| PCI-E |
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| October 2013 | |
A PC has a spine and this spine is evolving with time. This training note explains what the PC spine is, how it has evolved, and its impact on computer uses. The history of PC is quite short and is less than 50 years. Think of the PC as human without limbs. PC has a head and a spine. CPU is the head which can think and memorize. What is the spine? The spine provides the main skeleton of the body and accommodates the nerve to connect various parts of the body with the brain. In the early years, the spine had a few segments and they were called the North Bridge (GMCH), South Bridge (ICH), and BIOS (FWH). See the illustration. Each segment had its own path for communicating. This design is clumsy. Whilst we would expect the development to be towards a single segment spine, PC has gone a different direction and we are seeing multiple single segment spines! This direction is to allow the PC to have more limbs than human in order to serve human better. The good news is that all spines are based on one single design called PCIe for Peripheral Control Interconnect Express which makes a lot of sense. Note that the PCIe has emerged from a minor role to a major role. We are currently on PCIe 3.0 with a data transfer capability of 8Gbps per link and an aggregation capacity of 16 links per bridge. This means the aggregated data transfer is as high as 128Gbps between 2 points inside the PC. Assume 8 bits form 1Byte. We have 16GB/s. This is higher than the memory talking to the CPU (being 11GB/s for DDR3). This is impressive- the spine is faster than the memory. Note: Some systems have multiple DDR3 controllers and the remark would not be valid. Both the memory and the spine will evolve to the 4th generation: DDR4 and PCIe 4.0. DDR4 will have a single link transfer rate of 24GB/s and PCIe 4.0 an aggregated rate of 32GB/s. Owing to this trend, it has become practical for a PC to have more limbs to reach out to the outer world. Devices for reaching out consist of USB, LAN, and Thunderbolt. USB is going to version 3.1 soon with a data transfer rate of 10Gb/s whereas Ethernet LAN is going to hit 40Gb/s. Thunderbolt will reach 20Gb/s and is mainly a threat to USB for reaching out to peripherals. Ethernet will stay for inter-computer connections, and it competes with another standard called Infiniband for domination. Thunderbolt is designed like PCIe and is therefore a natural candidate for future peripheral connection. Similarly, a new idea has surfaced and it is called Switching over PCIe. A PCIe Switch is able to connect 2 PC for example via a PCIe card in each PC. The data transfer rate is 128Gb/s (see above underlined) which is readily higher than Ethernet 40Gb/s. This is a recent development and we are seeing some signs of technology unification which will provide a good basis for simplifying further developments. END |