The copper is another sub-component of Agnus; The name is short for “co-processor”. The copper is a programmable finite state machine that executes a programmed instruction stream, synchronized with the video hardware.
When it is turned on, the copper has three states; either reading an instruction, executing it, or waiting for a specific video beam position. The copper runs a program called the copper list in parallel with the main CPU. The copper runs in sync with the video beam, and it can be used to perform various operations which require video synchronization. Most commonly it is used to control video output, but it can write to most of the chipset registers and thus can be used to set audio registers or interrupt the CPU.
The copper list has three kinds of instructions, each one being a pair of two bytes, four bytes in total. Copper is most commonly used to set and reset the video hardware registers at the beginning of each frame.