Scoreboard

The scoreboard consists of three parts:

Instruction Status

keeps track of which steps of instruction execution have been completed.

Functional Unit Status

indicates the state of each functional unit as instructions are issued

Busy

indicates if the unit is busy or not

Op

indicates the operation to perform

Fi

the desitnation register specifier

Fj, Fk

the source register specifiers

Qj, Qk

which funtional units will produce the source register

Rj, Rk

flags to indicate if source operand is ready

Register Status

indicates which functional unit will produce each register value with an outstanding write.

What happens in the pipeline?

Issue

• Completes when:
  • functional unit is not busy
  • destination register not pending
• Updates:
  • functional unit status
  • register result status

Read operands

• Completes when:
  • Rj and Rk both available
• Updates:
  • resets Rj and Rk

Execution

• Completes when:
  • operation done
• Updates:
  • nothing

Write results

• Completes when:
  • no functional unit is waiting for this register from a different functional unit
• Updates:
  • sets all R fields for functional units waiting for this instruction.