21. The museum maintains an index register of all valuables that are kept in the facilities of the University. 22. Instructions that modified or stored index registers used a four-bit field ( xo ) to select that index register . 23. The machine's most significant innovation is generally considered to be its incorporation of index registers , commonplace on modern computers. 24. Instructions that modified or stored index registers used a four-bit field ( xo ) to select that index register. 25. The third index register was dedicated to pointing at the current procedure's stack frame on the call / return stack. 26. Another four-bit field in the instruction allowed for any AC other than AC0 to be used as an index register . 27. The Western Design Center also designed and produced the accumulator and index registers , as well as other 65C816 enhanced features. 28. There were conditional jump operations based on the values in the index registers specified in the " tag " field. 29. Op codes were 6 bits, registers 3 bits, index register ( modifier ) 2 bits and data address 13 bits. 30. Initially registers were limited to an University of Illinois, designed an index register as an extension to the original EDSAC hardware.
