Difference between revisions of "Copper"
(Created page with "The '''Copper''' is a simple programmed system which allows certain Next Registers to be altered automatically at certain scanline positions. "Copp...") |
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| %00 || STOP, copper does nothing | | %00 || STOP, copper does nothing | ||
|- | |- | ||
| − | | %01 || START, then | + | | %01 || START, then never reset |
|- | |- | ||
| %10 || START, loop continuously | | %10 || START, loop continuously | ||
|- | |- | ||
| − | | %11 || START, | + | | %11 || START, then reset at next VBlank |
|} | |} | ||
| + | |||
| + | If you are using one of the resetting modes (%01) (%11) then you can mark the "end" of the copper program by waiting for a line higher than 311. Since these lines will never be reached, the copper will essentially halt until reset. | ||
Revision as of 15:28, 13 October 2017
The Copper is a simple programmed system which allows certain Next Registers to be altered automatically at certain scanline positions.
"Copper" is a strangulation of "co-processor" and is a term taken from the Amiga which had a similar function. The copper on the Spectrum Next is not associated with the Raspberry Pi 'coprocessor'.
A program is uploaded to the Copper by sending it, byte by byte, to Copper Data ($60). Each Copper instruction is 16 bits wide and there are only 2 opcodes, so only a single bit in an instruction determines the operation. The two operations are:
| Opcode | Bit pattern | Effect |
|---|---|---|
| WAIT | %1hhhhhhv %vvvvvvvv | Wait for raster line v (0-311) and horizontal position h*8 (h is 0-55) |
| MOVE | %0rrrrrrr %vvvvvvvv | Write value v to Next register r |
The copper's program storage is 1k big; since each instruction is 2 bytes it can store exactly 512 instructions, numbered 0-511. The copper is controlled by a word accessed through Next Registers Copper Control Low Byte ($61) and Copper Control High Byte ($62).
The copper control word has the structure: %cc00000i %iiiiiiii. I is the instruction index (0-511), and c is used to set the behavior mode of the copper, as follows:
| C bits | Effect |
|---|---|
| %00 | STOP, copper does nothing |
| %01 | START, then never reset |
| %10 | START, loop continuously |
| %11 | START, then reset at next VBlank |
If you are using one of the resetting modes (%01) (%11) then you can mark the "end" of the copper program by waiting for a line higher than 311. Since these lines will never be reached, the copper will essentially halt until reset.