Enhanced Network Instruction Set

The original non-network Tierra had a five bit instruction set, providing thirty-two instructions [6]. For the network experiment, this has been extended to six bits, providing sixty-four instructions. These instructions are summarized at: http://www.hip.atr.co.jp/~ray/tierra/netreport/opcode.map. The enhancements to the instruction set are documented in [10].

However, it is worth repeating here, that the digital organisms have access to ``sensory'' data. The sensory mechanism provides a ``TPing'' data structure (named after the unix ping command) for each machine participating in the Tierra network. The TPing data structure includes the following elements, among others:

• Fresh When a TPing data structure arrives, it receives a time stamp on the local machine. This can be used to determine if the data is sufficiently fresh to be used reliably.
• Speed This is the number of Tierran virtual machine instructions per second, calculated during the last million instructions executed. Since the Tierra process runs as a low priority background process, the digital organisms only get left-over CPU cycles. This variable is a key measure of their energy supply.
• NumCells This tells how many organisms there are on the node. This is relevant since the CPU cycles (Speed) and the SoupSize will have to be divided among this many organisms.
• SoupSize This is the size in bytes, of the memory space of the ``soup'' on the node. This tells how much room there is on the node to hold organisms.
• TransitTime This measures the time in milliseconds, that it takes for the TPing request to cross the net and return. This gives a measure of the ``distance'' between nodes [7].
• Time This is currently implemented as the local 32 bit clock time. However, we intend to change it to the local 24 hour clock time. Creatures that choose to migrate around the planet staying on the dark side (or something like that) could use this kind of data.