Commodore Amiga - Putting Together a Copper Instruction List

The Copper control register, called COPCON, contains only one bit, bit #1. This bit,
called CDANG (for Copper Danger Bit) protects all registers numbered between $10 and
$IF inclusive. This range includes the blitter control registers. When CDANG is 0,
these registers cannot .be written by the Copper. \Vhen CDANG is
1, these registers can
be written by the Copper. Preventing the Copper from accessing the blitter control
registers prevents a "runaway" Copper (caused by a poorly formed instruction list) from
accidentally affecting system memory.
NOTE
The CDANG bit is cleared after a reset.
Putting Together a Copper Instruction List
The Copper instruction list contains all the register resetting done during the vertical
blanking interval and the register modifications necessary for making mid-screen altera-
tions. As you are planning what will happen during each display field, you may find
it
easier to think of each aspect of the display as a separate subsystem, such as playfields,
sprites, audio, interrupts, and so on. Then you can build a separate list of things
that
must be done for each subsystem individually at each video beam position.
When you have created all these intermediate lists of things to be done, you must merge
them together into a single instruction list
to be executed by the Copper once for each
display frame.
The alternative is to create this all-inclusive list directly, without the
intermediate steps.
For example, the bit-plane pointers used in playfield displays and the sprite pointers
must be rewritten during the vertical blanking interval so the
data will be properly
retrieved when the screen display
starts again. This can be done with a Copper instruc-
tion list
that does the following:
WAIT until first line of the display
MOVE
data to bit-plane pointer 1
MOVE
data to bit-plane pointer 2
MOVE
data to sprite pointer 1
and so on
As another example, the sprite DMA channels that create movable objects can be reused
multiple times during the same display field. You can change the size and shape of the
reuses of a sprite; however, every multiple reuse normally uses the same set of colors
Coprocessor Hardware
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