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Windows 95 and Windows NT support a robust approach to handling exceptions, called Structured Exception Handling, which involves cooperation of the operating system but also has direct support in the programming language.
An “exception” is an event that is unexpected or disrupts the ability of the process to proceed normally. Exceptions can be detected by both hardware and software
You can write more reliable code with Structured Exception Handling. You can ensure that resources, such as memory blocks and files, are properly closed in the event of unexpected termination.
A distinctive feature of Structured Exception Handling is that after an exception handler is installed, it can handle the exception no matter how many other functions are called. Thus, function A can handle an exception raised inside a function called by A.
The following macros make it easy to include Structure Exception Handling in your Assembly programs. Each assembly statement has comments describing the operation being performed.
| @TRY_BEGIN MACRO Handler | ||
| pushad | ;;Save Current State | |
| mov esi, offset Handler | ;;Address of New Exception Handler | |
| push esi | ;;Save Old Exception Handler | |
| push dword ptr fs:[0] | ;;Install New Handler | |
| mov dword ptr fs:[0], esp | ||
| ENDM | ||
| @TRY_EXCEPT MACRO Handler | ||
| jmp NoException&Handler | ;;No Exception Occured, so jump over | |
| Handler: | mov esp, [esp + 8] | ;;Exception Occured, Get old ESP |
| pop dword ptr fs:[0] | ;;Restore Old Exception Handler | |
| add esp, 4 | ;;ESP value before SEH was set | |
| popad | ;;Restore Old State | |
| ENDM | ||
| @TRY_END MACRO Handler | ||
| jmp ExceptionHandled&Handler | ;;Exception was handled by @TRY_EXCEPT | |
| NoException&Handler: | ;;No Exception Occured | |
| pop dword ptr fs:[0] | ;;Restore Old Exception Handler | |
| add esp, 32 + 4 | ;;ESP value before SEH was set. 32 for pushad and ... | |
| ExceptionHandled&Handler: | ;;...4 for push offset Handler. (No Restore State) | |
| ;;Exception has been handled, or no exception occured | ||
| ENDM | ||
The above macros are used in the following way
| @TRY_BEGIN HandlerName | |
| ;Code in this place will be checked for exceptions. | |
| @TRY_EXCEPT HandlerName | |
| ;Code in this place will be executed if an exception occurs. | |
| @TRY_END HandlerName | |
| ;Normal execution path |
| ;Structured Exception Handling in Assembly ;(c) 2000, Rohitab Batra ;software@rohitab.com ;This Code was downloaded from http://www.rohitab.com/ ; ;To Compile this program, you need the 32-bit version of Turbo Assembler ; ;TASM32 /ml SEH ;TLINK32 SEH,SEH,,IMPORT32.LIB .386p .model flat ,stdcall EXTRN ExitProcess:PROC EXTRN MessageBoxA:PROC ;Define the @TRY_BEGIN, @TRY_EXCEPT and @TRY_END Macros Here .data |
|||
| szCaption | db | 'SEH in Assembly', 0 | |
| szException | db | 'Exception has been handled !!', 0dh, 0ah | |
| db | 'Press OK to terminate gracefully', 0 | ||
| szNoException | db | 'No Exception occured', 0 | |
.code WinMain: @TRY_BEGIN Zero_Address_Access |
|||
| mov | ebx, 0 | ;Prepare to write to address 0 | |
| mov | [ebx], ebx | ;Write to address 0 (Access Violation) | |
| ;Comment the above line if to remove the exception | |||
| @TRY_EXCEPT Zero_Address_Access | |||
| ;This code will get executed if an exception occurs. | |||
| call | MessageBoxA, 0, offset szException, offset szCaption, 0 | ||
| jmp | ExitProgram | ||
| @TRY_END Zero_Address_Access | |||
| ;Normal Execution Path. | |||
| ExitProgram: | |||
| call | MessageBoxA, 0, offset szNoException, offset szCaption, 0 | ||
| call | ExitProcess, 0 | ||
| END WinMain | |||