Ericsson ZAT 19.2 Operation Manual page 105

4.1 What are S-registers
The S registers comprise the modem's active memory. These
contain the information that controls the operation of the modem.
There are 4 different S register memories in the modem. Each
memory contains 28 S registers (S0,S1,....$27) and 56 +S registers
(+$0,+S2,.. .., +$55).
As part of the description of each command the default value is
indicated with an underscore. These default values are carefully
selected in order to give the user a starting point from where it is
easy to use the modem for most applications. However, there can be
deviations from the standard default setting, due to national PTT
requirements, or as a result of customer requests. If in doubt about
this, the actual default values can be read out from the modem in the
following way:
Load factory profile with command
List all settings with command
ATISet|
4.1.1. Permanent memory:
This memory contains the factory configuration of the modem
and is not erasable. (Located in EPROM).
4.1.2. Active memory:
This S register memory controls the operation of the modem. It
is not permanent and will be destroyed in case of power down.
It is always the active memory that controls the operation of the
modem. The content of this memory may be changed by
commands.
4.1.3 Userdefined memory:
These are 2 memories in which the user may store userdefined
configurations. The memories are permanent (EEPROM).