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check out the thread at emutalk, i listed the exact amount of ram space invaders requires to function perfectly.
as for cycles, it runs at 2 mhz, divide by 60 to get cycles per frame(2,048,000 / 60) at this point i can say you only need 2 interrupts to be called every frame. way i did it(there i am sure more ways but this is mine lol) i called first half of the cycles than called first interrupt than vblank, second set than last interrupt, worked very well for me.
every opcode has an amount of cycles it takes to be called and you must keep track of this, by either adding or subtracting from a total.
the interrupts in space invaders do have an opcode but usually are not called directly by the program but by the emulator. but other systems have what i consider hardware interrupts that have no opcode.
the io in space invaders is controlled by io ports, info is listed on emutalk and if you need more feel free to ask, i can give you pages i found on them.
btw, ram mirror is not needed but you will be allowing some access into there as without it, the game will crash during the demo like what happened to me.
also i highly recommend that you completely read the info supplied on the emutalk thread, helps a lot. if you need more info let me know.
i also been thinking about a unique way of doing a opcode decoding process which i am unsure as of now would benefit greatly or cause issues but if done right, would completely get rid of the switch entirely.
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