mvbg

VM_RTC_LATCH

VM_RTC_LATCH is a GBVM instruction used to latch (capture) the current time values from the Game Boy Color’s (GBC) Real-Time Clock (RTC) into internal registers, making them available for reading.

Purpose: The RTC is a continuously running clock. To ensure that you get a consistent reading of the time (e.g., all components like seconds, minutes, hours, and days correspond to the exact same moment), you need to “latch” the values. VM_RTC_LATCH performs this operation. It is essential for:

• Consistent time readings: Preventing situations where, for example, you read the seconds, and then the minute ticks over before you read the minutes, leading to an inconsistent timestamp.
• Accurate time-based logic: Ensuring that all time-dependent calculations or events are based on a single, synchronized snapshot of the RTC.

When VM_RTC_LATCH is called, the current values of the RTC’s time components are copied into a set of internal, accessible registers. Subsequent VM_RTC_GET calls will read from these latched registers until VM_RTC_LATCH is called again.

Syntax:

VM_RTC_LATCH

VM_RTC_LATCH takes no arguments. It simply performs the latching operation.

Usage Example: Reading a Consistent Timestamp

Imagine you want to record the exact time a player saves their game, including seconds, minutes, and hours. To ensure these values are all from the same moment, you would latch the RTC before reading each component.

; In your game save routine:

; Variables to store the saved time components
VAR_SAVE_SECONDS:
  .R_INT8 0
VAR_SAVE_MINUTES:
  .R_INT8 0
VAR_SAVE_HOURS:
  .R_INT8 0

SAVE_GAME_ROUTINE:
  ; Latch the RTC to get a consistent snapshot of the current time
  VM_RTC_LATCH

  ; Read each time component from the latched RTC values
  VM_RTC_GET VAR_SAVE_SECONDS, .RTC_SECONDS
  VM_RTC_GET VAR_SAVE_MINUTES, .RTC_MINUTES
  VM_RTC_GET VAR_SAVE_HOURS, .RTC_HOURS

  ; Now, VAR_SAVE_SECONDS, VAR_SAVE_MINUTES, and VAR_SAVE_HOURS
  ; all represent the time at which VM_RTC_LATCH was called.

  ; ... proceed to save these time values along with other game data ...

  VM_LOAD_TEXT TEXT_GAME_SAVED_TIME
  VM_DISPLAY_TEXT
  VM_IDLE 60

  VM_RET

TEXT_GAME_SAVED_TIME:
  .TEXT "Game saved at: "
  .TEXT_END

; RTC component constants
.RTC_SECONDS:
  .R_INT8 0
.RTC_MINUTES:
  .R_INT8 1
.RTC_HOURS:
  .R_INT8 2

In this example, VM_RTC_LATCH is called once at the beginning of the save routine. This ensures that all subsequent VM_RTC_GET calls retrieve time components from the same moment in time, providing an accurate timestamp for the save file. Without VM_RTC_LATCH, there’s a small chance that the RTC could tick over between reading seconds and minutes, leading to an incorrect timestamp.

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 taking a snapshot of the system time or freezing a timestamp to ensure consistency when reading multiple time components. It’s a critical step for accurate time-based operations on hardware where the clock is continuously running and might update between successive reads.