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:
Paul.
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:
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$(ThisExceptionRecord.ExceptionAddress)
PRINT "here are the FPU details at the time of the exception"
PRINT "ControlWord ";HEX$(@[email protected])
PRINT "StatusWord ";HEX$(@[email protected])
PRINT "TagWord ";HEX$(@pException_Poin[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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