Scoreboarding


Scoreboarding is a centralized method, first used in the CDC 6600 computer, for dynamically scheduling a pipeline so that the instructions can execute out of order when there are no conflicts and the hardware is available.
In a scoreboard, the data dependencies of every instruction are logged. Instructions are released only when the scoreboard determines that there are no conflicts with previously issued and incomplete instructions. If an instruction is stalled because it is unsafe to continue, the scoreboard monitors the flow of executing instructions until all dependencies have been resolved before the stalled instruction is issued.

Stages

Instructions are decoded in order and go through the following four stages.
  1. Issue: The system checks which registers will be read and written by this instruction. This information is remembered as it will be needed in the following stages. In order to avoid output dependencies the instruction is stalled until instructions intending to write to the same register are completed. The instruction is also stalled when required functional units are currently busy.
  2. Read operands: After an instruction has been issued and correctly allocated to the required hardware module, the instruction waits until all operands become available. This procedure resolves read dependencies because registers which are intended to be written by another instruction are not considered available until they are actually written.
  3. Execution: When all operands have been fetched, the functional unit starts its execution. After the result is ready, the scoreboard is notified.
  4. Write Result: In this stage the result is about to be written to its destination register. However, this operation is delayed until earlier instructions—which intend to read registers this instruction wants to write to—have completed their read operands stage. This way, so called data dependencies can be addressed.

    Data structure

To control the execution of the instructions, the scoreboard maintains three status tables:
The detailed algorithm for the scoreboard control is described below:
function issue
wait until ; // FU can be any functional unit that can execute operation op
Busy ← Yes;
Op ← op;
Fi ← dst;
Fj ← src1;
Fk ← src2;
Qj ← Result;
Qk ← Result;
Rj ← Qj 0;
Rk ← Qk 0;
Result ← FU;
function read_operands
wait until ;
Rj ← No;
Rk ← No;
function execute
// Execute whatever FU must do
function write_back
wait until
foreach f do
if Qj=FU then Rj ← Yes;
if Qk=FU then Rk ← Yes;
Result ← No;

Remarks

The scoreboarding method must stall the issue stage when there is no functional unit available. In this case, future instructions that could potentially be executed will wait until the structural hazard is resolved. Some other techniques like Tomasulo algorithm can avoid the structural hazard and also resolve WAR and WAW dependencies with Register renaming.