Forwarding Can Save Ticks When We Have a Data Hazard

What is the problem?

• Look at the example that begins on page 304 of the book
• It is a sequence of 4 R-format instructions followed by a sw
• The first instruction (sub) writes into $2
• Each of the following instructions reads $2
• This causes a problem in two of the instructions:
  • The sub instruction writes $2 in its tick 5
  • Each of the other instructions reads $2 in its tick 2
  • Tick 2 of the immediately following instruction (and) coincides with tick 3 of the sub instruction
  • Tick 2 of the next instruction (or) coincides with tick 4 of the sub instruction
  • So the and and the or are reading $2 before the sub can write to it
  • This is called a data hazard
  • The last two instructions of the example don't have this problem
• Forwarding can help alleviate this problem
• In some cases, forwarding eliminates the problem completely
• In other cases, forwarding reduces the effect of the problem but does not eliminate it completely

When do we need forwarding?

• An instruction writes into a register
• One of the next two instructions reads from that same register

Here's a short example:

• the sub instruction writes into $18
• the and instruction writes into $5
• three succeeding instructions read from $18 (arrows)
• two succeeding instructions read from $5 (arrows)
• the red arrows mark places where forwarding will be necessary
• the green arrows mark places where forwarding will not be necessary
• the difference is that the red arrows point no more than two instructions down from the instruction that wrote into the register; the green arrows point at least three instructions down