Interrupt Sources

INT $40 — VBlank interrupt

This interrupt is requested every time the Game Boy enters VBlank (Mode 1).

The VBlank interrupt occurs ca. 59.7 times a second on a handheld Game Boy (DMG or CGB) or Game Boy Player and ca. 61.1 times a second on a Super Game Boy (SGB). This interrupt occurs at the beginning of the VBlank period (LY=144). During this period video hardware is not using VRAM so it may be freely accessed. This period lasts approximately 1.1 milliseconds.

INT $48 — STAT interrupt

There are various sources which can trigger this interrupt to occur as described in STAT register ($FF41).

The various STAT interrupt sources (modes 0-2 and LYC=LY) have their state (inactive=low and active=high) logically ORed into a shared “STAT interrupt line” if their respective enable bit is turned on.

A STAT interrupt will be triggered by a rising edge (transition from low to high) on the STAT interrupt line.

STAT blocking

If a STAT interrupt source logically ORs the interrupt line high while (or immediately after) it’s already set high by another source, then there will be no low-to-high transition and so no interrupt will occur. This phenomenon is known as “STAT blocking” (test ROM example).

As mentioned in the description of the STAT register, the PPU cycles through the different modes in a fixed order. So for example, if interrupts are enabled for two consecutive modes such as Mode 0 and Mode 1, then no interrupt will trigger for Mode 1 (since the STAT interrupt line won’t have a chance to go low between them).

Using the STAT interrupt

One very popular use is to indicate to the user when the video hardware is about to redraw a given LCD line. This can be useful for dynamically controlling the SCX/SCY registers ($FF43/$FF42) to perform special video effects.

Example application: set LYC to WY, enable LY=LYC interrupt, and have the handler disable objects. This can be used if you use the window for a text box (at the bottom of the screen), and you want objects (sprites) to be hidden by the text box.

INT $50 — Timer interrupt

The timer interrupt is requested every time that the timer overflows (that is, when TIMA exceeds $FF).

INT $58 — Serial interrupt

XXXXXX…

Transmitting and receiving serial data is done simultaneously. The received data is automatically stored in SB.

The serial I/O port on the Game Boy is a very simple setup and is crude compared to standard RS-232 (IBM-PC) or RS-485 (Macintosh) serial ports. There are no start or stop bits.

During a transfer, a byte is shifted in at the same time that a byte is shifted out. The rate of the shift is determined by whether the clock source is internal or external. The most significant bit is shifted in and out first.

When the internal clock is selected, it drives the clock pin on the game link port and it stays high when not used. During a transfer it will go low eight times to clock in/out each bit.

The state of the last bit shifted out determines the state of the output line until another transfer takes place.

If a serial transfer with internal clock is performed and no external Game Boy is present, a value of $FF will be received in the transfer.

The following code initiates the process of shifting $75 out the serial port and a byte to be shifted into $FF01:

   ld a, $75
   ld [$FF01], a
   ld a, $81
   ld [$FF02], a

The Game Boy does not support wake-on-LAN. Completion of an externally clocked serial transfer does not exit STOP mode.

INT $60 — Joypad interrupt

The Joypad interrupt is requested when any of P1 bits 0-3 change from High to Low. This happens when a button is pressed (provided that the action/direction buttons are enabled by bit 5/4, respectively), however, due to switch bounce, one or more High to Low transitions are usually produced when pressing a button.

Using the joypad interrupt

This interrupt is useful to identify button presses if we have only selected either action (bit 5) or direction (bit 4), but not both. If both are selected and, for example, a bit is already held Low by an action button, pressing the corresponding direction button would make no difference. The only meaningful purpose of the Joypad interrupt would be to terminate the STOP (low power) standby state. GBA SP, because of the different buttons used, seems to not be affected by switch bounce.

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