Instruction issue/overtaking

Fundamental principle:

• Split ID into two components:
  • IS (Issue)

    Each instruction is processed in order, and the scoreboard is interrogated and updated to reflect the data flow

  • RO (read operands)

    Buffer each issued instruction, wait until no data hazards, then initiate execution. Execution of delayed instructions can start in any order

Simple assumption (for scoreboarding):

• Operands are always read from registers -- ie there is no forwarding

The role of the scoreboard is to activate those appropriate instructions delayed in the RO stage, just as soon as their operands are delivered to their source registers.