Commodore Amiga A2000 - Jump Strobe Address; Control Register

NOTE
At the start of each vertical blanking interval, COP1LC is automatically used to start the
program counter. That is, no matter what the Copper is doing, when the end of vertical
blanking occurs, the Copper is automatically forced to restart its operations at the address
contained in COP1LC.
JUMP STROBE ADDRESS
When you write to a Copper strobe address, the Copper reloads its program counter from
the corresponding location register. The Copper can write its own location registers and
strobe addresses to perform programmed jumps. For instance, you might MOVE an
indirect address into the COP2LC location register. Then, any MOVE instruction that
addresses COPJMP2 strobes this indirect address into the program counter.
There are two jump strobe addresses:
COPJMP1 Restart Copper from address contained in COP1LC.
COPJMP2 Restart Copper from address contained in COP2LC.
CONTROL REGISTER
The Copper can access some special-purpose registers all of the time, some registers only
when a special control bit is set to a 1, some registers not at all. The registers that the
Copper can always affect are numbered $20 through $FF inclusive. Those it cannot affect
at all are numbered $00 to $0F inclusive. (See Appendix B for a list of registers
in address order.) The Copper control register is within this group ($00 to $0F). Thus it
takes deliberate action on the part of the 68000 to allow the Copper to write into a
specific range of the special-purpose registers.
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 $1F
inclusive. This range includes the blitter control registers. When CDANG is 0, these
registers cannot be written by the Copper. When 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.
- Coprocessor Hardware 21 -