Classifications of S registers
S registers are classified in several ways.
a. Valid registers have addresses that are between zero and some
upper limit (the upper limit for most modems is 255). Negative addresses
are invalid.
The modem remembers the last used address. After power-up or reset, the
last used address is 0. Attempts to point to an invalid address do not change
the last used address.
b. A register to which RAM space is assigned is said to exist. Only
registers whose addresses are valid exist.
c. Implemented registers are registers that exist and perform a defined
modem function, and these definitions are presented.
d. Unimplemented registers are those that exist but serve no function.
A modem may contain some unimplemented registers. Some are mandatory for
backward compatibility. Others are included as a convenience in the firmware
design.
e. Value-mapped registers are those whose values represent modem
parameters.
f. Bit-mapped registers are those whose individual bit values represent
modem parameters.
g. Read-Only registers are those whose values may be read but not
altered by the = command. These registers normally present the current status
or outcome of an event or process.
h. Non-volatile registers are those whose values remain after a loss
of power. Non-volatile registers are not stored by the &Wn command.
i. Stored registers are those which may be copied into a stored profile
by the &Wn command.
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