The ifsig instruction is not specifically designed to be a network instruction, but a general test for the arrival of any kind of signal. However, at present, only a single kind of signal has been implemented. This signal signifies that a TPing response has arrived for this cell, and is buffered in the IOS buffer.
The ifsig instruction specifies a signal type by a value in a CPU register (specified in the opcode.map file). If the signal type specified in the register, matches to the ``t'' (tag) field of any IOS structure in the IOS buffer, the ifsig instruction is considered true, and the next instruction will be executed in the next CPU cycle. If there is no match of the register to a tag in the buffer, the ifsig instruction is considered false, and the next instruction will be skipped.
This means that, in the case of the TPing response, ifsig will be true as long as there are unread TPing responses in the buffer, but it will become false once the TPing responses have been cleared from the buffer by the execution of tpingr instructions. The network ancestor uses ifsig to determine if it will read a TPing response using the tpingr instruction.