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Trapping FP overflows and other exceptions

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  • Paul Dixon
    replied
    It occurred to me that if anyone tries to add to this code that they might add in the latest win32api.inc which will then conflict with all the definitions I copied from the old .inc file so here's a modified version which now works with the PBCC6 .inc files.

    Code:
    'PBCC6.0 program
    'This code attempts to handle exceptions not normally trapped by PowerBASIC.
    'It consists of 3 routines.
    '1) PBMain just a short routine to show how to use the other 2 functions
    '2) SetupExceptionHandler which installs or removes the exception handler
    '3) ExceptionHandler  which is called, via the operating system, when an exception occurs.
    '
    'Floating point exceptions need to be explicitly allowed by clearing one or more of 6 mask bits in the FPU control word.
    'Other exceptions happen anyway but this program shows how to intercept them to do something useful before the default
    '  exception handler of the OS gets them and reports a the error.
    '
    ' It is possible, though sometimes difficult, to resume execution of the program from after the point of the exception.
    '  This code shows how to resume at the next address after a PrivelidgedInstruction exception.
    '  It also shows how to resume at another address such as an error handler. This should be in the same SUB/FUNCTION as the error.
    '
    'The default in this program is to enable all FP exceptions apart from INEXACT_RESULT as this will trip up most people
    '  more than it helps them! e.g. 1/3 will trigger the exception as the result cannot be exactly represented in binary.
    '  If you want to trap INEXACT_RESULT then set %FPTrapInexactResultException = 1   below.
    '
    'The only ASM here is to set the FPU control register bits so don't be put off by it.
    '
    'Three distict outcomes are shown in this code.
    ' If there is a priveliged instruction exception caused by CLI or STI (clear or set interrupt flag) then the inxtruction is ignored
    '  and execution continues at the next address as if the instruction wasn't there. Other priviliged instructions are passed to the
    '  default handler (usually the OS).
    '
    'If there is a floating point exception then it is reported along with a dump of the FP registers, it's then passed to the default handler.
    '
    'If there is an integer divide by zero then the error is reported and execution continues at the error handler specified in the call
    '   when the Exception Handler was installed.
    '
    'There are lots of potential problems here. This code is just posted to demonstrate how exceptions can be dealt with.
    'In particular, returning execution to the ResumeAddress will only work if the exception occurs in the same function as the ResumeAddress
    ' but calls to outside functions in maths routines are very common so care needs to be taken.
    '
    'Just for demonstration purposes I print things out in the ExceptionHandler to inform the user what has happened. Normally you wouldn't
    '  do this if you are returning execution to the program.
    
    
    #COMPILE EXE
    #DIM ALL
    #REGISTER NONE
    
    #INCLUDE "win32api.inc"
    
    
    %FPTrapInexactResultException = 0    'set to non zero if you want to trap FPInexactResults (specialised use only)
    %enable = 1
    %disable= 0
    
    %CLI= &hfa     'opcode of STI instruction    'these are just to test the PRIVILEGED_INSTRUCTION exception
    %STI= &hfb     'opcode of CLI instruction
    
    
    GLOBAL ExeptionHandlerResumeAddress AS DWORD
    
    
    
    FUNCTION SetupExceptionHandler(EnableTrap AS LONG,ResumeAddress AS DWORD) AS LONG
    'EnableTrap = 0 for uninstall handler, not 0 for install handler
    'ResumeAddress = address WITHIN THE ERRING FUNCTION/SUB to which you would like the code to return after the exception is handled
    '              =0 for don't resume.
    
    STATIC hHandler AS LONG
    
    LOCAL HandlerPointer AS LONG
    LOCAL NewControlWord, mask AS INTEGER    'control word is a 16 bit register
    LOCAL OriginalControlWord AS INTEGER
    
    ExeptionHandlerResumeAddress = ResumeAddress
    
        IF EnableTrap THEN
            'install the exception handler
            HandlerPointer = CODEPTR(ExceptionHandler)
            hHandler = AddVectoredExceptionHandler( 1  , HandlerPointer )
    
            IF hHandler = 0 THEN
               'Failed to add handler
                FUNCTION = 0
            ELSE
                'Handler installed
                FUNCTION = 1
    
                'now set the FPU control register to enable exceptions on all conditions
                !fstcw OriginalControlWord  'save the original control word
                !mov ax,OriginalControlWord 'get the control word
    
                #IF %FPTrapInexactResultException = 0
                !and ax,&hffe0              'zero the exception mask bits except for InexactResult
                #ELSE
                !and ax,&hffc0              'zero the exception mask bits (lower 6 bits or control word) to allow all 6 exceptions
                #ENDIF
    
                !mov NewControlWord,ax      'save the new control word
                !fldcw NewControlWord       'set the FPU to use the new control word
    
            END IF
    
        ELSE
            'remove the exception handler
            hHandler = RemoveVectoredExceptionHandler(hHandler)
            IF hHandler = 0 THEN
                'Couldn't remove exception handler
                FUNCTION = 0
    
            ELSE
                'Exception handler removed
                FUNCTION = 1
                !fldcw OriginalControlWord  'restore the original control word
    
            END IF
    
        END IF
    
    END FUNCTION
    
    
    
    FUNCTION ExceptionHandler(BYVAL pException_Pointers AS Exception_Pointers PTR) AS LONG
    
    LOCAL ThisExceptionPointer AS Exception_Pointers
    LOCAL ThisExceptionRecordPointer AS Exception_Record PTR
    LOCAL ThisExceptionRecord AS Exception_Record
    LOCAL temp AS BYTE PTR
    LOCAL r AS LONG
    LOCAL pt AS DWORD
    LOCAL FPControlWord AS INTEGER
    LOCAL FPControlWordNew AS INTEGER
    LOCAL WhatToDoNext  AS LONG
    
    
    ThisExceptionPointer = @pException_Pointers
    ThisExceptionRecordPointer = ThisExceptionPointer.ExceptionRecord
    ThisExceptionRecord = @ThisExceptionRecordPointer
    
    WhatToDoNext=%EXCEPTION_CONTINUE_SEARCH 'set the default for all exceptions I don't handle here.
                                            'the 2 options are:
                                            '%EXCEPTION_CONTINUE_SEARCH which passes the exception on to the next handler in line (the OS?)
                                            '%EXCEPTION_CONTINUE_EXECUTION which resumes execution of the code
                                            'If execution is resumed, the address to resume at can be set within the handler
    
    !fnstcw FPControlWord   'get FP control word
    !fnclex                 'clear all FP exeptions. Got to do this as following lines use the FPU and may cause a recurring exception
    !mov ax,FPControlWord
    !or ax,&h3f             'mask all exception while in the exception routine
    !mov FPControlWord,ax
    !fldcw FPControlWord    'load new FP control word
    
    'now handle each possible exception
    
    SELECT CASE ThisExceptionRecord.ExceptionCode
        CASE  BITS(DWORD,%STATUS_FLOAT_OVERFLOW)
    
            PRINT "There has been a FP overflow at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            PRINT "here are the FPU details at the time of the exception"
    
            PRINT "ControlWord   ";HEX$(@[email protected])
            PRINT "StatusWord    ";HEX$(@[email protected])
            PRINT "TagWord       ";HEX$(@[email protected])
            PRINT "ErrorOffset   ";HEX$(@[email protected])
            PRINT "ErrorSelector ";HEX$(@[email protected])
            PRINT "DataOffset    ";HEX$(@[email protected])
            PRINT "DataSelector  ";HEX$(@[email protected])
    
            pt=VARPTR(@[email protected](0))
    
            REDIM reg(0 TO 7) AS EXT AT pt
            FOR r& = 0 TO 7
                PRINT "ST(";r&;")=";reg(r&)
            NEXT
    
            PRINT
            PRINT "Press a key to pass error to default exception handler"
            WAITKEY$
    
    
        CASE BITS(DWORD,%STATUS_FLOAT_UNDERFLOW)
            PRINT "There has been a FP underflow at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            WAITKEY$
    
        CASE BITS(DWORD,%STATUS_FLOAT_INVALID_OPERATION)
            PRINT "There has been an Invalid FP Operation at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            WAITKEY$
    
        CASE BITS(DWORD,%STATUS_FLOAT_INEXACT_RESULT)
    
    
        CASE BITS(DWORD,%STATUS_FLOAT_DIVIDE_BY_ZERO)
    
    
        CASE BITS(DWORD,%STATUS_FLOAT_DENORMAL_OPERAND)
    
    
        CASE BITS(DWORD,%STATUS_ILLEGAL_INSTRUCTION)
    
    
        CASE BITS(DWORD,%STATUS_ACCESS_VIOLATION)
    
    
        CASE BITS(DWORD,%STATUS_PRIVILEGED_INSTRUCTION)
            PRINT "STATUS_PRIVILEGED_INSTRUCTION exception has occurred"
    
            temp = @[email protected]    'get the EIP register to see where the instruction was that caused the exception
    
            IF @temp = %STI OR  @temp = %CLI THEN   '@temp = the instruction itself
                'if its set/clear interrupt flag I'll just increment the return address pointer to ignore the exception
                INCR @[email protected]    'return execution to the byte after the one that caused the exception
    
                PRINT "You can't mess with the interrupt flag. I'll ignore it."
                PRINT "Execution will now continue"
                PRINT
                WhatToDoNext=%EXCEPTION_CONTINUE_EXECUTION
            ELSE
                'it's not an interrupt flag instruction so pass it to the default handler
                PRINT "You aren't allowed to use that instruction at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
                PRINT "I'm going to crash now."
    
                WhatToDoNext=%EXCEPTION_CONTINUE_SEARCH
            END IF
    
    
    
        CASE BITS(DWORD,%STATUS_STACK_OVERFLOW)
    
    
        CASE BITS(DWORD,%STATUS_FLOAT_MULTIPLE_FAULTS)
    
    
        CASE BITS(DWORD,%STATUS_INTEGER_OVERFLOW)
    
    
        CASE BITS(DWORD,%STATUS_INTEGER_DIVIDE_BY_ZERO)
            PRINT "Integer divide by zero at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            PRINT
            'as a test we'll try to return to the faulty function's error trap
            IF ExeptionHandlerResumeAddress <>0 THEN
                @[email protected] = ExeptionHandlerResumeAddress
            END IF
    
            WhatToDoNext=%EXCEPTION_CONTINUE_EXECUTION
    
    END SELECT
    
    
    IF WhatToDoNext =%EXCEPTION_CONTINUE_SEARCH THEN
        FUNCTION = %EXCEPTION_CONTINUE_SEARCH
    ELSE
        FUNCTION = %EXCEPTION_CONTINUE_EXECUTION
        !fldcw FPControlWord   'restore the FPUControl word if I'm going to return to executing my code
    
    END IF
    
    
    END FUNCTION
    
    
    
    FUNCTION PBMAIN () AS LONG
    
    LOCAL sum AS SINGLE
    LOCAL ResumeAddress AS LONG
    
    ResumeAddress = CODEPTR(ErrorTrap)
    
    
    IF SetupExceptionHandler(%enable,ResumeAddress) = 0 THEN
        PRINT "Failed to add exception handler"
    ELSE
        PRINT "Exception Handler installed "
        PRINT "Press a key to cause a Privileged Instruction exception"
        PRINT
        WAITKEY$
    
    '############################################
    '# cause a Privileged Instruction exception
    '############################################
    
    !cli
    
    
        PRINT "Press a key to cause an Integer Divide by zero exception"
        PRINT
        WAITKEY$
    
    '############################################
    '# cause an integer divide by zero exception
    '############################################
    LOCAL a,b,c AS LONG
    
    a=0
    b=1
    c=b\a
    
        'in this example the code doesn't reach this point but in real code you should uninstall the exception handler
        ''like this when you're finished with it.
        IF SetupExceptionHandler(%disable,0)= 0 THEN
            PRINT "Couldn't remove exception handler"
        ELSE
            PRINT "Exception handler removed"
        END IF
    
    
    END IF
    GOTO xit
    
    ErrorTrap:
    PRINT "I've resumed execution at the error trap."
    PRINT "press a key to cause a fatal FP overflow"
    PRINT
    WAITKEY$
    
    '############################################
    '# cause a FP overflow exception
    '############################################
    
    
        sum = 1
        DO
            sum = sum + sum        'cause an overflow exception by counting too high
        LOOP
    
    
    xit:
    WAITKEY$
    END FUNCTION

    Leave a comment:


  • Paul Dixon
    replied
    The original was written for PBCC4.
    Changes to the Win32api.inc files in later versions of the complier caused problems with the original so this version
    is the exact same code but with all the relevant definitions extracted from the PCBB4 .inc files and included directly in the source code instead.
    It now works in PBCC6.


    Code:
    'PBCC4.0 program
    'This code attempts to handle exceptions not normally trapped by PowerBASIC.
    'It consists of 3 routines.
    '1) PBMain just a short routine to show how to use the other 2 functions
    '2) SetupExceptionHandler which installs or removes the exception handler
    '3) ExceptionHandler  which is called, via the operating system, when an exception occurs.
    '
    'Floating point exceptions need to be explicitly allowed by clearing one or more of 6 mask bits in the FPU control word.
    'Other exceptions happen anyway but this program shows how to intercept them to do something useful before the default
    '  exception handler of the OS gets them and reports a the error.
    '
    ' It is possible, though sometimes difficult, to resume execution of the program from after the point of the exception.
    '  This code shows how to resume at the next address after a PrivelidgedInstruction exception.
    '  It also shows how to resume at another address such as an error handler. This should be in the same SUB/FUNCTION as the error.
    '
    'The default in this program is to enable all FP exceptions apart from INEXACT_RESULT as this will trip up most people
    '  more than it helps them! e.g. 1/3 will trigger the exception as the result cannot be exactly represented in binary.
    '  If you want to trap INEXACT_RESULT then set %FPTrapInexactResultException = 1   below.
    '
    'The only ASM here is to set the FPU control register bits so don't be put off by it.
    '
    'Three distict outcomes are shown in this code.
    ' If there is a priveliged instruction exception caused by CLI or STI (clear or set interrupt flag) then the inxtruction is ignored
    '  and execution continues at the next address as if the instruction wasn't there. Other priviliged instructions are passed to the
    '  default handler (usually the OS).
    '
    'If there is a floating point exception then it is reported along with a dump of the FP registers, it's then passed to the default handler.
    '
    'If there is an integer divide by zero then the error is reported and execution continues at the error handler specified in the call
    '   when the Exception Handler was installed.
    '
    'There are lots of potential problems here. This code is just posted to demonstrate how exceptions can be dealt with.
    'In particular, returning execution to the ResumeAddress will only work if the exception occurs in the same function as the ResumeAddress
    ' but calls to outside functions in maths routines are very common so care needs to be taken.
    '
    'Just for demonstration purposes I print things out in the ExceptionHandler to inform the user what has happened. Normally you wouldn't
    '  do this if you are returning execution to the program.
    
    
    #COMPILE EXE
    #DIM ALL
    #REGISTER NONE
    
    
    '#INCLUDE "win32api.inc"
    '################################################################################################################################################################
    'The following are extracted from the PBCC4 winapi.inc file.
    'It's differences in .inc files that is causing the original to fail to compile in later compilers
    DECLARE FUNCTION AddVectoredExceptionHandler LIB "KERNEL32.DLL" ALIAS "AddVectoredExceptionHandler" (BYVAL First AS DWORD, BYVAL Handler AS DWORD) AS LONG
    DECLARE FUNCTION RemoveVectoredExceptionHandler LIB "KERNEL32.DLL" ALIAS "RemoveVectoredExceptionHandler" (BYVAL Handler AS DWORD) AS LONG
    
    %EXCEPTION_EXECUTE_HANDLER    = 1
    %EXCEPTION_CONTINUE_SEARCH    = 0
    %EXCEPTION_CONTINUE_EXECUTION = -1
    
    %EXCEPTION_NONCONTINUABLE     = &H1  ' Noncontinuable exception
    %EXCEPTION_MAXIMUM_PARAMETERS = 15   ' maximum number of exception parameters
    
    %STATUS_WAIT_0                   = &H00000000???
    %STATUS_ABANDONED_WAIT_0         = &H00000080???
    %STATUS_USER_APC                 = &H000000C0???
    %STATUS_TIMEOUT                  = &H00000102???
    %STATUS_PENDING                  = &H00000103???
    %STILL_ACTIVE                    = %STATUS_PENDING
    %STATUS_DATATYPE_MISALIGNMENT    = &H80000002???
    %STATUS_BREAKPOINT               = &H80000003???
    %STATUS_SINGLE_STEP              = &H80000004???
    %STATUS_ACCESS_VIOLATION         = &HC0000005???
    %STATUS_IN_PAGE_ERROR            = &HC0000006???
    %STATUS_INVALID_HANDLE           = &HC0000008???
    %STATUS_NO_MEMORY                = &HC0000017???
    %STATUS_ILLEGAL_INSTRUCTION      = &HC000001D???
    %STATUS_NONCONTINUABLE_EXCEPTION = &HC0000025???
    %STATUS_INVALID_DISPOSITION      = &HC0000026???
    %STATUS_ARRAY_BOUNDS_EXCEEDED    = &HC000008C???
    %STATUS_FLOAT_DENORMAL_OPERAND   = &HC000008D???
    %STATUS_FLOAT_DIVIDE_BY_ZERO     = &HC000008E???
    %STATUS_FLOAT_INEXACT_RESULT     = &HC000008F???
    %STATUS_FLOAT_INVALID_OPERATION  = &HC0000090???
    %STATUS_FLOAT_OVERFLOW           = &HC0000091???
    %STATUS_FLOAT_STACK_CHECK        = &HC0000092???
    %STATUS_FLOAT_UNDERFLOW          = &HC0000093???
    %STATUS_INTEGER_DIVIDE_BY_ZERO   = &HC0000094???
    %STATUS_INTEGER_OVERFLOW         = &HC0000095???
    %STATUS_PRIVILEGED_INSTRUCTION   = &HC0000096???
    %STATUS_STACK_OVERFLOW           = &HC00000FD???
    %STATUS_CONTROL_C_EXIT           = &HC000013A???
    %STATUS_FLOAT_MULTIPLE_FAULTS    = &HC00002B4???
    %STATUS_FLOAT_MULTIPLE_TRAPS     = &HC00002B5???
    %STATUS_ILLEGAL_VLM_REFERENCE    = &HC00002C0???
    %STATUS_REG_NAT_CONSUMPTION      = &HC00002C9???
    %STATUS_SXS_EARLY_DEACTIVATION   = &HC015000F???
    %STATUS_SXS_INVALID_DEACTIVATION = &HC0150010???
    
    %SIZE_OF_80387_REGISTERS = 80
    
    TYPE FLOATING_SAVE_AREA
      ControlWord AS DWORD
      StatusWord AS DWORD
      TagWord AS DWORD
      ErrorOffset AS DWORD
      ErrorSelector AS DWORD
      DataOffset AS DWORD
      DataSelector AS DWORD
      RegisterArea(0 TO %SIZE_OF_80387_REGISTERS - 1) AS BYTE
      Cr0NpxState AS DWORD
    END TYPE
    
    
    TYPE CONTEXT
      '
      ' The flags values within this flag control the contents of
      ' a CONTEXT record.
      '
      ' If the context record is used as an input parameter, then
      ' for each portion of the context record controlled by a flag
      ' whose value is set, it is assumed that that portion of the
      ' context record contains valid context. If the context record
      ' is being used to modify a threads context, then only that
      ' portion of the threads context will be modified.
      '
      ' If the context record is used as an IN OUT parameter to capture
      ' the context of a thread, then only those portions of the thread's
      ' context corresponding to set flags will be returned.
      '
      ' The context record is never used as an OUT only parameter.
      '
      ContextFlags AS DWORD
    
      ' This section is specified/returned if CONTEXT_DEBUG_REGISTERS is
      ' set in ContextFlags.  Note that CONTEXT_DEBUG_REGISTERS is NOT
      ' included in CONTEXT_FULL.
      Dr0 AS DWORD
      Dr1 AS DWORD
      Dr2 AS DWORD
      Dr3 AS DWORD
      Dr6 AS DWORD
      Dr7 AS DWORD
    
      ' This section is specified/returned if the
      ' ContextFlags word contians the flag CONTEXT_FLOATING_POINT.
      FloatSave AS FLOATING_SAVE_AREA
    
      ' This section is specified/returned if the
      ' ContextFlags word contians the flag CONTEXT_SEGMENTS.
      regGs AS DWORD
      regFs AS DWORD
      regEs AS DWORD
      regDs AS DWORD
    
      ' This section is specified/returned if the
      ' ContextFlags word contians the flag CONTEXT_INTEGER.
      regEdi AS DWORD
      regEsi AS DWORD
      regEbx AS DWORD
      regEdx AS DWORD
      regEcx AS DWORD
      regEax AS DWORD
    
      ' This section is specified/returned if the
      ' ContextFlags word contians the flag CONTEXT_CONTROL.
      regEbp AS DWORD
      regEip AS DWORD
      regCs AS DWORD      ' MUST BE SANITIZED
      regFlag AS DWORD    ' MUST BE SANITIZED
      regEsp AS DWORD
      regSs AS DWORD
    END TYPE
    
    
    TYPE EXCEPTION_RECORD
      ExceptionCode AS DWORD
      ExceptionFlags AS DWORD
      pExceptionRecord AS EXCEPTION_RECORD PTR
      ExceptionAddress AS DWORD
      NumberParameters AS DWORD
      ExceptionInformation(0 TO %EXCEPTION_MAXIMUM_PARAMETERS - 1) AS DWORD
    END TYPE
    
    TYPE EXCEPTION_POINTERS
      pExceptionRecord AS EXCEPTION_RECORD PTR
      ContextRecord AS CONTEXT PTR
    END TYPE
    
    'End of data extracted from PBCC4 Win32api.inc
    '################################################################################################################################################################
    
    
    %FPTrapInexactResultException = 0    'set to non zero if you want to trap FPInexactResults (specialised use only)
    %enable = 1
    %disable= 0
    
    %CLI= &hfa     'opcode of STI instruction    'these are just to test the PRIVILEGED_INSTRUCTION exception
    %STI= &hfb     'opcode of CLI instruction
    
    
    GLOBAL ExeptionHandlerResumeAddress AS DWORD
    
    
    
    FUNCTION SetupExceptionHandler(EnableTrap AS LONG,ResumeAddress AS DWORD) AS LONG
    'EnableTrap = 0 for uninstall handler, not 0 for install handler
    'ResumeAddress = address WITHIN THE ERRING FUNCTION/SUB to which you would like the code to return after the exception is handled
    '              =0 for don't resume.
    
    STATIC hHandler AS LONG
    
    LOCAL HandlerPointer AS LONG
    LOCAL NewControlWord, mask AS INTEGER    'control word is a 16 bit register
    LOCAL OriginalControlWord AS INTEGER
    
    ExeptionHandlerResumeAddress = ResumeAddress
    
        IF EnableTrap THEN
            'install the exception handler
            HandlerPointer = CODEPTR(ExceptionHandler)
            hHandler = AddVectoredExceptionHandler( 1  , HandlerPointer )
    
            IF hHandler = 0 THEN
               'Failed to add handler
                FUNCTION = 0
            ELSE
                'Handler installed
                FUNCTION = 1
    
                'now set the FPU control register to enable exceptions on all conditions
                !fstcw OriginalControlWord  'save the original control word
                !mov ax,OriginalControlWord 'get the control word
    
                #IF %FPTrapInexactResultException = 0
                !and ax,&hffe0              'zero the exception mask bits except for InexactResult
                #ELSE
                !and ax,&hffc0              'zero the exception mask bits (lower 6 bits or control word) to allow all 6 exceptions
                #ENDIF
    
                !mov NewControlWord,ax      'save the new control word
                !fldcw NewControlWord       'set the FPU to use the new control word
    
            END IF
    
        ELSE
            'remove the exception handler
            hHandler = RemoveVectoredExceptionHandler(hHandler)
            IF hHandler = 0 THEN
                'Couldn't remove exception handler
                FUNCTION = 0
    
            ELSE
                'Exception handler removed
                FUNCTION = 1
                !fldcw OriginalControlWord  'restore the original control word
    
            END IF
    
        END IF
    
    END FUNCTION
    
    
    
    FUNCTION ExceptionHandler(BYVAL pException_Pointers AS Exception_Pointers PTR) AS LONG
    
    LOCAL ThisExceptionPointer AS Exception_Pointers
    LOCAL ThisExceptionRecordPointer AS Exception_Record PTR
    LOCAL ThisExceptionRecord AS Exception_Record
    LOCAL temp AS BYTE PTR
    LOCAL r AS LONG
    LOCAL pt AS DWORD
    LOCAL FPControlWord AS INTEGER
    LOCAL FPControlWordNew AS INTEGER
    LOCAL WhatToDoNext  AS LONG
    
    
    ThisExceptionPointer = @pException_Pointers
    ThisExceptionRecordPointer = ThisExceptionPointer.pExceptionRecord
    ThisExceptionRecord = @ThisExceptionRecordPointer
    
    WhatToDoNext=%EXCEPTION_CONTINUE_SEARCH 'set the default for all exceptions I don't handle here.
                                            'the 2 options are:
                                            '%EXCEPTION_CONTINUE_SEARCH which passes the exception on to the next handler in line (the OS?)
                                            '%EXCEPTION_CONTINUE_EXECUTION which resumes execution of the code
                                            'If execution is resumed, the address to resume at can be set within the handler
    
    !fnstcw FPControlWord   'get FP control word
    !fnclex                 'clear all FP exeptions. Got to do this as following lines use the FPU and may cause a recurring exception
    !mov ax,FPControlWord
    !or ax,&h3f             'mask all exception while in the exception routine
    !mov FPControlWord,ax
    !fldcw FPControlWord    'load new FP control word
    
    'now handle each possible exception
    SELECT CASE ThisExceptionRecord.ExceptionCode
        CASE  %STATUS_FLOAT_OVERFLOW
    
            PRINT "There has been a FP overflow at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            PRINT "here are the FPU details at the time of the exception"
    
            PRINT "ControlWord   ";HEX$(@[email protected])
            PRINT "StatusWord    ";HEX$(@[email protected])
            PRINT "TagWord       ";HEX$(@[email protected])
            PRINT "ErrorOffset   ";HEX$(@[email protected])
            PRINT "ErrorSelector ";HEX$(@[email protected])
            PRINT "DataOffset    ";HEX$(@[email protected])
            PRINT "DataSelector  ";HEX$(@[email protected])
    
            pt=VARPTR(@[email protected](0))
    
            REDIM reg(0 TO 7) AS EXT AT pt
            FOR r& = 0 TO 7
                PRINT "ST(";r&;")=";reg(r&)
            NEXT
    
            PRINT
            PRINT "Press a key to pass error to default exception handler"
            WAITKEY$
    
    
        CASE %STATUS_FLOAT_UNDERFLOW
            PRINT "There has been a FP underflow at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            WAITKEY$
    
        CASE %STATUS_FLOAT_INVALID_OPERATION
            PRINT "There has been an Invalid FP Operation at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            WAITKEY$
    
        CASE %STATUS_FLOAT_INEXACT_RESULT
    
    
        CASE %STATUS_FLOAT_DIVIDE_BY_ZERO
    
    
        CASE %STATUS_FLOAT_DENORMAL_OPERAND
    
    
        CASE %STATUS_ILLEGAL_INSTRUCTION
    
    
        CASE %STATUS_ACCESS_VIOLATION
    
    
        CASE %STATUS_PRIVILEGED_INSTRUCTION
            PRINT "STATUS_PRIVILEGED_INSTRUCTION exception has occurred"
    
            temp = @[email protected]    'get the EIP register to see where the instruction was that caused the exception
    
            IF @temp = %STI OR  @temp = %CLI THEN   '@temp = the instruction itself
                'if its set/clear interrupt flag I'll just increment the return address pointer to ignore the exception
                INCR @[email protected]    'return execution to the byte after the one that caused the exception
    
                PRINT "You can't mess with the interrupt flag. I'll ignore it."
                PRINT "Execution will now continue"
                PRINT
                WhatToDoNext=%EXCEPTION_CONTINUE_EXECUTION
            ELSE
                'it's not an interrupt flag instruction so pass it to the default handler
                PRINT "You aren't allowed to use that instruction at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
                PRINT "I'm going to crash now."
    
                WhatToDoNext=%EXCEPTION_CONTINUE_SEARCH
            END IF
    
    
    
        CASE %STATUS_STACK_OVERFLOW
    
    
        CASE %STATUS_FLOAT_MULTIPLE_FAULTS
    
    
        CASE %STATUS_INTEGER_OVERFLOW
    
    
        CASE %STATUS_INTEGER_DIVIDE_BY_ZERO
            PRINT "Integer divide by zero at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            PRINT
            'as a test we'll try to return to the faulty function's error trap
            IF ExeptionHandlerResumeAddress <>0 THEN
                @[email protected] = ExeptionHandlerResumeAddress
            END IF
    
            WhatToDoNext=%EXCEPTION_CONTINUE_EXECUTION
    
    END SELECT
    
    
    IF WhatToDoNext =%EXCEPTION_CONTINUE_SEARCH THEN
        FUNCTION = %EXCEPTION_CONTINUE_SEARCH
    ELSE
        FUNCTION = %EXCEPTION_CONTINUE_EXECUTION
        !fldcw FPControlWord   'restore the FPUControl word if I'm going to return to executing my code
    
    END IF
    
    
    END FUNCTION
    
    
    
    FUNCTION PBMAIN () AS LONG
    
    LOCAL sum AS SINGLE
    LOCAL ResumeAddress AS LONG
    
    ResumeAddress = CODEPTR(ErrorTrap)
    
    
    IF SetupExceptionHandler(%enable,ResumeAddress) = 0 THEN
        PRINT "Failed to add exception handler"
    ELSE
        PRINT "Exception Handler installed "
        PRINT "Press a key to cause a Privileged Instruction exception"
        PRINT
        WAITKEY$
    
    '############################################
    '# cause a Privileged Instruction exception
    '############################################
    
    !cli
    
    
        PRINT "Press a key to cause an Integer Divide by zero exception"
        PRINT
        WAITKEY$
    
    '############################################
    '# cause an integer divide by zero exception
    '############################################
    LOCAL a,b,c AS LONG
    
    a=0
    b=1
    c=b\a
    
        'in this example the code doesn't reach this point but in real code you should uninstall the exception handler
        ''like this when you're finished with it.
        IF SetupExceptionHandler(%disable,0)= 0 THEN
            PRINT "Couldn't remove exception handler"
        ELSE
            PRINT "Exception handler removed"
        END IF
    
    
    END IF
    GOTO xit
    
    ErrorTrap:
    PRINT "I've resumed execution at the error trap."
    PRINT "press a key to cause a fatal FP overflow"
    PRINT
    WAITKEY$
    
    '############################################
    '# cause a FP overflow exception
    '############################################
    
    
        sum = 1
        DO
            sum = sum + sum        'cause an overflow exception by counting too high
        LOOP
    
    
    xit:
    WAITKEY$
    END FUNCTION

    Leave a comment:


  • Arthur Gomide
    replied
    withdraw
    Last edited by Arthur Gomide; 16 Jan 2013, 12:02 PM.

    Leave a comment:


  • Paul Dixon
    started a topic Trapping FP overflows and other exceptions

    Trapping FP overflows and other exceptions

    This PBCC4 program is intended to show how to install and use an exception handler.
    It's not complete but should provide a useful framework for those interested in trapping their own exceptions.
    As it stands it demonstrates 3 different ways to handle exceptions.
    Exceptions include FP overflows and divide by zero errors.

    Discussion thread:
    http://www.powerbasic.com/support/pb...212#post289212

    Paul.
    Code:
    'PBCC4.0 program
    'This code attempts to handle exceptions not normally trapped by PowerBASIC.
    'It consists of 3 routines.
    '1) PBMain just a short routine to show how to use the other 2 functions
    '2) SetupExceptionHandler which installs or removes the exception handler
    '3) ExceptionHandler  which is called, via the operating system, when an exception occurs.
    '
    'Floating point exceptions need to be explicitly allowed by clearing one or more of 6 mask bits in the FPU control word.
    'Other exceptions happen anyway but this program shows how to intercept them to do something useful before the default
    '  exception handler of the OS gets them and reports a the error.
    '
    ' It is possible, though sometimes difficult, to resume execution of the program from after the point of the exception.
    '  This code shows how to resume at the next address after a PrivelidgedInstruction exception.
    '  It also shows how to resume at another address such as an error handler. This should be in the same SUB/FUNCTION as the error.
    '
    'The default in this program is to enable all FP exceptions apart from INEXACT_RESULT as this will trip up most people
    '  more than it helps them! e.g. 1/3 will trigger the exception as the result cannot be exactly represented in binary.
    '  If you want to trap INEXACT_RESULT then set %FPTrapInexactResultException = 1   below.
    '
    'The only ASM here is to set the FPU control register bits so don't be put off by it.
    '
    'Three distict outcomes are shown in this code.
    ' If there is a priveliged instruction exception caused by CLI or STI (clear or set interrupt flag) then the inxtruction is ignored
    '  and execution continues at the next address as if the instruction wasn't there. Other priviliged instructions are passed to the
    '  default handler (usually the OS).
    '
    'If there is a floating point exception then it is reported along with a dump of the FP registers, it's then passed to the default handler.
    '
    'If there is an integer divide by zero then the error is reported and execution continues at the error handler specified in the call
    '   when the Exception Handler was installed.
    '
    'There are lots of potential problems here. This code is just posted to demonstrate how exceptions can be dealt with.
    'In particular, returning execution to the ResumeAddress will only work if the exception occurs in the same function as the ResumeAddress
    ' but calls to outside functions in maths routines are very common so care needs to be taken.
    '
    'Just for demonstration purposes I print things out in the ExceptionHandler to inform the user what has happened. Normally you wouldn't
    '  do this if you are returning execution to the program.
    
    
    #COMPILE EXE
    #DIM ALL
    #REGISTER NONE
    
    #INCLUDE "win32api.inc"
    
    DECLARE FUNCTION AddVectoredExceptionHandler LIB "KERNEL32.DLL" ALIAS "AddVectoredExceptionHandler" (BYVAL First AS DWORD, BYVAL Handler AS DWORD) AS LONG
    DECLARE FUNCTION RemoveVectoredExceptionHandler LIB "KERNEL32.DLL" ALIAS "RemoveVectoredExceptionHandler" (BYVAL Handler AS DWORD) AS LONG
    
    %FPTrapInexactResultException = 0    'set to non zero if you want to trap FPInexactResults (specialised use only)
    %enable = 1
    %disable= 0
    
    %CLI= &hfa     'opcode of STI instruction    'these are just to test the PRIVILEGED_INSTRUCTION exception
    %STI= &hfb     'opcode of CLI instruction
     
     
    GLOBAL ExeptionHandlerResumeAddress AS DWORD
    
    
    
    FUNCTION SetupExceptionHandler(EnableTrap AS LONG,ResumeAddress AS DWORD) AS LONG
    'EnableTrap = 0 for uninstall handler, not 0 for install handler
    'ResumeAddress = address WITHIN THE ERRING FUNCTION/SUB to which you would like the code to return after the exception is handled
    '              =0 for don't resume.
    
    STATIC hHandler AS LONG
    
    LOCAL HandlerPointer AS LONG
    LOCAL NewControlWord, mask AS INTEGER    'control word is a 16 bit register
    LOCAL OriginalControlWord AS INTEGER
    
    ExeptionHandlerResumeAddress = ResumeAddress
    
        IF EnableTrap THEN
            'install the exception handler
            HandlerPointer = CODEPTR(ExceptionHandler)
            hHandler = AddVectoredExceptionHandler( 1  , HandlerPointer )
    
            IF hHandler = 0 THEN
               'Failed to add handler
                FUNCTION = 0
            ELSE
                'Handler installed
                FUNCTION = 1
             
                'now set the FPU control register to enable exceptions on all conditions
                !fstcw OriginalControlWord  'save the original control word
                !mov ax,OriginalControlWord 'get the control word
                
                #IF %FPTrapInexactResultException = 0
                !and ax,&hffe0              'zero the exception mask bits except for InexactResult
                #ELSE
                !and ax,&hffc0              'zero the exception mask bits (lower 6 bits or control word) to allow all 6 exceptions
                #ENDIF
                
                !mov NewControlWord,ax      'save the new control word
                !fldcw NewControlWord       'set the FPU to use the new control word
    
            END IF
    
        ELSE
            'remove the exception handler
            hHandler = RemoveVectoredExceptionHandler(hHandler)
            IF hHandler = 0 THEN
                'Couldn't remove exception handler
                FUNCTION = 0
    
            ELSE
                'Exception handler removed
                FUNCTION = 1
                !fldcw OriginalControlWord  'restore the original control word
    
            END IF
    
        END IF
    
    END FUNCTION
    
    
    
    FUNCTION ExceptionHandler(BYVAL pException_Pointers AS Exception_Pointers PTR) AS LONG
    
    LOCAL ThisExceptionPointer AS Exception_Pointers
    LOCAL ThisExceptionRecordPointer AS Exception_Record PTR
    LOCAL ThisExceptionRecord AS Exception_Record
    LOCAL temp AS BYTE PTR
    LOCAL r AS LONG
    LOCAL pt AS DWORD
    LOCAL FPControlWord AS INTEGER
    LOCAL FPControlWordNew AS INTEGER
    LOCAL WhatToDoNext  AS LONG
    
    
    ThisExceptionPointer = @pException_Pointers
    ThisExceptionRecordPointer = ThisExceptionPointer.pExceptionRecord
    ThisExceptionRecord = @ThisExceptionRecordPointer
    
    WhatToDoNext=%EXCEPTION_CONTINUE_SEARCH 'set the default for all exceptions I don't handle here.
                                            'the 2 options are:
                                            '%EXCEPTION_CONTINUE_SEARCH which passes the exception on to the next handler in line (the OS?)
                                            '%EXCEPTION_CONTINUE_EXECUTION which resumes execution of the code
                                            'If execution is resumed, the address to resume at can be set within the handler
                                            
    !fnstcw FPControlWord   'get FP control word
    !fnclex                 'clear all FP exeptions. Got to do this as following lines use the FPU and may cause a recurring exception
    !mov ax,FPControlWord
    !or ax,&h3f             'mask all exception while in the exception routine
    !mov FPControlWord,ax
    !fldcw FPControlWord    'load new FP control word
    
    'now handle each possible exception
    SELECT CASE ThisExceptionRecord.ExceptionCode
        CASE  %STATUS_FLOAT_OVERFLOW
    
            PRINT "There has been a FP overflow at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            PRINT "here are the FPU details at the time of the exception"
    
            PRINT "ControlWord   ";HEX$(@pExcep[email protected])
            PRINT "StatusWord    ";HEX$(@[email protected])
            PRINT "TagWord       ";HEX$(@[email protected])
            PRINT "ErrorOffset   ";HEX$(@[email protected])
            PRINT "ErrorSelector ";HEX$(@[email protected])
            PRINT "DataOffset    ";HEX$(@[email protected])
            PRINT "DataSelector  ";HEX$(@[email protected])
    
            pt=VARPTR(@[email protected](0))
    
            REDIM reg(0 TO 7) AS EXT AT pt
            FOR r& = 0 TO 7
                PRINT "ST(";r&;")=";reg(r&)
            NEXT
    
            PRINT
            PRINT "Press a key to pass error to default exception handler"
            WAITKEY$
            
            
        CASE %STATUS_FLOAT_UNDERFLOW
            PRINT "There has been a FP underflow at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            WAITKEY$
            
        CASE %STATUS_FLOAT_INVALID_OPERATION
            PRINT "There has been an Invalid FP Operation at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            WAITKEY$
        
        CASE %STATUS_FLOAT_INEXACT_RESULT
            
        
        CASE %STATUS_FLOAT_DIVIDE_BY_ZERO
            
        
        CASE %STATUS_FLOAT_DENORMAL_OPERAND
            
        
        CASE %STATUS_ILLEGAL_INSTRUCTION
            
        
        CASE %STATUS_ACCESS_VIOLATION
    
    
        CASE %STATUS_PRIVILEGED_INSTRUCTION
            PRINT "STATUS_PRIVILEGED_INSTRUCTION exception has occurred"
    
            temp = @[email protected]    'get the EIP register to see where the instruction was that caused the exception
    
            IF @temp = %STI OR  @temp = %CLI THEN   '@temp = the instruction itself
                'if its set/clear interrupt flag I'll just increment the return address pointer to ignore the exception
                INCR @[email protected]    'return execution to the byte after the one that caused the exception
    
                PRINT "You can't mess with the interrupt flag. I'll ignore it."
                PRINT "Execution will now continue"
                PRINT
                WhatToDoNext=%EXCEPTION_CONTINUE_EXECUTION
            ELSE
                'it's not an interrupt flag instruction so pass it to the default handler
                PRINT "You aren't allowed to use that instruction at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
                PRINT "I'm going to crash now."
    
                WhatToDoNext=%EXCEPTION_CONTINUE_SEARCH
            END IF
    
    
            
        CASE %STATUS_STACK_OVERFLOW
            
        
        CASE %STATUS_FLOAT_MULTIPLE_FAULTS
            
        
        CASE %STATUS_INTEGER_OVERFLOW
            
        
        CASE %STATUS_INTEGER_DIVIDE_BY_ZERO
            PRINT "Integer divide by zero at address &h";HEX$(ThisExceptionRecord.ExceptionAddress)
            PRINT
            'as a test we'll try to return to the faulty function's error trap
            IF ExeptionHandlerResumeAddress <>0 THEN
                @[email protected] = ExeptionHandlerResumeAddress
            END IF
    
            WhatToDoNext=%EXCEPTION_CONTINUE_EXECUTION
        
    END SELECT
    
    
    IF WhatToDoNext =%EXCEPTION_CONTINUE_SEARCH THEN
        FUNCTION = %EXCEPTION_CONTINUE_SEARCH
    ELSE
        FUNCTION = %EXCEPTION_CONTINUE_EXECUTION
        !fldcw FPControlWord   'restore the FPUControl word if I'm going to return to executing my code
    
    END IF
    
    
    END FUNCTION
    
    
    
    FUNCTION PBMAIN () AS LONG
    
    LOCAL sum AS SINGLE
    LOCAL ResumeAddress AS LONG
    
    ResumeAddress = CODEPTR(ErrorTrap)
    
    
    IF SetupExceptionHandler(%enable,ResumeAddress) = 0 THEN
        PRINT "Failed to add exception handler"
    ELSE
        PRINT "Exception Handler installed "
        PRINT "Press a key to cause a Privileged Instruction exception"
        PRINT
        WAITKEY$
    
    '############################################
    '# cause a Privileged Instruction exception
    '############################################
    
    !cli
    
    
        PRINT "Press a key to cause an Integer Divide by zero exception"
        PRINT
        WAITKEY$
    
    '############################################
    '# cause an integer divide by zero exception
    '############################################
    LOCAL a,b,c AS LONG
    
    a=0
    b=1
    c=b\a
    
        'in this example the code doesn't reach this point but in real code you should uninstall the exception handler
        ''like this when you're finished with it.
        IF SetupExceptionHandler(%disable,0)= 0 THEN
            PRINT "Couldn't remove exception handler"
        ELSE
            PRINT "Exception handler removed"
        END IF
    
    
    END IF
    GOTO xit
    
    ErrorTrap:
    PRINT "I've resumed execution at the error trap."
    PRINT "press a key to cause a fatal FP overflow"
    PRINT
    WAITKEY$
    
    '############################################
    '# cause a FP overflow exception
    '############################################
    
    
        sum = 1
        DO
            sum = sum + sum        'cause an overflow exception by counting too high
        LOOP
    
         
    xit:
    WAITKEY$
    END FUNCTION
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