IBM 7090: Difference between revisions

 

The basic instruction format was a 3-bit ''prefix'', 15-bit ''decrement'', 3-bit ''tag'', and 15-bit ''address''. The prefix field specified the class of instruction. The decrement field often contained an immediate operand to modify the results of the operation, or was used to further define the instruction type. The three bits of the tag specified three ''[[index register]]s'', the contents of which were '''''subtracted''''' from the address to produce an ''effective address''. The address field either contained an address or an immediate operand.