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
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