Interconnection structure of computer

The interconnection structure is the collection of paths that connect the main modules of a computer (CPU, memory and I/O modules) so that they can exchange data and control information. Its design depends on what each module needs to send and receive.

Modules and their transfers

   +-----+  data / address / control  +--------+
   | CPU |<==========================>| Memory |
   +-----+                            +--------+
      ^                                   ^
      |        +------------+             |
      +------->| I/O module |<------------+
               +------------+     (DMA)
                 |        |
              Peripheral devices

• Memory module: has N words with unique addresses. It accepts Read and Write control signals, an address and data.
• I/O module: seen from inside the computer, it works like memory (read and write), but it also controls one or more external devices and can send interrupt signals to the CPU.
• CPU: reads instructions and data, writes data, sends control signals, and receives interrupts.

Types of transfers (operations)

1. Memory to CPU: the CPU reads an instruction or data.
2. CPU to memory: the CPU writes data.
3. I/O to CPU: the CPU reads data from an I/O device through an I/O module.
4. CPU to I/O: the CPU sends data to an I/O device.
5. I/O to or from memory: direct memory access (DMA), without the CPU.

Types of interconnections

• Bus structure: a shared communication path made of address, data and control lines. Only one device can send at a time. This is the most common structure.
• Multiple buses (bus hierarchy): a local bus, a system bus and an expansion bus, joined by bridges, to reduce the load on each bus.
• Point-to-point (crossbar or switch): each pair of modules has its own direct link. This is used in modern processors (for example QuickPath, HyperTransport, PCI Express).

Operation on a bus

To send data, a module must (1) obtain the use of the bus (arbitration), then (2) place the address, control signals and data on the lines. For a read, the CPU sends the address and a Read signal, and the memory places the data on the data bus. For a write, the CPU sends the address, the data and a Write signal, and the memory stores the data.