Suppose a consciousness-producing computer program which experiences its own isolated, deterministic world. There must be some critical instruction in the program which causes consciousness to occur; an instruction such that, if we halt the program immediately before it is executed, consciousness will not occur, and if we halt immediately after it is executed, consciousness will occur.

There needn't be any such thing. Consciousness is not an all or nothing thing, as is already evident from ordinary experience. As well ask how many atoms make life.