HP OpenVMS Programming Concepts Manual

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9.3 Exception Conditions

Exceptions can be generated by any of the following:

Hardware
Software

Hardware-generated exceptions always result in conditions that require special action if program execution is to continue.

Software-generated exceptions may result in error or warning conditions. These conditions and their message descriptions are documented in the online Help Message utility and in the OpenVMS system messages documentation. To access online message descriptions, use the HELP/MESSAGE command.

More information on using the Help Message utility is available in OpenVMS System Messages: Companion Guide for Help Message Users. That document describes only those messages that occur when the system is not fully operational and you cannot access Help Message.

Some examples of exception conditions are as follows:

Arithmetic exception condition in a user-written program detected and signaled by hardware (for example, floating-point overflow)
Error in a user argument to a run-time library routine detected by software and signaled by calling LIB$STOP (for example, a negative square root)
Error in a run-time library language-support routine, such as an I/O error or an error in a data-type conversion
RMS success condition stating that the record is already locked
RMS success condition stating that the created file superseded an existing version

There are two standard methods for an HP- or user-written routine to indicate that an exception condition has occurred:

Return a completion code to the calling program using the function value mechanism
Most general-purpose run-time library routines indicate exception conditions by returning a condition value in R0 (R8 for I64). The calling program then tests bit 0 of R0 (R8 for I64) for success or failure. This method allows better programming structure, because the flow of control can be changed explicitly after the return from each call. If the actual function value returned is greater than 32 bits, then use both R0 and R1 (R8, R9 for I64).
On Alpha systems, if the actual function returned is a floating-point value, the floating-point value is returned in F0, or F0 and F1.
On I64 systems, if the actual function returned is a floating-point value, the floating-point value is returned in F8, F9.
Signal the exception condition
A condition can be signaled by calling the RTL routine LIB$SIGNAL or LIB$STOP. Any condition handlers that were enabled are then called by the CHF. See Figure 9-5 for the order in which CHF invokes condition handlers.
Exception conditions raised by hardware or software are signaled to the routine identically.
For more details, see Section 9.8 and Section 9.8.1.

9.3.1 Conditions Caused by Exceptions

Table 9-1 and Table 9-2 summarize common conditions caused by exceptions. The condition names are listed in the first column. The second column explains each condition more fully by giving information about the type, meaning, and arguments relating to the condition. The condition type is either trap or fault. For more information about traps and faults, refer to the VAX Architecture Reference Manual, the Alpha Architecture Reference Manual, and the Intel® Itanium® Architecture Software Developer's Manual. The meaning of the exception condition is a short description of each condition. The arguments for the condition handler are listed where applicable; they give specific information about the condition.

Table 9-1 Summary of Exception Conditions
Condition Name Explanation

SS$_ACCVIO Type: Fault.

Description Acess Violation.

Arguments:

Reason for access violation. This is a mask with the following format:
Bit <0> = type of access violation
0 = page table entry protection code did not permit intended access
1 = P0LR, P1LR, or SLR length violation ¹

Bit <1> = page table entry reference
0 = specified virtual address not accessible
1 = associated page table entry not accessible

Bit <2> = intended access
0 = read
1 = modify

Bit <16> = indicates fault on the pre-fetch of the instruction ²
0 = successful execution
1 = fault on fetch

Bit <17> =indicates whether instruction is marked as no execute ²
0 = not market
1 = indicates instruction is marked as a fault on execute in its page table entry

Virtual address to which access was attempted or, on some processors, virtual address within the page to which access was attempted.

SS$_ARTRES ² Type:
Description:
Arguments:
Trap.
Reserved arithmetic trap.
None.

SS$_ASTFLT Type: Trap.

Description: Stack invalid during attempt to deliver an AST.

Arguments:

Stack pointer value when fault occurred.
AST parameter of failed AST.
Program counter (PC) at AST delivery interrupt.
Processor status longword (PSL) for VAX or processor status (PS) for Alpha at AST delivery interrupt. ³ For PS, it is the low-order 32 bits.
Program counter (PC) to which AST would have been delivered. ³
Processor status longword (PSL) for VAX or processor status (PS) for Alpha to which AST would have been delivered. ³ For PS, it is the low-order 32 bits.

SS$_BREAK Type:
Description:
Arguments:
Fault.
Breakpoint instruction encountered.
None.

SS$_CMODSUPR Type:
Description:
Arguments:
Trap.
Change mode to supervisor instruction encountered. ⁴
Change mode code. The possible values are --32,768 through 32,767.

SS$_CMODUSER Type:
Description:
Arguments:
Trap.
Change mode to user instruction encountered. ⁴
Change mode code. The possible values are --32,768 through 32,767.

SS$_COMPAT ¹ Type: Fault.

Description: Compatibility-mode exception. This exception condition can occur only when executing in compatibility mode. ⁵

Arguments: Type of compatibility exception. The possible values are as follows:
0 = Reserved instruction execution
1 = BPT instruction executed
2 = IOT instruction executed
3 = EMT instruction executed
4 = TRAP instruction executed
5 = Illegal instruction executed
6 = Odd address fault
7 = TBIT trap.

SS$_DECOVF ¹ ² Type:
Description:
Arguments:
Trap.
Decimal overflow.
None.

SS$_FLTDIV ¹ ² ⁶ Type:
Description:
Arguments:
Trap.
Floating/decimal divide-by-zero.
None.

SS$_FLTDIV_F ¹ Type:
Description:
Arguments:
Fault.
Floating divide-by-zero.
None.

SS$_FLTINE ⁶ Type:
Description:
Arguments:
Trap.
Floating inexact result.
None.

SS$_FLTINE_F Type:
Description:
Arguments:
Trap.
Floating inexact result fault.
None.

SS$_FLTINV ⁶ Type:
Description:
Arguments:
Trap.
Floating invalid operation.
None.

SS$_FLTINV_F Type:
Description:
Arguments:
Trap.
Floating invalid operation fault.
None.

SS$_FLTOVF ¹ ² ⁶ Type:
Description:
Arguments:
Trap.
Floating-point overflow.
None.

SS$_FLTOVF_F ¹ Type:
Description:
Arguments:
Fault.
Floating-point overflow fault.
None.

SS$_FLTUND ¹ ² ⁶ Type:
Description:
Arguments:
Trap.
Floating-point underflow.
None.

SS$_FLTUND_F ¹ Type:
Description:
Arguments:
Fault.
Floating-point underflow fault.
None.

SS$_INTDIV ¹ ² Type:
Description:
Arguments:
Trap.
Integer divide-by-zero.
None.

SS$_INTOVF ¹ ² Type:
Description:
Arguments:
Trap.
Integer overflow.
None.

SS$_OPCCUS ¹ Type:
Description:
Arguments:
Fault.
Opcode reserved for customer fault.
None.

SS$_OPCDEC Type:
Description:
Arguments:
Fault.
Opcode reserved for HP fault.
None.

SS$_PAGRDERR Type: Fault.

Description: Read error occurred during an attempt to read a faulted page from disk.

Arguments:

Translation not valid reason. This is a mask with the following format:
Bit <0> = 0
Bit <1> = page table entry reference
0 = specified virtual address not valid
1 = associated page table entry not valid

Bit <2> = intended access
0 = read
1 = modify

Virtual address of referenced page.

SS$_RADRMOD ¹ Type:
Description:
Arguments:
Fault.
Attempt to use a reserved addressing mode.
None.

SS$_ROPRAND Type:
Description:
Arguments:
Fault.
Attempt to use a reserved operand.
None.

SS$_SSFAIL Type:
Description:
Arguments:
Fault.
System service failure (when system service failure exception mode is enabled). Conditon occured as result of the use of the obsolete feature that was enabled by using $SETSFM service.
Status return from system service (R0). (The same value is in R0 of the mechanism array.)

SS$_SUBRNG ¹ ² Type:
Description:
Arguments:
Trap.
Subscript range trap.
None.

SS$_TBIT ¹ Type:
Description:
Arguments:
Fault.
Trace bit is pending following an instruction.
None.

**Table 9-1 Summary of Exception Conditions**
Condition Name	Explanation
SS$_ACCVIO	Type:	Fault.
	Description	Acess Violation.
	Arguments:	Reason for access violation. This is a mask with the following format: Bit <0> = type of access violation 0 = page table entry protection code did not permit intended access 1 = P0LR, P1LR, or SLR length violation ¹ Bit <1> = page table entry reference 0 = specified virtual address not accessible 1 = associated page table entry not accessible Bit <2> = intended access 0 = read 1 = modify Bit <16> = indicates fault on the pre-fetch of the instruction ² 0 = successful execution 1 = fault on fetch Bit <17> =indicates whether instruction is marked as no execute ² 0 = not market 1 = indicates instruction is marked as a fault on execute in its page table entry Virtual address to which access was attempted or, on some processors, virtual address within the page to which access was attempted.
SS$_ARTRES ²	Type: Description: Arguments:	Trap. Reserved arithmetic trap. None.
SS$_ASTFLT	Type:	Trap.
	Description:	Stack invalid during attempt to deliver an AST.
	Arguments:	Stack pointer value when fault occurred. AST parameter of failed AST. Program counter (PC) at AST delivery interrupt. Processor status longword (PSL) for VAX or processor status (PS) for Alpha at AST delivery interrupt. ³ For PS, it is the low-order 32 bits. Program counter (PC) to which AST would have been delivered. ³ Processor status longword (PSL) for VAX or processor status (PS) for Alpha to which AST would have been delivered. ³ For PS, it is the low-order 32 bits.
SS$_BREAK	Type: Description: Arguments:	Fault. Breakpoint instruction encountered. None.
SS$_CMODSUPR	Type: Description: Arguments:	Trap. Change mode to supervisor instruction encountered. ⁴ Change mode code. The possible values are --32,768 through 32,767.
SS$_CMODUSER	Type: Description: Arguments:	Trap. Change mode to user instruction encountered. ⁴ Change mode code. The possible values are --32,768 through 32,767.
SS$_COMPAT ¹	Type:	Fault.
	Description:	Compatibility-mode exception. This exception condition can occur only when executing in compatibility mode. ⁵
	Arguments:	Type of compatibility exception. The possible values are as follows: 0 = Reserved instruction execution 1 = BPT instruction executed 2 = IOT instruction executed 3 = EMT instruction executed 4 = TRAP instruction executed 5 = Illegal instruction executed 6 = Odd address fault 7 = TBIT trap.
SS$_DECOVF ¹ ²	Type: Description: Arguments:	Trap. Decimal overflow. None.
SS$_FLTDIV ¹ ² ⁶	Type: Description: Arguments:	Trap. Floating/decimal divide-by-zero. None.
SS$_FLTDIV_F ¹	Type: Description: Arguments:	Fault. Floating divide-by-zero. None.
SS$_FLTINE ⁶	Type: Description: Arguments:	Trap. Floating inexact result. None.
SS$_FLTINE_F	Type: Description: Arguments:	Trap. Floating inexact result fault. None.
SS$_FLTINV ⁶	Type: Description: Arguments:	Trap. Floating invalid operation. None.
SS$_FLTINV_F	Type: Description: Arguments:	Trap. Floating invalid operation fault. None.
SS$_FLTOVF ¹ ² ⁶	Type: Description: Arguments:	Trap. Floating-point overflow. None.
SS$_FLTOVF_F ¹	Type: Description: Arguments:	Fault. Floating-point overflow fault. None.
SS$_FLTUND ¹ ² ⁶	Type: Description: Arguments:	Trap. Floating-point underflow. None.
SS$_FLTUND_F ¹	Type: Description: Arguments:	Fault. Floating-point underflow fault. None.
SS$_INTDIV ¹ ²	Type: Description: Arguments:	Trap. Integer divide-by-zero. None.
SS$_INTOVF ¹ ²	Type: Description: Arguments:	Trap. Integer overflow. None.
SS$_OPCCUS ¹	Type: Description: Arguments:	Fault. Opcode reserved for customer fault. None.
SS$_OPCDEC	Type: Description: Arguments:	Fault. Opcode reserved for HP fault. None.
SS$_PAGRDERR	Type:	Fault.
	Description:	Read error occurred during an attempt to read a faulted page from disk.
	Arguments:	Translation not valid reason. This is a mask with the following format: Bit <0> = 0 Bit <1> = page table entry reference 0 = specified virtual address not valid 1 = associated page table entry not valid Bit <2> = intended access 0 = read 1 = modify Virtual address of referenced page.
SS$_RADRMOD ¹	Type: Description: Arguments:	Fault. Attempt to use a reserved addressing mode. None.
SS$_ROPRAND	Type: Description: Arguments:	Fault. Attempt to use a reserved operand. None.
SS$_SSFAIL	Type: Description: Arguments:	Fault. System service failure (when system service failure exception mode is enabled). Conditon occured as result of the use of the obsolete feature that was enabled by using $SETSFM service. Status return from system service (R0). (The same value is in R0 of the mechanism array.)
SS$_SUBRNG ¹ ²	Type: Description: Arguments:	Trap. Subscript range trap. None.
SS$_TBIT ¹	Type: Description: Arguments:	Fault. Trace bit is pending following an instruction. None.

¹On VAX systems, this condition is generated by hardware.
²On Alpha systems, this condition is generated by software.
³The PC and PSL (or PS) normally included in the signal array are not included in this argument list. The stack pointer of the access mode receiving this exception is reset to its initial value.
⁴If a change mode handler has been declared for user or supervisor mode with the Declare Change Mode or Compatibility Mode Handler (SYS$DCLCMH) system service, that routine receives control when the associated trap occurs.
⁵If a compatibility-mode handler has been declared with the Declare Change Mode or Compatibility Mode Handler (SYS$DCLCMH) system service, that routine receives control when this fault occurs.
⁶On I64 systems, this condition is generated by hardware.

Table 9-2 I64-Specific Exception Conditions
Condition Name Explanation

SS$_NATFAULT Type: Fault

Description Register NaT consumption fault - A non-speculative operation, (load, store, control register access, instruction fetch, and so forth), read a NaT source register, NaTVal source register, or referenced a NaTPage.

Arguments: Reason mask:
Bit <0> Execute exception - interruption is associated with an instruction fetch
Bit <2> Write exception - interruption is associated with a write operation.
Bit <19> Register Stack - interruption is associated with a mandatory RSE fill or spill.

SS$_FLTDENORMAL Type: Fault

Description Normal/unnormal operand exception

Arguments:

SS$_BREAK_SYS Type: Fault

Description An attempt was made to execute an Itanium break instruction.

Arguments: Break code is implementation specific. See Vol.2 of the Intel® Itanium® Architecture Software Developer's Manual.

SS$_BREAK_ARCH Type: Fault

Description An attempt was made to execute an Itanium break instruction.

Arguments: Break code is one of SS$_ROPRAND, SS$_INTDIV, SS$_INTOVF, SS$_SUBRNG, SS$_NULPTRERR, SS$_DECOVF, SS$_DECDIV, SS$_DECINV, or SS$_STKOVF

SS$_BREAK_APPL Type: Fault

Description An attempt was made to execute an Itanium break instruction.

Arguments: Break code is one of SS$_ROPRAND, SS$_INTDIV, SS$_INTOVF, SS$_SUBRNG, SS$_NULPTRERR, SS$_DECOVF, SS$_DECDIV, SS$_DECINV, or SS$_STKOVF

SS$_DEBUG_FAULT Type: Fault

Description Debug fault - Either the instruction address matches the parameters set up in the instruction debug registers, or the data address of a load, store, semaphore, or mandatory RSE fill or spill matches the parameters set up in the data debug registers.

Arguments:

Reason mask =
Bit <0> Execute exception - interruption is associated with an instruction fetch
Bit <2> Write exception - interruption is associated with a write operation.
Bit <19> Register Stack - interruption is associated with a mandatory RSE fill or spill.

Va = The address of the data being referenced.

**Table 9-2 I64-Specific Exception Conditions**
Condition Name	Explanation
SS$_NATFAULT	Type:	Fault
	Description	Register NaT consumption fault - A non-speculative operation, (load, store, control register access, instruction fetch, and so forth), read a NaT source register, NaTVal source register, or referenced a NaTPage.
	Arguments:	Reason mask: Bit <0> Execute exception - interruption is associated with an instruction fetch Bit <2> Write exception - interruption is associated with a write operation. Bit <19> Register Stack - interruption is associated with a mandatory RSE fill or spill.
SS$_FLTDENORMAL	Type:	Fault
	Description	Normal/unnormal operand exception
	Arguments:
SS$_BREAK_SYS	Type:	Fault
	Description	An attempt was made to execute an Itanium break instruction.
	Arguments:	Break code is implementation specific. See Vol.2 of the Intel® Itanium® Architecture Software Developer's Manual.
SS$_BREAK_ARCH	Type:	Fault
	Description	An attempt was made to execute an Itanium break instruction.
	Arguments:	Break code is one of SS$_ROPRAND, SS$_INTDIV, SS$_INTOVF, SS$_SUBRNG, SS$_NULPTRERR, SS$_DECOVF, SS$_DECDIV, SS$_DECINV, or SS$_STKOVF
SS$_BREAK_APPL	Type:	Fault
	Description	An attempt was made to execute an Itanium break instruction.
	Arguments:	Break code is one of SS$_ROPRAND, SS$_INTDIV, SS$_INTOVF, SS$_SUBRNG, SS$_NULPTRERR, SS$_DECOVF, SS$_DECDIV, SS$_DECINV, or SS$_STKOVF
SS$_DEBUG_FAULT	Type:	Fault
	Description	Debug fault - Either the instruction address matches the parameters set up in the instruction debug registers, or the data address of a load, store, semaphore, or mandatory RSE fill or spill matches the parameters set up in the data debug registers.
	Arguments:	Reason mask = Bit <0> Execute exception - interruption is associated with an instruction fetch Bit <2> Write exception - interruption is associated with a write operation. Bit <19> Register Stack - interruption is associated with a mandatory RSE fill or spill. Va = The address of the data being referenced.

Change-Mode and Compatibility-Mode Handlers

Two types of hardware exception can be handled in a way different from the normal condition-handling mechanism described in this chapter. The two types of hardware exception are as follows:

Traps caused by change-mode-to-user or change-mode-to-supervisor instructions
On VAX systems, compatibility mode faults

You can use the Declare Change Mode or Compatibility Mode Handler (SYS$DCLCMH) system service to establish procedures to receive control when one of these conditions occurs. The SYS$DCLCMH system service is described in the HP OpenVMS System Services Reference Manual.

9.3.2 Exception Conditions

On Alpha systems, the condition values that your condition-handling routine expects to receive on VAX systems may no longer be meaningful, even though the format of the 32-bit condition value and its location in the signal array are the same as they are on VAX systems. Because of architectural differences, some exception conditions that are returned on VAX systems are not supported on Alpha systems. In addition, some Alpha exception conditions exist on I64 and some do not, and there are some new I64-specific exception conditions.

On Alpha systems, because hardware exceptions are more architecture specific than software exceptions, only a subset of the hardware exceptions supported on VAX systems are also supported on Alpha systems. In addition, the Alpha architecture defines several additional exceptions that are not supported on VAX systems. Some Alpha hardware exceptions exist on I64 and some do not, and there are some new I64-specific hardware exceptions.

Table 9-3 lists the Alpha exceptions that are not supported on VAX systems and VAX hardware exceptions that are not supported on Alpha systems. For some arithmetic exceptions, Alpha software produces VAX compatible exceptions that are not supported by the hardware itself. See Section 9.3.3 for a discussion of SS$_HPARITH, a generic Alpha exception condition that software may replace with specific VAX exceptions.

Note

SS$_HPARITH is not used on I64 instead; VAX arithmetic conditions are used. There are also I64-specific exceptions: SS$_NATFAULT, SS$_DEBUG_FAULT, SS$_BREAK_SYS, SS$_BREAK_ARCH, SS$_BREAK_APPL, and SS$_FLTDENORMAL.

Table 9-3 Architecture-Specific Hardware Exceptions
Exception Condition Code Comment

New Alpha Exceptions

SS$_HPARITH--High-performance arithmetic exception Generated for most Alpha arithmetic exceptions (see Section 9.3.3)

SS$_ALIGN--Data alignment trap No VAX equivalent

VAX-Specific Hardware Exceptions

SS$_ARTRES--Reserved arithmetic trap No Alpha system equivalent

SS$_COMPAT--Compatibility fault No Alpha system equivalent

SS$_DECOVF--Decimal overflow ¹ Replaced by SS$_HPARITH on Alpha
(see Section 9.3.3)

SS$_FLTDIV--Float divide-by-zero (trap) ¹ Replaced by SS$_HPARITH on Alpha
(see Section 9.3.3)

SS$_FLTDIV_F--Float divide-by-zero (fault) ² Replaced by SS$_HPARITH on Alpha (see Section 9.3.3)

SS$_FLTOVF--Float overflow (trap) ¹ Replaced by SS$_HPARITH on Alpha
(see Section 9.3.3)

SS$_FLTOVF_F--Float overflow (fault) ² Replaced by SS$_HPARITH on Alpha (see Section 9.3.3)

SS$_FLTUND--Float underflow (trap) ¹ Replaced by SS$_HPARITH on Alpha
(see Section 9.3.3)

SS$_FLTUND_F--Float underflow (fault) ² Replaced by SS$_HPARITH on Alpha
(see Section 9.3.3)

SS$_INTDIV--Integer divide-by-zero ¹ Replaced by SS$_HPARITH on Alpha
(see Section 9.3.3)

SS$_INTOVF--Integer overflow ¹ Replaced by SS$_HPARITH on Alpha
(see Section 9.3.3)

SS$_TBIT--Trace pending ² No Alpha equivalent

SS$_OPCCUS--Opcode reserved to customer No Alpha equivalent

SS$_RADMOD--Reserved addressing mode No Alpha equivalent

SS$_SUBRNG--INDEX subscript range check No Alpha equivalent

**Table 9-3 Architecture-Specific Hardware Exceptions**
Exception Condition Code	Comment
New Alpha Exceptions
SS$_HPARITH--High-performance arithmetic exception	Generated for most Alpha arithmetic exceptions (see Section 9.3.3)
SS$_ALIGN--Data alignment trap	No VAX equivalent
VAX-Specific Hardware Exceptions
SS$_ARTRES--Reserved arithmetic trap	No Alpha system equivalent
SS$_COMPAT--Compatibility fault	No Alpha system equivalent
SS$_DECOVF--Decimal overflow ¹	Replaced by SS$_HPARITH on Alpha (see Section 9.3.3)
SS$_FLTDIV--Float divide-by-zero (trap) ¹	Replaced by SS$_HPARITH on Alpha (see Section 9.3.3)
SS$_FLTDIV_F--Float divide-by-zero (fault) ²	Replaced by SS$_HPARITH on Alpha (see Section 9.3.3)
SS$_FLTOVF--Float overflow (trap) ¹	Replaced by SS$_HPARITH on Alpha (see Section 9.3.3)
SS$_FLTOVF_F--Float overflow (fault) ²	Replaced by SS$_HPARITH on Alpha (see Section 9.3.3)
SS$_FLTUND--Float underflow (trap) ¹	Replaced by SS$_HPARITH on Alpha (see Section 9.3.3)
SS$_FLTUND_F--Float underflow (fault) ²	Replaced by SS$_HPARITH on Alpha (see Section 9.3.3)
SS$_INTDIV--Integer divide-by-zero ¹	Replaced by SS$_HPARITH on Alpha (see Section 9.3.3)
SS$_INTOVF--Integer overflow ¹	Replaced by SS$_HPARITH on Alpha (see Section 9.3.3)
SS$_TBIT--Trace pending ²	No Alpha equivalent
SS$_OPCCUS--Opcode reserved to customer	No Alpha equivalent
SS$_RADMOD--Reserved addressing mode	No Alpha equivalent
SS$_SUBRNG--INDEX subscript range check	No Alpha equivalent

¹On Alpha systems, this condition may be generated by software.
²On I64 systems, this condition may be generated by software.

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