Enable - Commodore Amiga Hardware Reference Manual

The notes about horizontal and vertical beam position found in the discussion of the
WAIT instruction apply also to the SKIP instruction.
The following example SKIP instruction skips the instruction following it if VP (vertical
beam position) is greater than or equal to 100 ($64).
DC.W $6401,$FFOI ;If VP
>=
100, skip next instruction (ignore HPJ
COPPER LOOPS AND BRANCHES AND COMPARISON ENABLE
You can change the value in the location registers at any time and use this value to con-
struct loops in the instruction list. Before the next vertical blanking time, however, the
COPILC registers must be repointed to the beginning of the appropriate Copper list.
The value in the COPILC location registers will be restored to the Copper's program
counter at the start of the vertical blanking period.
Bits 14-1 of instruction word 2 in the WAIT and SKIP instructions specify which bits of
the horizontal and vertical position are to be used for the beam counter comparison.
The position in instruction word 1 and the compare enable bits in instruction word 2 are
tested against the actual beam counters before any further action is taken. A position
bit in instruction word 1 is used in comparing the positions with the actual beam
counters
if
and only if the corresponding enable bit in instruction word 2 is set to
1.
If
the corresponding enable bit is 0, the comparison is always true. For instance, if you
care only about the value in the last four bits of the vertical position, you set only the
last four compare enable bits, bits (11-8) in instruction word 2.
As another example, suppose you want to issue an interrupt each time a total of 16
vertical scan lines has occurred. In addition, you want the interrupts only between lines
80 and 160. The Copper instruction sequence below would do this. The enable "masks"
are specified with the instructions.
Before the Copper is told to begin this set of instructions, you would use the 68000 to
write the address of LOOP to COP2LC.
Coprocessor Hardware 21