mvbg

VM_RTC_START

VM_RTC_START is a GBVM instruction used to start or stop the Game Boy Color’s (GBC) Real-Time Clock (RTC).

Purpose: While the RTC typically runs continuously, there might be specific scenarios where you need to pause or resume its operation. VM_RTC_START provides this control. It is essential for:

• Pausing time-based events: Temporarily halting the progression of in-game time-sensitive events (e.g., during a pause menu, a cutscene, or when the game is suspended).
• Controlling time flow: If your game has mechanics that involve manipulating time (e.g., time travel, time-stop abilities), this instruction could be part of their implementation.
• Debugging/Testing: Freezing the RTC at a specific time to test time-dependent features without waiting for real-world time to pass.

When VM_RTC_START is called, it sends a command to the RTC hardware to either begin counting time or to halt its counting. This affects the progression of seconds, minutes, hours, and days.

Syntax:

VM_RTC_START START

• START: A constant that specifies whether the RTC should start or stop:
  • .RTC_STOP: Halts the RTC. The time will no longer progress.
  • .RTC_START: Resumes the RTC. The time will continue to progress from where it left off.

Usage Example: Pausing In-Game Time During a Pause Menu

Imagine your game has a pause menu. When the player pauses the game, you want to stop all time-based events (like a day/night cycle or a timed quest) by pausing the RTC. When they unpause, the RTC should resume.

; In your game's pause menu script:

; RTC control constants
.RTC_STOP:
  .R_INT8 0 ; Example value for stopping RTC
.RTC_START:
  .R_INT8 1 ; Example value for starting RTC

PAUSE_GAME_ROUTINE:
  ; Stop the RTC when the game is paused
  VM_RTC_START .RTC_STOP

  VM_LOAD_TEXT TEXT_GAME_PAUSED
  VM_DISPLAY_TEXT
  VM_INPUT_WAIT .INPUT_START ; Wait for Start button to unpause

  ; Start the RTC when the game is unpaused
  VM_RTC_START .RTC_START

  VM_LOAD_TEXT TEXT_GAME_UNPAUSED
  VM_DISPLAY_TEXT
  VM_IDLE 30

  VM_RET

TEXT_GAME_PAUSED:
  .TEXT "Game Paused"
  .TEXT_END

TEXT_GAME_UNPAUSED:
  .TEXT "Game Resumed"
  .TEXT_END

In this example, VM_RTC_START .RTC_STOP is called when the pause menu is activated, effectively freezing the in-game time. When the player unpauses, VM_RTC_START .RTC_START is called to resume the RTC, allowing time-based events to continue their progression. This provides a consistent and expected behavior for pausing a game with time-sensitive elements.

Analogy to Game Boy Development: This is a direct interface with the Game Boy Color’s Real-Time Clock hardware. In modern programming, it’s analogous to pausing or resuming a timer or a system clock. It provides a way to control the flow of time within your game, which is crucial for mechanics that rely on real-world time progression.