Copper

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.