Some adapters understand only the SMBus (System Management Bus) protocol,
which is a subset from the I2C protocol. Fortunately, many devices use
only the same subset, which makes it possible to put them on an SMBus.
If you write a driver for some I2C device, please try to use the SMBus
commands if at all possible (if the device uses only that subset of the
I2C protocol). This makes it possible to use the device driver on both
SMBus adapters and I2C adapters (the SMBus command set is automatically
translated to I2C on I2C adapters, but plain I2C commands can not be
handled at all on a pure SMBus adapter).

Below is a list of SMBus commands.

Key to symbols
==============

S     (1 bit) : Start bit
P     (1 bit) : Stop bit
Rd/Wr (1 bit) : Read/Write bit. Rd equals 1, Wr equals 0.
A, NA (1 bit) : Accept and reverse accept bit. 
Addr  (7 bits): I2C 7 bit address. Note that this can be expanded as usual to 
                get a 10 bit I2C address.
Comm  (8 bits): Command byte, a data byte which often selects a register on
                the device.
Data  (8 bits): A plain data byte. Sometimes, I write DataLow, DataHigh
                for 16 bit data.
Count (8 bits): A data byte containing the length of a block operation.

[..]: Data sent by I2C device, as opposed to data sent by the host adapter.


SMBus Write Quick
=================

This sends a single byte to the device, at the place of the Rd/Wr bit.
There is no equivalent Read Quick command.

A Addr Rd/Wr [A] P


SMBus Read Byte
===============

This reads a single byte from a device, without specifying a device
register. Some devices are so simple that this interface is enough; for
others, it is a shorthand if you want to read the same register as in
the previous SMBus command.

S Addr Rd [A] [Data] NA P


SMBus Write Byte
================

This is the reverse of Read Byte: it sends a single byte to a device.
See Read Byte for more information.

S Addr Wr [A] Data NA P


SMBus Read Byte Data
====================

This reads a single byte from a device, from a designated register.
The register is specified through the Comm byte.

S Addr Wr [A] Comm [A] S Addr Rd [A] [Data] NA P


SMBus Read Word Data
====================

This command is very like Read Byte Data; again, data is read from a
device, from a designated register that is specified through the Comm
byte. But this time, the data is a complete word (16 bits).

S Addr Wr [A] Comm [A] S Addr Rd [A] [DataLow] A [DataHigh] NA P


SMBus Write Byte Data
=====================

This writes a single byte to a device, to a designated register. The
register is specified through the Comm byte. This is the opposite of
the Read Byte Data command.

S Addr Wr [A] Comm [A] Data [A] P


SMBus Write Word Data
=====================

This is the opposite operation of the Read Word Data command. 16 bits
of data is read from a device, from a designated register that is 
specified through the Comm byte. 

S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] P


SMBus Process Call
==================

This command selects a device register (through the Comm byte), sends
16 bits of data to it, and reads 16 bits of data in return.

S Addr Wr [A] Comm [A] DataLow [A] DataHigh [A] 
                             S Addr Rd [A] [DataLow] A [DataHigh] NA P


SMBus Block Read
================

This command reads a block of upto 32 bytes from a device, from a 
designated register that is specified through the Comm byte. The amount
of data is specified by the device in the Count byte.

S Addr Wr [A] Comm [A] 
           S Addr Rd [A] [Count] A [Data] A [Data] A ... A [Data] NA P


SMBus Block Write
=================

The opposite of the Block Read command, this writes upto 32 bytes to 
a device, to a designated register that is specified through the
Comm byte. The amount of data is specified in the Count byte.

S Addr Wr [A] Comm [A] Count [A] Data [A] Data [A] ... [A] Data [A] P