---

The Cross Bar Switch

---

• Interconecting multiple CPUs and Memory modules
  • History:

    One of the important problem that must be solved to make shared memory multi-processors possible is: Design a fast (and as cheap as possible) data transfer network

    ---

    ---

  • The first solution was the cross bar switch

    It is fast, but very expensive to make

    ---

    ---

  • Cheaper and effective solutions came later in the form of:

    Multi-stage Inter-connection Networks (MINs)

  ---

  ---

  ---

  ---

• The Crossbar Switch
  • A crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner.

  • Architecture of a uni-directional crossbar switch:
    1. The crossbar switch can switch inputs (yellow color squares) to the outputs (cyan color squares) - imagine each box is a CPU or a memory module.

    2. The setting of the switching fabric is under the control of a switching logic circuit

    3. The sources (inputs) first send request signals to the switching logic circuit.

       The switching logic circuit will set the switching fabric to route the requests from the input to their destinations.

    ---

  • Example:
    • CPU 0 wants to access memory module 3 (needs to send address value to memory module 3)
    • CPU 1 wants to access memory module 1 (needs to send address value to memory module 1)
    • CPU 2 wants to access memory module 0 (needs to send address value to memory module 0)
    • CPU 3 wants to access memory module 2 (needs to send address value to memory module 2)

    Setting of the switching control to enable the transmission of the memory addresses from CPU to the memory modules:

  ---

  ---

• Conflicting requests
  • Different inputs can request the same output.

  • Example:
    • CPU 0 wants to access memory module 3 (needs to send address value to memory module 3)
    • CPU 1 wants to access memory module 3 (needs to send address value to memory module 3)
    • CPU 2 wants to access memory module 0 (needs to send address value to memory module 0)
    • CPU 3 wants to access memory module 2 (needs to send address value to memory module 2)

  • The switching logic must resolve conflicts and determine which request will be forwarded (e.g., lowest CPU id will get through):

    The "denied" CPU will get a WAIT signal and will try its request again.

  ---

• The Non-Blocking property
  • A switching fabric is called non-blocking if multiple simultaneous requests destined for different outputs/destinations can always be forwarded.

  • Clearly, the Cross Bar switching fabric is non-blocking in terms of the number of inputs (that's too fast...)

  ---

• Advantages and disadvantages of the cross bar switch
  • Advantage: non-blocking

  • Disvantage: size switching fabric grows quadratically in terms of the number of inputs (that's too fast...)

  ---

  ---

• Bi-directional transfer
  • Bi-directional transfer can be easily accomplished by using a second cross bar switch connecting from memory modules to the CPUs