SpinRite tests the data surfaces of writeable magnetic disks, including IDE, SATA, and floppy disks. To be useful, memory cells must be readable and writeable. The standard defines clones, which are writeable, and snapshots, which are read-only. Any writeable firmware in the computer can be infected by malicious code. For example, the program counter is not directly writeable in many processor architectures. The service exposes a read and writeable packet for sharing TCNs. Instead of triggers are for making views writeable.