The basic instruction format is a three-bit "prefix", fifteen-bit "decrement", three-bit "tag", and fifteen-bit "address". The prefix field specifies the class of instruction. The decrement field often contains an immediate operand to modify the results of the operation, or is used to further define the instruction type. The three bits of the tag specify three (seven in the 7094) "index registers", the contents of which are subtracted from the address to produce an "effective address". The address field either contains an address or an immediate operand.