Multi cycle DataPath

Transcript

1. 36
   One Alternative to Single-Cycle DP: Multi-Cycle DP
   • Cuts datapath into multiple stages (5 here), isolated using registers
   • FSM control “walks” insns thru stages (by staging control signals)
   + Insns can bypass stages and exit early
   P
   C
   Insn
   Mem
   Register
   File
   S
   X
   s1 s2 d
   Data
   Mem
   a
   d
   +
   4
   <<
   2
   I
   R D
   O
   B
   A
   s3
   s3
   s3 s4
   s5
   s5
   s5
   

   View Slide

2. 46
   Multi-Cycle Datapath Example: Add
   P
   C
   Insn
   Mem
   Register
   File
   S
   X
   s1 s2 d
   Data
   Mem
   a
   d
   +
   4
   <<
   2
   I
   R D
   O
   B
   A
   • Example: Add
   • Cycle 1: Read IMEM
   

   View Slide

3. 47
   Multi-Cycle Datapath Example: Add
   P
   C
   Insn
   Mem
   Register
   File
   S
   X
   s1 s2 d
   Data
   Mem
   a
   d
   +
   4
   <<
   2
   I
   R D
   O
   B
   A
   • Example: Add
   • Cycle 1: Read IMEM
   • Cycle 2: Decode + Read RF
   

   View Slide

4. 48
   Multi-Cycle Datapath Example: Add
   P
   C
   Insn
   Mem
   Register
   File
   S
   X
   s1 s2 d
   Data
   Mem
   a
   d
   +
   4
   <<
   2
   I
   R D
   O
   B
   A
   • Example: Add
   • Cycle 1: Read IMEM
   • Cycle 2: Decode + Read RF
   • Cycle 3: ALU
   

   View Slide

5. 49
   Multi-Cycle Datapath Example: Add
   P
   C
   Insn
   Mem
   Register
   File
   S
   X
   s1 s2 d
   Data
   Mem
   a
   d
   +
   4
   <<
   2
   I
   R D
   O
   B
   A
   • Example: Add
   • Cycle 1: Read IMEM
   • Cycle 2: Decode + Read RF
   • Cycle 3: ALU
   • Cycle 4: Writeback + Increment PC
   

   View Slide

6. 50
   Multi-Cycle Datapath Performance
   • Opposite performance split of single-cycle datapath
   + Short clock period
   – High CPI
   P
   C
   Insn
   Mem
   Register
   File
   S
   X
   s1 s2 d
   Data
   Mem
   a
   d
   +
   4
   <<
   2
   I
   R D
   O
   B
   A
   

   View Slide