This chapter is primarily intended for assembly language programmers. Some
additional detail is provided for application developers who may have a need to
use the stack unwind library routines.
Stack unwinding refers to the processes of procedure trace-back and context
restoration, both of which have several possible system and user-level
applications. A software stack unwinding convention is necessary on PA-RISC
because in the event of an interruption of execution, there is insufficient
information directly available to perform a comprehensive stack trace. The
stack trace is the basic operation performed in context restoration.
Some important tools are heavily dependent on the presence of the stack
unwinding facility. For example, system dump analysis tools examine all system
processes that were running at the time of a system crash, an operation which
involves multiple stack traces. Symbolic debuggers require the ability to
display the state of the call stack at any point during a program's execution.
Many language-specific features such as the ESCAPE mechanism in HP
Pascal also require stack unwinding capabilities.
The information necessary to perform a stack trace, such as the size of each
frame, is not available on the stack. One approach to supporting stack unwind
would have been to generate information at run time and store it in the frame
marker. This would have required extra instructions to be executed for most
procedure calls. The designers of the PA-RISC calling convention chose not to
use this approach under the assumption that stack unwinding is infrequently
necessary.
Alternatively, stack unwind information is generated once at compile time and
stored in a static data structure called the unwind table. An unwind table is
automatically built into each program file by the linker.
Each entry in the unwind table contains two addresses which describe a region
of code, typically the starting and ending address of a procedure. Each entry
also contains an unwind descriptor which holds information about the frame and
register usage of that region. When an unwind operation is required, the unwind
table is searched to find the region containing the instruction where the
exception or interrupt occurred.
The implementation details of stack unwinding are unimportant for most
high-level programs and programmers. Assembly programmers, however, need to
follow the documented guidelines outlined in this chapter to ensure that
programs can be successfully unwound.
Further details are available in Appendix C and D. Appendix C explains the
functionality of the PA-RISC unwind library routines. These can be used by
programmers who want or need to access stack unwind functionality from their
applications. Appendix D provides a more detailed description of what is
involved in implementing customized stack unwind routines.
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