Virus

A virus typically deletes or otherwise corrupts files; less destructively, it may result in a message being displayed at some predetermined prompt. Let us see how to model all such eventualities at a given computer.

Since all those behaviours correspond to the computer not functioning as it should, we choose for state space (of that part of Cyberspace in which we are interested) the set $X$ of pairs $(c,s)$, where $c$ denotes the current program state of the computer and $s$ denotes the state as specified: as it should to be. An action of the computer relates state $(c,s)$ to state $(c',s')$ iff that action in fact transforms $c$ to $c'$ but ought to transform $s$ to $s'$.

The level function we choose measures the deviation of the current state from its value as it ought to be. Thus $\lambda (c,s)$ equals the number of bits (say) in the program state not equal to their correct value (i.e. as determined by the second component of our system state). Even if the virus remains quiescent for some time after its invasion, it is still present as an incorrect component of the system's state and so is accurately captured by that definition of $\lambda$. The pre-order defined from that level function yields an entropy structure with respect to which a virus increases entropy. For the only way it can affect the computer is by modifying the computer's program state in some way.

Note that the function $\lambda$ provides more detail than we need. Indeed even a single wrong bit in certain parts of a computer could have unpredictable consequences. So in this instance there seems little value attached to the ordering on actions introduced in section 4.2.