SerialExpress heritage

General principles
SerialExpress can be viewed as an extension of the
processor/memory bus, extending outside of the motherboard chassis.
Although point-to-point cabling is used, the protocols reflect a
backplane heritage, with the following characteristics:
- Read/write transactions. Read and write transactions are used
to transfer data between nodes.
- Standard CSRs. Standard control and status registers are used
to initialize and configure nodes.
- Resets. After a bus reset, nodes appear at well defined
local-bus addresses.
- Bridges. Addresses can be extended and read/write transactions
are forwardable through bridges.
The assumption of a memory-mapped read/write interface
significantly different from networks, like Ethernet, or channels
(like Fibre Channel). The intent is to efficiently support small and
large data transfers, eliminating the processor from the task of
intervening in most transfers.
Many of the SerialExpress protocols represent refinements of other
cable-friendly IEEE 'bus' Standards, including:
- IEEE Std 1596 Scalable Coherent Interface (SCI)
- IEEE Std 1394 High Performance Serial Bus.

Leveraged Serial Bus protocols
Properties of SerialExpress that were derived from Serial Bus,
rather than SCI, include the following:
- Unaligned. Larger byte-aligned transfers are supported.
- Isochronous. Fixed-rate limited-latency isochronous data
transfers are supported.
- Duplex. Duplex cables and adaptive electronics reduces
cable-installation complexities. SerialExpress even includes loop
breaking protocols, not supported by Serial Bus.
- Multispeed. Multiple speeds can be supported on different
links.

Leveraged SCI properties
Properties of SerialExpress that were derived from SCI, rather
than Serial Bus, include the following:
- Scalability. Arbitration and acknowledge timeouts adapt to a
wide range of cable lengths.
- Concurrent transfers. Data can be concurrently sent over
nonoverlapping segments.
- Small packets. Large transfers are fragmented into smaller
64-byte packets.
- Fast arbitration. Arbitration is immediate when no packets are
passing through; latencies only affect arbitration recovery time
after transmission conflicts.
- Ringlets. A logical bus is implemented as a small ringlet,
around which packets are sent.
- Multicast isochronous. Flow-controlled multicasts simply
distribute information to multiple nodes.

Distinct SerialExpress properties
Properties of SerialExpress that were derived from SCI, rather
than Serial Bus, include the following:
- Reverse arbitration. Reverse flow arbitration efficiently
resolves arbitration conflicts.
- Sequential ordering. Command codes allow selective enforcement
of ordering constraints.
- Local fault retry. Ringlet local hardware fault retry
protocols are simple yet robust.
- Dual ringlets. Counter-rotating ringlets double the throughput
in closed daisy-chain topologies.
- Instant restart. Nodes use context info to instantly and
safely restart after bus reset conditions.
- Speed matching buffers. SerialExpress uses speed matching
buffers, rather than having links dynamically adapt to multiple
speed transmissions.
- Precise time. Wall-clock synchronization protocols compensate
for wiring delays, assuming identical out-and-back delays,
yielding improved clock-synchronization accuracies.

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Last modified: May 4, 1997