System Debug provides a large collection of predefined environment variables,
the names of which are reserved and may not be replaced by user-defined
variables with the VAR and LOC commands.
Several environment variables provide access to the current System Debug execution
environment. Examples of these variables include the current input radix
and the prompt string.
Other environment variables are used to access key components of the state
of the machine being examined.
For example, all of the machine registers defined in the HP 3000 and
HP Precision Architectures are available as environment variables.
Subject to the context of use, some of these
variables may be set by the user with the ENV command.
The environment variables that correspond to the CM and NM machine
registers are also accessible through the MR (modify register)
and DR (display register) commands.
All environment variables may be read (accessed) as expression operands.
Some environment variables also require privileged mode for modification
access.
The ENV command in chapter 4 gives a detailed description of each of the
predefined environment variables and specifies which ones may be modified
and which ones are read-only.