Read, Write and Execute Privileges.

Tierran programs only have write access within their own `cell membrane' (apart from when they are in the process of creating a daughter cell, when they also have write access to a specific additional chunk of memory, which has been allocated by the Tierra operating system). A similar situation exists in Cosmos. However, Tierran programs have read and execute privileges for all areas of instruction memory, so that they can directly examine the code of other programs, and even execute this code. Cosmos cells, on the other hand, only have direct read and execute privileges within their own cell membrane, and must rely on the system's communication facilities to interact with other cells (see Section 4.6.1). This restriction in Cosmos is related to the guiding analogy of the biological cell, which cannot directly read the genetic code of a neighbouring cell.