The material in this section is intended for advanced Amiga users. This information assumes a working knowledge of the Amiga operating system and a familiarity with the Amiga ROM Kernel Manuals.
The ARexx interpreter, RexxMast, provides a uniform execution environment by running each program as a separate process in the Amiga's multitasking operating system. This allows for a flexible interface between an external host program and RexxMast. The host program can proceed concurrently with its operations or can wait for the interpreted ARexx program to finish. Each ARexx program has both an external and internal environment.
The external environment includes its process structure, input and output streams, and current directory. When each ARexx process is created, it inherits the input and output streams and current directory from its client, the external program that invoked the ARexx program. For example, if an ARexx program was started from a Shell, the ARexx program will inherit the input and output stream and current directory of that Shell. The current directory is used as the starting point in any search for a program or data file. External functions are limited to a maximum of 15 arguments.
The internal environment of an ARexx program consists of a static global structure and one or more storage environments. The global data values are fixed (static) at the time the program is invoked. These values include the program source code, static data strings, and argument strings. Once the program is running, these values cannot be changed.
ARexx programs invoked as commands usually have only one argument string, although the command tokenization option may provide more than one. A program invoked as an internal function can have any number of arguments. These arguments persist for the duration of the program.
The storage environment includes the symbol table used for variable values, numeric options, trace options, and host address strings. While the global environment is unique, there may be many storage environments during the course of the program execution. Each time an internal function is called, a new storage environment is activated and initialized. The initial values for most fields are inherited from the previous environment, but values may be changed afterwards without affecting the caller's environment. The new environment persists until control returns form the function.
Every storage environment includes a symbol table to store the value strings that have been assigned to variables. This symbol table is organized as a two-level binary tree. The primary level stores entries for simple and stem symbols. The secondary level is used for compound symbols. All of the compound symbols associated with a particular stem are stored in one tree, with the entry for the stem being the root of the tree.
Symbols are not entered into the table until an assignment is made to the symbol. Once created, entries at the primary level are never removed, even if the symbol subsequently becomes uninitialized. Secondary trees are released whenever a new assignment is made to the stem associated with that tree.
ARexx provides complete tracking for all of the dynamically allocated resources that it uses to execute a program. These resources include memory space, DOS files and related structures, and the message port structure. The tracking system was designed to allow a program to shut down at any point without leaving any resources hanging.
It is possible to go outside of the Amiga operating system from within an ARexx program. It is the programmer's responsibility to trace and return any resources allocated outside of the ARexx resource tracking system. ARexx provides a special interrupt facility so that a program can retain control after an execution error, perform the required cleanup, and exit.
