Dynamic Scheduling

1

• Dynamic Scheduling
• How have we dealt with data hazards so far?
• Forward logic to reduce effective pipeline
  latency
• Hazard detection hardware stalls
• Compiler scheduling of instructions, known as
  static scheduling
• But the instructions are issued in order

2

• Dynamic Scheduling (Contd)
• DIVD F0, F2, F4
• ADDD F10, F0, F8
• SUBD F12, F8, F14
• Due to the stalls between DIVD and ADDD, SUBD
  cannot execute, although it has no dependence on
  the above two instructions. This is a performance
  limitation
• Dynamic scheduling overcomes this limitation
• The hardware dynamically changes the order of
  instruction execution to reduce the stalls.
• The pipeline could do out-of-order execution and
  out-of-order completion

3

• Dynamic Scheduling with a Scoreboard
• (Figure 4.3)
• Scoreboarding is a technique that allows
  instructions to execute out of order
• WAR hazards may arise when instructions are
  issued out of order
• DIVD F0, F2, F4
• ADDD F10, F0, F8
• SUBD F8, F8, F14

4

• Dynamic Scheduling with a Scoreboard (Contd)
• There is an antidependence between ADDD and SUBD
• A WAR hazard exists if SUBD executes before ADDD
• Similarly, if the destination of SUBD were F10, a
  WAW hazard would occur
• Both these types of hazards are to be detected
  and avoided in a scoreboard

5
Scoreboard Example LD F6, 34 (R2) LD F2,
45 (R3) MULTD F0, F2, F4 SUBD F8, F6,
F2 DIVD F10, F0, F6 ADDD F6, F8,
F2 (Figure 4.4, 4.5, 4.6 and 4.7)