Commodore Amiga Hardware Reference Manual page 222

Vertical Blanking Interrupt
Bit 5, VERTB, causes an interrupt at line 0 (start of vertical blank) of the video display
frame. The system is often required to perform many different tasks during the vertical
blanking interval. Among these tasks are the updating of various pointer registers,
rewriting lists of Copper tasks when necessary, and other system-control operations.
The minimum time of vertical blanking is 20 horizontal scan lines (begins at line 0 and
ends at line 20). You also have control over where (after line 20) the display actually
starts by using the DIWSTRT (display window start) register (see chapter 3, "Playfield
Hardware"). This can extend the effective vertical blanking time.
If you find that you still require additional time during vertical blanking, you can use
the Copper to create a level 3 interrupt. This Copper interrupt would be timed to occur
just after the last line of display on the screen (after the display window stop which you
have defined by using the DIWSTOP register).
Copper Interrupt
Bit 4, COPER, is used by the Copper to issue a level 3 interrupt. The Copper can
change the content of
any
of the bits of this register, as it can write any value into most
of the machine registers. However, this bit has been reserved for specifically identifying
the Copper as the interrupt source.
Generally, you use this bit when you want to sense that the display beam has reached a
specific position on the screen, and you wish to change something in memory based on
this occurrence.
Audio Interrupts
Bits 10 - 7, AUD3 - 0, are assigned to the audio channels. They are called AUD3, AUD2,
AUDl, and AUDO and are assigned to channels 3, 2, 1, and 0, respectively.
This level 4 interrupt signals "audio block done." When the audio DMA is operating in
automatic mode, this interrupt occurs when the last word in an audio data stream has
been accessed. In manual mode, it occurs when the audio data register is ready to
accept another word of data.
210 System Control Hardware