Table of Contents

Types of shared objects

In our refactoring of RTEMS we've created several different types of shared objects. All are ELF dynamic objects in overall structure but each plays a different role:

• RTEMS object (rtems.so). This contains most of RTEMS, newlib and other basic runtime support. It's unique in that it's not loaded by the dynamic linker; instead it's loaded by U-Boot at a fixed location, the start of the Run-Time Support Region of memory. U-Boot loads each segment of rtems.so at  the so-called physical address of the segment so we have to use a special linker script to set those properly. The RTEMS object is permanently resident, aka installed.
• Symbol-Value Tables (*.svt).The dynamic symbol table of this type of object is used as a read-only lookup table whose content is defined by C source code. Each symbol's value is the location of a data object, generally a string or a struct. There can be up to 32 SVTs numbered 0-31, with number 31, the System table sys.svt, and number 30, the Application table app.svt, being loaded at system startup.
• Ordinary shared objects, aka shareables (*.so). Basically add-on libraries that are installed when needed, e.g., nfs.so and console.so.
• Tasks (*.exe). A task is roughly equivalent to a main program. The loader and dynamic linker cooperate to load a task and, when necessary, to load and install any shareables it needs. A task also has an associated thread which executes the task code starting at its entry point, always present and always named ask_Entry(). The properties of the task such as name, priority, etc., are usually looked up in an SVT. One or more tasks are automatically chosen, using more information stored in SVTs, to be loaded and run at system startup. Note that tasks are never themselves installed.
• Device drivers (*.drv). These objects perform device initialization and may register RTEMS device drivers. They are always installed.

All types of objects except rtems.so:

• Are loaded into storage that is allocated dynamically from the RTS Region.
• May have a function called lnk_prelude() which is called after the object is fully linked and initialized but before the entry point, if any, is called. For instance driver objects have no entry point but do their work in their lnk_prelude() functions.

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