2. Function Unit Status

1. * * The ADDF is Next to Complete. The changes are noted in Red. Question ADDF will try to write back. Why can it not do this now? 2. Function Unit Status FU Busy Op Dest Sop1 Sop2 Unit1 Unit2 Rdy1 Rdy2 1[i] N 2[*1] Y Mult F0 F2 F4 No Yes 3[*2] N 4[+] Y Add F6 F8 F2 5[/] Y Div F10 F0 F6 2[*1] No Yes

2. Answer ADDF tries to write back to F6, but since the DIVF instruction has not yet read its operands (WAR), the scoreboarding technique will stall the write back until DIVF has read its operands.

3. * * * * The MULTF Finally Completes. The changes are noted in Red. 2. Function Unit Status FU Busy Op Dest Sop1 Sop2 Unit1 Unit2 Rdy1 Rdy2 1[i] N 2[*1] Y Mult F0 F2 F4 3[*2] N 4[+] Y Add F6 F8 F2 5[/] Y Div F10 F0 F6 2[*1] No Yes

4. * * The MULTF Writes Results. The changes are noted in Red. 2. Function Unit Status FU Busy Op Dest Sop1 Sop2 Unit1 Unit2 Rdy1 Rdy2 1[i] N 2[*1] N 3[*2] N 4[+] Y Add F6 F8 F2 5[/] Y Div F10 F0 F6 Yes Yes

5. The DIVF can now read its operands. The changes are noted in Red. 2. Function Unit Status FU Busy Op Dest Sop1 Sop2 Unit1 Unit2 Rdy1 Rdy2 1[i] N 2[*1] N 3[*2] N 4[+] Y Add F6 F8 F2 * * 5[/] Y Div F10 F0 F6 * *

6. The ADDF writes results and DIVF starts. The changes are noted in Red. 2. Function Unit Status FU Busy Op Dest Sop1 Sop2 Unit1 Unit2 Rdy1 Rdy2 1[i] N 2[*1] N 3[*2] N 4[+] N 5[/] Y Div F10 F0 F6 * *

7. The DIVF will finish execution and write its results. The changes are noted in Red. 2. Function Unit Status FU Busy Op Dest Sop1 Sop2 Unit1 Unit2 Rdy1 Rdy2 1[i] N 2[*1] N 3[*2] N 4[+] N 5[/] N