mvbg

VM_PRINTER_DETECT

VM_PRINTER_DETECT is a GBVM instruction used to check for the presence and readiness of a connected Game Boy Printer.

Purpose: Before attempting to print anything, it’s crucial to verify that a Game Boy Printer is actually connected and functioning correctly. VM_PRINTER_DETECT allows your GBVM script to perform this check, enabling:

• User feedback: Informing the player if a printer is not connected, preventing frustration from failed print attempts.
• Conditional printing: Only offering print options if a printer is detected.
• Error handling: Providing a mechanism to gracefully handle situations where the printer is unavailable.

This instruction attempts to communicate with the Game Boy Printer. It will wait for a specified DELAY (timeout) for a response from the printer. The result of the detection (success or failure) is stored in a target variable.

Syntax:

VM_PRINTER_DETECT ERROR, DELAY

• ERROR: A target variable that will receive the result of the detection:
  • 0: Printer detected successfully.
  • Non-zero value: An error occurred (e.g., printer not found, timeout).
• DELAY: The detection timeout (an integer). This specifies how long (in Game Boy frames or internal ticks) the system should wait for a response from the printer before giving up. A longer delay increases the chance of detection but also increases the wait time.

Usage Example: Checking for Printer Before Offering Print Option

Imagine your game has a menu option to print a high score. You only want to enable this option if a Game Boy Printer is connected.

; In your menu script, before displaying the print option:

; Assume VAR_PRINTER_STATUS is a variable to store the detection result
VAR_PRINTER_STATUS:
  .R_INT8 0

; Define a reasonable delay for printer detection (e.g., 60 frames = 1 second)
PRINTER_DETECT_DELAY:
  .R_INT16 60

CHECK_PRINTER_AVAILABILITY:
  ; Attempt to detect the printer
  VM_PRINTER_DETECT VAR_PRINTER_STATUS, PRINTER_DETECT_DELAY

  ; Check the detection result
  VM_IF_CONST .EQ, VAR_PRINTER_STATUS, 0, PRINTER_FOUND, 0
    ; Printer not found, disable print option and display message
    VM_LOAD_TEXT TEXT_PRINTER_NOT_FOUND
    VM_DISPLAY_TEXT
    VM_IDLE 60
    VM_SET_CONST VAR_PRINT_OPTION_ENABLED, 0 ; Disable print option
    VM_JUMP END_PRINTER_CHECK

PRINTER_FOUND:
  ; Printer found, enable print option
  VM_LOAD_TEXT TEXT_PRINTER_READY
  VM_DISPLAY_TEXT
  VM_IDLE 60
  VM_SET_CONST VAR_PRINT_OPTION_ENABLED, 1 ; Enable print option

END_PRINTER_CHECK:
  ; ... continue menu display logic, showing/hiding print option based on VAR_PRINT_OPTION_ENABLED ...

  VM_RET

TEXT_PRINTER_NOT_FOUND:
  .TEXT "Game Boy Printer not found."
  .TEXT_END

TEXT_PRINTER_READY:
  .TEXT "Game Boy Printer ready!"
  .TEXT_END

In this example, VM_PRINTER_DETECT is called to check for the printer. If VAR_PRINTER_STATUS is 0, it means the printer was detected, and the game can enable the print option. Otherwise, a message is displayed, and the print option remains disabled. This provides a robust way to integrate Game Boy Printer functionality into your game, ensuring a better user experience.

Analogy to Game Boy Development: This is a direct interface with a specific Game Boy peripheral. In modern programming, it’s analogous to checking for the availability of a hardware device (e.g., a USB printer, a network connection) before attempting to use it. It’s a crucial step in robust hardware integration, allowing your software to adapt to the presence or absence of external devices.