games often has specific points (e.g., a beat drop, a melody change, the end of a loop) where you might want to trigger visual effects, gameplay changes, or other events. VM_MUSIC_ROUTINE provides a way to synchronize your game logic with the music. It is essential for:

• Rhythm-based gameplay: Creating actions that must be performed in time with the music.
• Dynamic visualizers: Making on-screen elements react to the music (e.g., lights flashing on a beat).
• Storytelling: Triggering cutscene events or character animations at precise musical cues.
• Atmospheric effects: Changing environmental elements (e.g., weather, lighting) in response to music.

This instruction allows you to define up to four different music event routines (identified by ROUTINE ID 0-3). When the music driver reaches a point in the music data that triggers one of these routines, the associated script will be executed.

Syntax:

VM_MUSIC_ROUTINE ROUTINE, BANK, ADDR

• ROUTINE: The routine ID (an integer between 0 and 3). This identifies which of the four available music event routines you are configuring.
• BANK: The memory bank number where the script (ADDR) for this music routine is located. This is crucial for accessing scripts that might be in different ROM banks.
• ADDR: The address (label) of the script (subroutine) that will be executed when this music routine is triggered by the music driver.

Usage Example: Flashing Lights on a Beat

Imagine you want the screen to flash briefly (e.g., change background color) every time a specific beat or musical cue occurs in your game’s music. Your music data would need to be authored to include triggers for these routines.

; In your game initialization or scene setup:

; Define the script to flash the screen
FLASH_SCREEN_SCRIPT:
  ; Assume PALETTE_FLASH_BG is a bright, temporary palette
  ; Assume PALETTE_NORMAL_BG is the regular background palette

  VM_LOAD_PALETTE .PALETTE_BACKGROUND_0, PALETTE_FLASH_BG
  VM_IDLE 2 ; Flash for a very short duration
  VM_LOAD_PALETTE .PALETTE_BACKGROUND_0, PALETTE_NORMAL_BG
  VM_RET

PALETTE_FLASH_BG:
  .CGB_PAL 31,31,31, 31,31,31, 31,31,31, 31,31,31 ; All white/bright

PALETTE_NORMAL_BG:
  .CGB_PAL 31,31,31, 20,20,20, 10,10,10, 0,0,0 ; Normal palette

; Attach FLASH_SCREEN_SCRIPT to Music Routine 0
VM_MUSIC_ROUTINE 0, BANK(FLASH_SCREEN_SCRIPT), FLASH_SCREEN_SCRIPT

; Start playing music that has triggers for Routine 0
VM_MUSIC_PLAY MUSIC_BATTLE_THEME_BANK, MUSIC_BATTLE_THEME, 0

; ... rest of game loop ...

MUSIC_BATTLE_THEME_BANK:
  .R_INT8 BANK(MUSIC_BATTLE_THEME)
MUSIC_BATTLE_THEME:
  ; ... music data with embedded triggers for routine 0 ...
  ; (e.g., a specific byte sequence in the music data that the driver interprets as a call to routine 0)

In this example, VM_MUSIC_ROUTINE 0, BANK(FLASH_SCREEN_SCRIPT), FLASH_SCREEN_SCRIPT sets up the FLASH_SCREEN_SCRIPT to be called whenever the music driver encounters a trigger for Routine 0 in the MUSIC_BATTLE_THEME. This allows the screen to flash in perfect synchronization with the music, creating a dynamic visual effect that enhances the battle experience.

Analogy to Game Boy Development: This is a direct interface with the Game Boy’s music driver, allowing for custom callbacks from the music engine. In modern game development, it’s analogous to using an audio event system (like FMOD or Wwise) to trigger gameplay events based on markers or parameters within an audio track. It provides a powerful way to tightly integrate audio and gameplay, creating more immersive and responsive experiences.