Editing Twistor memory (section)

===Piggyback twistor===
Another form of twistor ROM replaced the permanent magnet cards with a second magnetic tape wrapped around the first on the twistor lines, in a "piggyback" configuration. This tape was coated with coballoy instead of permalloy, which is much "harder" magnetically, requiring about twice the field in order to flip. To make the system even harder, the coballoy tape was about two and a half times thicker than the permalloy one, so the resulting field strength was five times. The external current required to flip the state of the coballoy tape was about 15 times that of the normal operational current.

Read operations in the piggyback are identical to the permanent magnet version. Writes were slightly more complex, due to the fact that piggyback twistors all featured the magnetic tape along the entire length of the X wire. This meant that any one solenoid was wrapping both the bit that is being written as well as the one on the section of return wire. To set the one both and not the other, the solenoid was first powered in one direction and then the other, while the current in the twistor line remained constant. This created two magnetic fields in turn, one aligned with the first section of wire and then the second. All reads and writes were carried out on paired bits in this fashion.