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Arm Compiler armasm User Guide : Exception tables and Unwind tables

Exception tables and Unwind tables

You use FRAME directives to enable the assembler to generate unwind tables.


Not supported for AArch64 state.

Exception tables are necessary to handle exceptions thrown by functions in high-level languages such as C++. Unwind tables contain debug frame information which is also necessary for the handling of such exceptions. An exception can only propagate through a function with an unwind table.

An assembly language function is code enclosed by either PROC and ENDP or FUNC and ENDFUNC directives. Functions written in C++ have unwind information by default. However, for assembly language functions that are called from C++ code, you must ensure that there are exception tables and unwind tables to enable the exceptions to propagate through them.

An exception cannot propagate through a function with a nounwind table. The exception handling runtime environment terminates the program if it encounters a nounwind table during exception processing.

The assembler can generate nounwind table entries for all functions and non-functions. The assembler can generate an unwind table for a function only if the function contains sufficient FRAME directives to describe the use of the stack within the function. To be able to create an unwind table for a function, each POP or PUSH instruction must be followed by a FRAME POP or FRAME PUSH directive respectively. Functions must conform to the conditions set out in the Exception Handling ABI for the Arm® Architecture (EHABI), section 9.1 Constraints on Use. If the assembler cannot generate an unwind table it generates a nounwind table.