US Patent 7831817 Two-level branch prediction apparatus

A two-level branch prediction apparatus includes branch bias logic for predicting whether a branch instruction will result in a branch being taken. Upon receipt of a first address portion of a branch instruction's address, a first store outputs a corresponding first branch bias value. A history store stores history data identifying an actual branch outcome for preceding branch instructions. A second store stores multiple entries, each entry including a replacement branch bias value and a TAG value. An index derived from the history data causes the second store to output a corresponding entry. The first branch bias value is selected unless the TAG value corresponds to a comparison TAG value derived from a second address portion of the branch instruction's address, in which event, the replacement branch bias value is selected.