Real-Time Vital Signs Monitor

Summary

A real-time based vital signs monitor was implemented that works independently of the PPU frames. This monitor prints the system status (PC, LCDC, LY, total cycles) every 1 second of real time, even when the LCD is off and no frames are generated. Additionally, when the LCD is off, it displays the VRAM checksum to diagnose if the game is loading graphics. Removed previous boot monitor (20 prints) to clean up output.

Hardware Concept

The previous boot monitor confirmed that the CPU executes instructions correctly (PC progresses from 0x0100 to 0x1F68 and beyond). However, when the LCD is off (`LCDC=00`), the PPU does not advance `LY` (stays at 0), no full frames are generated, and therefore the frame-based "Heartbeat" never appears. The emulator is running "in the dark": the game may be copying data to VRAM with the screen off (correct behavior), but we have no visibility into what is happening.

To diagnose the status of the emulator while the screen is off, we need a working monitor based onreal time(system clock seconds), not frames. This monitor should show:

• PC (Program Counter):Where the code is currently running
• LCDC (LCD Control, 0xFF40):Whether the LCD is on (bit 7) and other controls
• LY (Scanline):PPU current line (must be 0 if LCD is off)
• Total cycles:How many cycles have been executed since the beginning
• VRAM Checksum:Sum of all bytes in VRAM (0x8000-0x9FFF) - if it increases, the game is loading graphics

Fountain:Pan Docs - LCD Control Register, VRAM Access

Implementation

The method was modifiedrun()of the classViboyto add a real-time based vital signs monitor that works independently of PPU frames.

Modified components

• Viboy (`src/viboy.py`):
  • Removed previous boot monitor (counterdebug_step_counterand the 20 prints of "BOOT STEP")
  • A variable was addedlast_realtime_log = time.time()before main loop
  • Inside the loop, after executing each instruction, it checks if 1.0 seconds have passed since the last report
  • If 1.0 seconds have passed, a message is printed with the current status: PC, LCDC, LY, and total cycles
  • If the LCD is off (`LCDC & 0x80 == 0`), the VRAM checksum is additionally displayed to diagnose if the game is loading graphics

Design decisions

1.0 second interval:An interval of 1 second was chosen to balance between useful information and not cluttering the console. This interval is enough to see if the PC is scrolling or static, and if the VRAM checksum is increasing (indicating graphics loading).

Using logging.info() instead of print():Although the boot monitor usedprint()To ensure visibility, this monitor useslogging.info()because it is less intrusive and allows you to control the logging level. If the user needs guaranteed visibility, they can set the logging level to INFO.

VRAM diagnostic when LCD is off:When the LCD is off, the game is usually loading graphics data into VRAM. Showing the VRAM checksum allows you to check if the game is actually copying data or if it is in an infinite loop doing nothing. If the checksum increases between reports, the game is loading graphics. If the checksum is 0 and does not change, the game is in an infinite loop.

Removing Boot Monitor:The previous boot monitor (20 prints) was useful for diagnosing immediate deadlocks, but it cluttered the console. Now that we know the CPU is working properly, the real-time monitor is more useful for diagnosing long-term problems.

Affected Files

• src/viboy.py- Removed boot monitor and added real-time based vital signs monitor

Tests and Verification

This monitor is a diagnostic tool that will be validated by running the emulator with ROMs that show crashing issues (undefined black screen).

• Test ROM:Pokémon Red (user-contributed ROM, not distributed) - shows black screen when LCD is off
• Execution mode:UI with monitor on, run for 10-20 seconds
• Success Criterion:The monitor must display information every 1 second with the current state of the system, allowing you to identify:
  • Whether the PC is advancing or static (indicating whether there is an infinite loop)
  • If the LCDC changes from 0x00 to 0x80 or 0x91 (indicating that the game is trying to boot video)
  • If the VRAM checksum increases (indicating that the game is loading graphics)
  • If the VRAM checksum is 0 and does not change (indicating that the game is in an infinite loop without copying data)
• Expected observation:
  • If the VRAM checksum increases: The game is loading graphics, just wait for it to finish and turn on the LCD
  • If the VRAM checksum is 0 and does not change: The game is in an infinite loop and is NOT copying data (possible DMA or logic failure)
  • If the LCDC changes to 0x80 or 0x91: The game tries to start the video
• Result: draft- Pending verification with actual emulator execution

Legal note:The Pokémon Red ROM is provided by the user for local testing. It is not distributed or included in the repository.

Sources consulted

• Bread Docs:LCD Control Register
• Bread Docs:VRAM (Video RAM)

Educational Integrity

Next Steps

• [ ] Run the emulator with Pokémon Red and wait for the black screen to appear
• [ ] Wait 10-20 seconds to see at least 10-20 monitor reports
• [ ] Analyze reports to identify:
  • Whether the PC is advancing or static
  • If the VRAM checksum increases (indicating graphics load)
  • If the LCDC changes from 0x00 to 0x80 or 0x91 (indicating video start)
• [ ] If the VRAM checksum increases: Wait for it to finish loading and check if the LCD turns on
• [ ] If the VRAM checksum is 0 and does not change: Investigate why the game is not copying data (possible DMA or logic failure)
• [ ] Once the problem is resolved, remove or disable the temporary monitor