Program Loading (Processor-Specific)

As the system creates or augments a process image, the system logically copies a file's segment to a virtual memory segment. When, and if, the system physically reads the file depends on the program's execution behavior, system load, and so forth.

A process does not require a physical page unless the process references the logical page during execution. Processes commonly leave many pages unreferenced. Therefore, delaying physical reads can improve system performance. To obtain this efficiency in practice, executable files and shared object files must have segment images whose file offsets and virtual addresses are congruent, modulo the page size.

Virtual addresses and file offsets for 32–bit segments are congruent modulo 64K (0x10000). Virtual addresses and file offsets for 64–bit segments are congruent modulo 1 megabyte (0x100000). By aligning segments to the maximum page size, the files are suitable for paging regardless of physical page size.

By default, 64–bit SPARC programs are linked with a starting address of 0x100000000. The whole program is located above 4 gigabytes, including its text, data, heap, stack, and shared object dependencies. This helps ensure that 64–bit programs are correct because the program will fault in the least significant 4 gigabytes of its address space if the program truncates any of its pointers. While 64–bit programs are linked above 4 gigabytes, you can still link programs below 4 gigabytes by using a mapfile and the -M option to the link-editor. See /usr/lib/ld/sparcv9/map.below4G.

The following figure presents the SPARC version of the executable file.

Figure 13-1 SPARC: Executable File (64K alignment)

The following table defines the loadable segment elements for the previous figure.

Table 13-4 SPARC: ELF Program Header Segments (64K alignment)

The following figure presents the x86 version of the executable file.

Figure 13-2 32-bit x86: Executable File (64K alignment)

The following table defines the loadable segment elements for the previous figure.

Table 13-5 32-bit x86: ELF Program Header Segments (64K alignment)

The example's file offsets and virtual addresses are congruent modulo the maximum page size for both text and data. Up to four file pages hold impure text or data depending on page size and file system block size.

• The first text page contains the ELF header, the program header table, and other information.

• The last text page holds a copy of the beginning of data.

• The first data page has a copy of the end of text.

• The last data page can contain file information not relevant to the running process. Logically, the system enforces the memory permissions as if each segment were complete and separate The segments addresses are adjusted to ensure that each logical page in the address space has a single set of permissions. In the previous examples, the region of the file holding the end of text and the beginning of data is mapped twice: at one virtual address for text and at a different virtual address for data.

The end of the data segment requires special handling for uninitialized data, which the system defines to begin with zero values. If a file's last data page includes information not in the logical memory page, the extraneous data must be set to zero, not the unknown contents of the executable file.

Impurities in the other three pages are not logically part of the process image. Whether the system expunges these impurities is unspecified. The memory image for this program is shown in the following figures, assuming 4 Kbyte (0x1000) pages. For simplicity, these figures illustrate only one page size.

Figure 13-3 32-bit SPARC: Process Image Segments

Figure 13-4 x86: Process Image Segments

One aspect of segment loading differs between executable files and shared objects. Executable file segments typically contain absolute code. For the process to execute correctly, the segments must reside at the virtual addresses used to create the executable file. The system uses the p_vaddr values unchanged as virtual addresses.

On the other hand, shared object segments typically contain position-independent code. This code enables a segment's virtual address change between different processes, without invalidating execution behavior.

Though the system chooses virtual addresses for individual processes, it maintains the relative positions of the segments. Because position-independent code uses relative addressing between segments, the difference between virtual addresses in memory must match the difference between virtual addresses in the file.

The following tables show possible shared object virtual address assignments for several processes, illustrating constant relative positioning. The tables also include the base address computations.

Table 13-6 32-bit SPARC: ELF Example Shared Object Segment Addresses

Table 13-7 32-bit x86: ELF Example Shared Object Segment Addresses

Program Interpreter

A dynamic executable or shared object that initiates dynamic linking can have one PT_INTERP program header element. During exec(2), the system retrieves a path name from the PT_INTERP segment and creates the initial process image from the interpreter file's segments. The interpreter is responsible for receiving control from the system and providing an environment for the application program.

In the Oracle Solaris OS, the interpreter is known as the runtime linker, ld.so.1(1).