Data forwarding example

  Consider the five stage pipeline architecture:

  IF instruction fetch, PC is address into memory fetching instruction
  ID instruction decode and register read out of two values
  EX execute instruction or compute data memory address
  M  data memory access to store or fetch a data word
  WB write back value into general register


         IF       ID       EX        M       WB
    +--+     +--+     +--+     +--+     +--+
    |  |     |  |     |  |-|\  |  |     |  |
    |  |     |  |    /|  | \ \_|  |     |  |
    |PC|-(M)-|IR|-(R) |  | / / |  |-(M)-|  |--+
    |  |     |  |  ^ \|  |-|/  |  |     |  |  |
    +--+     +--+  |  +--+     +--+     +--+  |
     ^        ^    |   ^   ALU  ^        ^    |
     |        |    |   |        |        |    |
 clk-+--------+--------+--------+--------+    |
                   |                          |
                   +--------------------------+

  Now consider the instruction sequence:

  400  lw  $1,100($0)  load general register 1 from memory location 100
  404  lw  $2,104($0)  load general register 2 from memory location 104
  408  add $3,$1,$2    add contents of registers 1 and 2, sum into register 3
  40C  add $4,$3,$1    add contents of registers 3 and 1, sum into register 4
  410  beq $3,$4,-100  branch if contents of register 3 and 4 are equal to 314
  404  add $3,$3,$3    add ..., this is the "delayed branch slot" always exec.

  The pipeline stage table with NO data forwarding is:

  lw   IF ID EX M  WB
  lw      IF ID EX M  WB
  add        IF ID -  -  EX M  WB
  add           IF -  -  ID -  -  EX M  WB
  beq                    IF -  -  ID -  -  EX*M  WB     * becomes nop
  add                             IF -  -  ID EX M  WB

  time 1  2  3  4  5  6  7  8  9  10 11 12 13 14 15 16
  a dash represents a stall of the pipeline

  The pipeline stage table with data forwarding is:

  lw   IF ID EX M  WB
  lw      IF ID EX M  WB
  add        IF ID -  EX M  WB
  add              IF ID EX M  WB
  beq                 IF ID -  EX*M  WB     * becomes nop
  add                       IF ID EX M  WB 

  time 1  2  3  4  5  6  7  8  9  10 11
  a dash represents a stall of the pipeline

  Note the inherent stall for using data immediately after a load.
  Note the inherent stall for beq using data from the ALU.

  The hazard unit must know what the data forwarding unit can do.
  With data forwarding there is no hazard from the first 'add' to
  the second 'add' instruction.