Document revision date: 19 July 1999

Exceptions can be grouped into the following six classes:

• Arithmetic traps and faults
• Memory management exceptions
• Exceptions detected during operand reference
• Tracing
• Serious system failures

This section contains the descriptions of the exceptions that occur as the result of performing an arithmetic or conversion operation. They are mutually exclusive and are all assigned the same vector in the system control block (SCB) and the same signal "reason" code. Each exception indicates that an instruction has been completed (trap) or backed up (fault). An appropriate distinguishing exception type code is pushed onto the stack as a longword. Table E-1 lists the arithmetic exception type codes.

Table E-1 Arithmetic Exception Type Codes

Exception Type	Mnemonic	Decimal Value	Hexadecimal Value
Traps
integer overflow	SS$_INTOVF	1	1
integer divide-by-zero	SS$_INTDIV	2	2
floating overflow	SS$_FLTOVF	3	3
floating or decimal divide-by-zero	SS$_FLTDIV	4	4
floating underflow	SS$_FLTUND	5	5
decimal overflow	SS$_DECOVF	6	6
subscript range	SS$_SUBRNG	7	7
Faults
floating underflow	SS$_FLTOVF_F	8	8
floating divide-by-zero	SS$_FLTDIV_F	9	9
floating underflow	SS$_FLTUND_F	10	A

E.1.1 Integer Overflow Trap

An integer overflow trap is an exception indicating that the last instruction executed had an integer overflow, which set the processor status longword (PSL) V bit, and that the integer overflow was enabled (the IV bit in the PSL was set). The stored result is the low-order part of the correct result. The N and Z bits in the PSL are set according to the stored result. The type code pushed onto the stack is 1 (SS$_INTOVF).

E.1.2 Integer Divide-by-Zero Trap

An integer divide-by-zero trap is an exception indicating that the last instruction executed had an integer zero divisor. The stored result is equal to the dividend, and condition code V bit in the PSL is set. The type code pushed onto the stack is 2 (SS$_INTDIV).

E.1.3 Floating Overflow Trap

A floating overflow trap is an exception indicating that the last instruction executed resulted in an exponent greater than the largest representable exponent for the data type after normalization and rounding. The stored result contains a one in the sign field and zeros in the exponent and fraction fields. This is a reserved operand. It causes a reserved operand fault if used in a subsequent floating-point instruction. The N and V condition code bits in the PSL are set, and the Z and C bits in the PSL are cleared. The type code pushed onto the stack is 3 (SS$_FLTOVF).

E.1.4 Divide-by-Zero Trap

A floating divide-by-zero trap is an exception indicating that the last instruction executed had a floating zero divisor. The stored result is the reserved operand described previously for the floating overflow trap. The condition codes are set as they are for the floating overflow trap.

A decimal string divide-by-zero trap is an exception indicating that the last instruction executed had a decimal-string zero divisor. The destination, R0 to R5, and condition codes are UNPREDICTABLE. The zero divisor can be either +0 or -0.

The type code pushed onto the stack for both types of divide-by-zero is 4 (SS$_FLTDIV).

E.1.5 Floating Underflow Trap

A floating underflow trap is an exception indicating that the last instruction executed resulted in an exponent less than the smallest representable exponent for the data type after normalization and rounding, and that floating underflow was enabled (FU set). The stored result is zero. The N, V, and C condition codes bits in the PSL are cleared, and the Z bit in the PSL is set, except for the polynomial evaluation instruction POLYx. In POLYx, the trap occurs on completion of the instruction, which may be many operations after the underflow. The condition codes are set on the final result in POLYx. The type code pushed onto the stack is 5 (SS$_FLTUND).

E.1.6 Decimal String Overflow Trap

A decimal string overflow trap is an exception indicating that the last instruction executed had a decimal-string result too large for the destination string provided, and that decimal overflow was enabled (the DV bit in the PSL was set). The V condition code bit in the PSL is always set. The type code pushed onto the stack is 6 (SS$_DECOVF).

E.1.7 Subscript-Range Trap

A subscript range trap is an exception indicating that the last instruction was an INDEX instruction with a subscript operand that failed the range check. The value of the subscript operand is lower than the low operand or greater than the high operand. The result is stored in indexout, and the condition codes are set as if the subscript were within range. The type code pushed onto the stack is 7 (SS$_SUBRNG).

E.1.8 Floating Overflow Fault

A floating overflow fault is an exception indicating that the last instruction executed resulted in an exponent greater than the largest representable exponent for the data type after normalization and rounding. The destination was unaffected, and the saved condition codes are UNPREDICTABLE. The saved program counter (PC) points to the instruction causing the fault. The POLYx instruction is suspended with the first-part-done bit (FPD) set. The type code pushed onto the stack is 8 (SS$_FLTOVF_F).

E.1.9 Divide-by-Zero Floating Fault

A floating divide-by-zero fault is an exception indicating that the last instruction executed had a floating zero divisor. The quotient operand was unaffected and the saved condition codes are UNPREDICTABLE. The saved PC points to the instruction causing the fault. The type code pushed onto the stack is 9 (SS$_FLTDIV_F).

E.1.10 Floating Underflow Fault

A floating underflow fault is an exception indicating that the last instruction executed resulted in an exponent less than the smallest representable exponent for the data type after normalization and rounding, and that floating underflow was enabled (the FU bit was set). The destination operand is unaffected. The saved condition codes are UNPREDICTABLE. The saved PC points to the instruction causing the fault. The POLYx instruction is suspended with FPD set. The type code pushed onto the stack is 10 (SS$_FLTUND_F).

E.2 Memory Management Exceptions

A memory management exception can be either an access control violation fault or a translation not valid fault.

E.2.1 Access Control Violation Fault

An access control violation fault is an exception indicating that the process attempted a reference not allowed at the current access mode.

E.2.2 Translation Not Valid Fault

A translation not valid fault is an exception indicating that the process attempted a reference to a page for which the valid bit in the page table had not been set.

Note that if a process attempts to reference a page for which the page table entry specifies both translation not valid fault and access control violation, an access control violation fault occurs.

E.3 Exceptions Detected During Operand Reference

Two exceptions are possible during operand reference: the reserved addressing mode fault and the reserved operand exception.

E.3.1 Reserved Addressing Mode Fault

A reserved addressing mode fault is an exception indicating that an operand specifier attempted to use an addressing mode that is disallowed. No parameters are pushed.

E.3.2 Reserved Operand Exception

A reserved operand exception is an exception indicating that an accessed operand has a format reserved for future use by Digital. No parameters are pushed onto the stack. This exception always backs up the saved PC to point to the opcode. The exception service routine may determine the type of operand by examining the opcode using the saved PC.

Note that only the changes made by instruction fetch and the changes made because of operand specifier evaluation may be restored. Therefore, some instructions are not restartable. These exceptions are labeled as aborts rather than as faults. The saved PC is always restored properly unless the instruction attempted to modify it in a manner that results in UNPREDICTABLE results.

The reserved operand exceptions are caused by the following:

• Bit field too wide
• Invalid combination of bits in PSL restored by the return from interrupt (REI) instruction (fault)
• Invalid combination of bits in PSW mask longword during a return from procedure (RET) instruction (fault)
• Invalid combination of bits in the bit set PSW (BISPSW) or bit clear PSW (BICPSW) instructions (fault)
• Invalid call procedure with stack argument list (CALLS) or call procedure with general argument list (CALLG) instructions entry mask (fault)
• Invalid register number in the move from processor register (MFPR) instruction or move to processor register (MTPR) instruction (fault)
• Invalid PCB contents in the load processor context (LDPCTX) instruction for some implementations (abort)
• Unaligned operand in the add aligned word interlocked (ADAWI) instruction (fault)
• Invalid register contents in the move to processor register (MTPR) instruction for some implementations (fault)
• Invalid operand addresses in insert and remove queue interlocked (INSQHI, INSQTI, REMQHI, or REMQTI) instructions (fault)
• A floating-point number that has the sign bit set and the exponent zero in the polynomial evaluation (POLY) instruction table (fault)
• POLY degree too large (fault)
• Decimal string too long (abort)
• Invalid digit in convert trailing numeric to packed (CVTTP) or convert separate numeric to packed (CVTSP) instructions (abort)
• Reserved pattern operator in the edit packed to character string (EDITPC) instruction (fault)
• Incorrect source string length at completion of EDITPC (abort)

The following exceptions may occur as a consequence of instruction execution:

• Reserved or privileged instruction fault
• Opcode reserved to customers fault
• Instruction emulation exceptions
• Compatibility mode exception
• Change mode trap
• Breakpoint fault

Each is described in the following subsections.

E.4.1 Reserved or Privileged Instruction Fault

A reserved or privileged instruction fault occurs when the processor encounters an opcode that is not specifically defined or requires higher privileges than the current mode. No parameters are pushed onto the stack. Opcode FFFF (hex) will always fault.

E.4.2 Operand Reserved to Customers Fault

An opcode reserved to customers fault is an exception that occurs when an opcode reserved to customers is executed. The operation is identical to the reserved or privileged instruction fault, except that the event is caused by a different set of opcodes and faults through a different vector. All opcodes reserved to customers start with FC (hex), which is the XFC instruction. If the special instruction must generate a unique exception, one of the reserved-to-customer vectors should be used. An example might be an unrecognized second byte of the instruction.

The XFC fault is intended primarily for use with writable control store to implement installation-dependent instructions. The method used to enable and disable the handling of an XFC fault in user-written microcode is implementation dependent. Some implementations may transfer control to microcode without checking bits <1:0> of the exception vector.

E.4.3 Instruction Emulation Exceptions

When a subset processor executes a string instruction that is omitted from its instruction set, an emulation exception results. An emulation exception can occur through either of two system control block (SCB) vectors, depending on whether or not the first-part-done (FPD) bit in the program status longword was set at the beginning of the instruction. If the FPD bit is clear, a subset emulation trap occurs through the SCB vector at offset CB (hex), and a subset emulation trap frame is pushed onto the current stack. If the FPD bit is set, a suspended emulation fault occurs through the SCB vector at offset CC (hex), and the PC and the PSL are pushed onto the current stack.

E.4.4 Compatibility Mode Exception

A compatibility mode exception is an exception that occurs when the processor is in compatibility mode. A longword of information containing a code that indicates the exception type is pushed onto the stack. Figure E-1 shows the stack frame, which is the same as that for arithmetic exceptions.

Figure E-1 Compatibility Mode Exception Stack Frame

The compatibility type codes are shown in Table E-2.

Table E-2 Compatibility Mode Exception Type Codes

Exception Type	Decimal Value
Faults
reserved opcode	0
BPT instruction	1
IOT instruction	2
EMT instruction	3
TRAP instruction	4
illegal instruction	5
Aborts
odd address	6

All other exceptions in compatibility mode, including the access control violation fault, the translation not valid fault, and the machine check abort, occur by means of the regular native-mode vector.

E.4.5 Change Mode Trap

A change mode trap is an exception occurring when one of the change mode instructions (CHMK, CHME, CHMS, or CHMU) is executed. The instruction operand is pushed onto the exception stack.

E.4.6 Breakpoint Fault

A breakpoint fault is an exception that occurs when the breakpoint instruction (BPT) is executed. The BPT instruction pushes the current PSL onto the stack.

To proceed from a breakpoint fault, a debugger or tracing program does the following:

1. Restores the original contents of the location containing the BPT instruction.
2. Sets the T bit in the PSL saved by the BPT fault. The PSL is on the stack.
3. Resumes operation of the main instruction stream.

When the instruction that has a breakpoint completes execution, a trace exception occurs. At this point, the tracing program takes control and does the following:

1. Reinserts the BPT instruction.
2. Restores the T bit to its original state (usually zero).
3. Resumes operation of the main instruction stream.

Note that if both tracing and breakpointing are in progress (if the PSL T bit was set at the time of the BPT), both the BPT restoration and a normal trace exception should be processed on the trace exception by the trace handler.

E.5 Trace Fault

Program tracing is used for many purposes. Debugging programs and evaluating program performance are the most common uses of program tracing.

A trace fault is an exception that occurs between instructions when trace is enabled. One trace fault occurs before the execution of each traced instruction. The address in the PC saved when a trace fault occurs is the address of the instruction after the trace fault that would normally be executed. The trace exception for an instruction takes precedence over all other exceptions. The detection of reserved instruction faults occurs after the trace fault. If a trace fault and a memory management fault (or an odd address abort during a compatibility mode instruction fetch) occur simultaneously, exceptions are taken in UNPREDICTABLE order.

To ensure that exactly one trace occurs per instruction despite other traps and faults, the PSL contains the trace enable (T) and trace pending (TP) bits.

The PSL TP bit generates a fault before any other processing at the start of the next instruction.

The following are rules of operation for trace:

1. At the beginning of an instruction, if the trace pending (TP) bit is set, it is cleared and a trace fault is taken.
2. The value of the trace enable (T) bit is loaded into the trace pending (TP) bit.
3. The detection of interrupts and other exceptions can occur during instruction execution. In this case, TP is cleared before the exception or interrupt is initiated. The system saves the entire PSL including the T bit and TP bit on interrupt or exception initiation and restores the PSL at the end with an REI. This makes interrupts and benign exceptions totally transparent to the executing program.
   The following are conditions and results that might occur during instruction execution or before the next instruction:
   1. If the instruction faults or an interrupt is serviced, the PSL TP bit is cleared before the PSL is saved on the stack. The saved PC (the next lower word on the stack after the saved PSL) is set to the start of the faulting or interrupted instruction. Instruction execution is resumed at step 1.
   2. If the instruction aborts or takes an arithmetic trap, the PSL TP bit is not changed before the PSL is saved on the stack.
   3. If an interrupt is serviced after instruction completion and arithmetic traps but before the presence of tracing is checked at the start of the next instruction, the PSL TP bit is not changed before the PSL is saved on the stack.

The routine entered by a change mode (CHMx) instruction is not traced because change mode clears T and TP in the new PSL that is used for whichever new mode is entered. However, if the T bit was set in the old PSW (the one to be saved) at the beginning of the change mode instruction, the system sets both the T and the TP bit in the saved PSL. Trace faults resume with the instruction that follows other returns from interrupt (REI) in the routine entered by the CHMx instruction. An instruction following an REI faults if T was set when the REI was executed, or if the TP bit in the saved PSL is set. In both cases, TP is set after the REI.

E.5.2 Trace Operation Upon Return From Interrupt

Note that a trace fault that occurs for an instruction following an REI instruction that had set the TP will be taken with the new PSL restored by the REI instruction. Thus, special care must be taken if exception or interrupt routines are traced.

E.5.3 Trace Operation After a BISPSW Instruction

If the T bit is set by a BISPSW instruction, trace faults begin with the second instruction after the BISPSW.

E.5.4 Trace Operation After a CALLS or CALLG Instruction

The CALLS and CALLG instructions save a clear T bit, although the T bit in the PSL is unchanged. This is done so that a debugger or trace program proceeding from a BPT fault does not get a spurious trace from the RET that matches the CALL.

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