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
The serial interrupt is requested upon completion of a serial data transfer. In other words, eight serial clock cycles after starting a transfer (by setting SC bit 7), the incoming data will be in SB and the interrupt will be requested.
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.