OpenVMS RTL Library (LIB$) Manual

Document revision date: 30 March 2001

OpenVMS RTL Library (LIB$) Manual

Contents

Index

For further information, see the section called Call Format for a Signal Routine.

context

OpenVMS usage:	context
type:	unspecified
access:	read only
mechanism:	by value

Context in which the exception occurs, including the register and PSL contents, to be used when calling the signal-procedure. The context argument contains the value of this context.

unspecified-user-argument

OpenVMS usage:	user_arg
type:	longword (unsigned)
access:	read only
mechanism:	by value

Optional argument passed to LIB$DECODE_FAULT. If the argument was not specified, the value zero is substituted. The unspecified-user-argument argument contains the value of this optional argument.

original-registers

OpenVMS usage:	vector_longword_unsigned
type:	longword (unsigned)
access:	modify
mechanism:	by reference, array reference

Array containing the values of registers R0 through R15 (PC) at the time of the fault, before operand processing. The original-registers argument is the address of this 16-longword array.

If the action routine specifies that the instruction should restart or that a fault should be generated, the registers are restored from original-registers. See also the description of registers above.

Condition Values Returned from the User Action Routine

The user action routine can return the following condition values to LIB$DECODE_FAULT:

Condition Value Description

SS$_CONTINUE If the user action routine returns a value of SS$_CONTINUE, instruction execution will continue as specified by the current contents of the registers element for the PC.

SS$_RESIGNAL If the user action routine returns SS$_RESIGNAL, the original exception is resignaled, with the only changes reflected being those specified by registers elements for R0 and R1 (which are stored in the mechanism arguments vector), PC, and PSL. All other registers are restored from original registers.

LIB$_RESTART If the user action routine returns LIB$_RESTART, the current instruction is restarted with registers restored from original-registers and a PSL from PSL. This feature is useful for writing trace handlers.

Condition Value	Description
SS$_CONTINUE	If the user action routine returns a value of SS$_CONTINUE, instruction execution will continue as specified by the current contents of the registers element for the PC.
SS$_RESIGNAL	If the user action routine returns SS$_RESIGNAL, the original exception is resignaled, with the only changes reflected being those specified by registers elements for R0 and R1 (which are stored in the mechanism arguments vector), PC, and PSL. All other registers are restored from original registers.
LIB$_RESTART	If the user action routine returns LIB$_RESTART, the current instruction is restarted with registers restored from original-registers and a PSL from PSL. This feature is useful for writing trace handlers.

Call Format for a Signal Routine

Your action routine calls the signal routine using this format:

signal-procedure fault-flag ,context ,signal-arguments

fault-flag

OpenVMS usage: mask_longword

type: longword (unsigned)

access: read only

mechanism: by reference

Longword flag whose low-order bit determines whether the exception is to be signaled as a fault or as a trap. The fault-flag argument contains the address of this longword.
If the low-order bit of fault-flag is set to 1, the exception is signaled as a fault. If the low-order bit of fault-flag is set to 0, the exception is signaled as a trap; the current contents of the registers array are used. In either case, the current contents of PSL are used to set the exception PSL.
context

OpenVMS usage: context

type: unspecified

access: read only

mechanism: by reference

Context in which the new exception is to occur, as passed to your user action routine by LIB$DECODE_FAULT. The context argument is the address of this context value.
signal-arguments

OpenVMS usage: arg_list

type: longword (unsigned)

access: read only

mechanism: by reference, array reference

Signal arguments to be used. The signal-arguments argument is the address of an array of longwords that contains these signal arguments.
The first longword contains the number of following longwords; the remainder of the list contains signal names and arguments. Unlike the signal argument list passed to a condition handler, no PC or PSL is present.
Before the exception is signaled, the stack frames are unwound back to the original exception. You should be careful when causing a new signal that a loop of faults is not inadvertently generated. For example, the condition handler that called LIB$DECODE_FAULT will usually be called for the second signal. If the handler does not analyze the second signal as such, it may cycle through the identical path as for the first signal.
To resignal the current exception, have the user action routine return a value of SS$_RESIGNAL instead of calling the signal routine (unless you want previously called condition handlers to be called again).

Condition Values Returned

SS$_RESIGNAL Resignal condition to next handler. The exception described by signal-arguments was not an instruction fault handled by LIB$DECODE_FAULT. If LIB$DECODE_FAULT can process the fault, it does not return to its caller.

Condition Value Signaled

LIB$_INVARG Invalid argument to Run-Time Library. The instruction definition contained more than 16 operands or an operand definition contained an invalid data type or access code. This message is signaled after the stack frames have been unwound so that it appears to have been signaled from a routine that was called by the instruction that faulted.

Example

The following Fortran example implements a simple recovery scheme for floating underflow and overflow faults, replacing the result of the instruction with the correctly signed, smallest possible value for underflows or largest possible value for overflows.


C+ C Example condition handler and user-action routine using C LIB$DECODE_FAULT. This example demonstrates the use of C most of the features of LIB$DECODE_FAULT. Its purpose C is to handle floating underflow and overflow faults, C replacing the result of the instruction with the correctly C signed smallest possible value for underflows, or greatest C possible value for overflows. C C For simplicity, faults involving the POLYx instructions are C not handled. C C* C FIXUP_RESULT is the condition handler enabled by the program C desiring the fixup of overflows and underflows. C* C- INTEGER4 FUNCTION FIXUP_RESULT(SIGARGS, MECHARGS) IMPLICIT NONE INCLUDE '($SSDEF)' ! SS$_ symbols INCLUDE '($LIBDCFDEF)' ! LIB$DECODE_FAULT symbols INTEGER4 SIGARGS(1:) ! Signal arguments list INTEGER4 MECHARGS(1:) ! Mechanism arguments list C+ C This is a sample redefinition of MULH3 instruction. C- BYTE OPTABLE(8) /'FD'X,'65'X, ! MULH3 opcode 1 LIB$K_DCFOPR_RH, ! Read H_floating 2 LIB$K_DCFOPR_RH, ! Read H_floating 3 LIB$K_DCFOPR_WH, ! Write H_floating 4 LIB$K_DCFOPR_END, ! End of operands 5 'FF'X,'FF'X/ ! End of instructions INTEGER4 LIB$DECODE_FAULT ! External function EXTERNAL FIXUP_ACTION ! Action routine to do the fixup C+ C Determine if the exception is one we want to handle. C- IF ((SIGARGS(2) .EQ. SS$_FLTOVF_F) .OR. 1 (SIGARGS(2) .EQ. SS$_FLTUND_F)) THEN C+ C We think we can handle the fault. Call C LIB$DECODE_FAULT and pass it the signal arguments and C the address of our action routine and opcode table. C- FIXUP_RESULT = LIB$DECODE_FAULT (SIGARGS, 1 MECHARGS, %DESCR(FIXUP_ACTION),, OPTABLE) RETURN END IF C+ C We can only get here if we couldn't handle the fault. C Resignal the exception. C- FIXUP_RESULT = SS$_RESIGNAL RETURN END C+ C User action routine to handle the fault. C- INTEGER4 FUNCTION FIXUP_ACTION (OPCODE,INSTR_PC,PSL, 1 REGISTERS,OP_COUNT, 2 OP_TYPES,READ_OPS, 3 WRITE_OPS,SIGARGS, 4 SIGNAL_ROUT,CONTEXT, 5 USER_ARG,ORIG_REGS) IMPLICIT NONE INCLUDE '($SSDEF)' ! SS$_ definitions INCLUDE '($PSLDEF)' ! PSL$ definitions INCLUDE '($LIBDCFDEF)' ! LIB$DECODE_FAULT ! definitions INTEGER4 OPCODE ! Instruction opcode INTEGER4 INSTR_PC ! PC of this instruction INTEGER4 PSL ! Processor status ! longword INTEGER4 REGISTERS(0:15) ! R0-R15 contents INTEGER4 OP_COUNT ! Number of operands INTEGER4 OP_TYPES(1:) ! Types of operands INTEGER4 READ_OPS(1:) ! Addresses of read operands INTEGER4 WRITE_OPS(1:) ! Addresses of write operands INTEGER4 SIGARGS(1:) ! Signal argument list INTEGER4 SIGNAL_ROUT ! Signal routine address INTEGER4 CONTEXT ! Signal routine context INTEGER4 USER_ARG ! User argument value INTEGER4 ORIG_REGS(0:15) ! Original registers C+ C Declare and initialize table of class codes for each of the C "real" opcodes. We'll index into this by the first byte of C one-byte opcodes, the second byte of two-byte opcodes. The C class codes will be used in a computed GOTO (CASE). The C codes are: C 0 - Unsupported C 1 - ADD C 2 - SUB C 3 - MUL,DIV C 4 - ACB C 5 - CVT C 6 - EMOD C C The class mainly determines how we compute the sign of the C result, except for ACB. C- BYTE INST_CLASS_TABLE(0:255) DATA INST_CLASS_TABLE / 1 480, ! 00-2F 2 0,0,0,5,0,0,0,0,0,0,0,0,0,0,0,0, ! 30-3F 3 1,1,2,2,3,3,3,3,0,0,0,0,0,0,0,4, ! 40-4F 4 0,0,0,0,6,0,0,0,0,0,0,0,0,0,0,0, ! 50-5F 5 1,1,2,2,3,3,3,3,0,0,0,0,0,0,0,4, ! 60-6F 6 0,0,0,0,6,0,5,0,0,0,0,0,0,0,0,0, ! 70-7F 7 1120, ! 80-EF 8 0,0,0,0,0,0,5,5,0,0,0,0,0,0,0,0/ ! F0-FF C+ C Table of operand sizes in 8-bit bytes, indexed by the C datatype code contained in the OP_TYPES array. Only floating C types matter. C- BYTE OP_SIZES(9) /0,0,0,0,0,4,8,8,16/ INTEGER4 LIB$EXTV ! External function INTEGER4 RESULT_NEGATIVE ! -1 if result negative, ! 0 if positive INTEGER4 SIGN1,SIGN2,SIGN3 ! Signs of operands INTEGER4 INST_BYTE ! Current opcode byte INTEGER4 INST_CLASS ! Class of instruction ! from table INTEGER4 OP_DTYPE ! Datatype of operand INTEGER4 OP_SIZE ! Size of operand in ! 8-bit bytes INTEGER4 RESULT_OP ! Position of result ! in WRITE_OPS array LOGICAL4 OVERFLOW ! TRUE if SS$_FLTOVF_F LOGICAL4 SMALLER ! Function which ! compares operands PARAMETER ESCD = '0FD'X ! First byte of G,H instructions INTEGER2 SMALL_F(2) ! Smallest F_floating DATA SMALL_F /'0080'X,0/ INTEGER2 SMALL_D(4) ! Smallest D_floating DATA SMALL_D /'0080'X,0,0,0/ INTEGER2 SMALL_G(4) ! Smallest G_floating DATA SMALL_G /'0010'X,0,0,0/ INTEGER2 SMALL_H(8) ! Smallest H_floating DATA SMALL_H /'0001'X,0,0,0,0,0,0,0/ INTEGER2 BIGGEST(8) ! Biggest value (all datatypes) DATA BIGGEST /'7FFF'X,7'FFFF'X/ INTEGER4 SIGNAL_ARRAY(2) ! Array for signalling new ! exception C+ C C NOTE: Because the operands arrays contain the locations of C the operands, rather than the operands themselves, C we must call a routine using the %VAL function to C "fool" the called routine into considering the C contents of an operands array element as the address C of an item. This would not be necessary in a C language that understood the concept of pointer C variables, such as PASCAL. C C C If FPD is set in the PSL, signal SS$_ROPRAND (reserved operand). In C reality this shouldn't happen since none of the instructions we C handle can set FPD, but do it as an example. C- IF (BTEST(PSL,PSL$V_FPD)) THEN SIGNAL_ARRAY(1) = 1 ! Count of signal arguments SIGNAL_ARRAY(2) = SS$_ROPRAND ! Error status value CALL SIGNAL_ROUT ( 1 1, ! Fault flag - signal as fault 2 SIGNAL_ARRAY, ! Signal arguments array 3 CONTEXT) ! Context as passed to us ! Call will never return END IF C+ C Set OVERFLOW according to the exception type. We assume that C the only alternatives are SS$_FLTOVF_F and SS$_FLTUND_F. C- OVERFLOW = (SIGARGS(2) .EQ. SS$_FLTOVF_F) C+ C Determine the datatype of the instruction by that of its C second operand, since that is always the type of the C destination. C- OP_DTYPE = IBITS(OP_TYPES(2),LIB$V_DCFTYP,LIB$S_DCFTYP) C+ C Get the size of the datatype in words. C- OP_SIZE = OP_SIZES (OP_DTYPE) C+ C Determine the class of instruction and dispatch to the C appropriate routine. C- INST_BYTE = IBITS(OPCODE,0,8) ! Get first byte IF (INST_BYTE .EQ. ESCD) INST_BYTE = IBITS(OPCODE,8,8) INST_CLASS = INST_CLASS_TABLE(INST_BYTE) GO TO (1000,2000,3000,4000,5000,6000),INST_CLASS C+ C If we get here, the instruction's entry in the C INST_CLASS_TABLE is zero. This might happen if the instruction was C a POLYx, or was some other unsupported instruction. Resignal the C original exception. C- FIXUP_ACTION = SS$_RESIGNAL ! Resignal condition to next handler RETURN ! Return to LIB$DECODE_FAULT C+ C 1000 - ADDF2, ADDF3, ADDD2, ADDD3, ADDG2, ADDG3, ADDH2, ADDH3 C C Result's sign is the same as that of the first operand, C unless this is an underflow, in which case the magnitudes of C the values may change the sign. C- 1000 RESULT_NEGATIVE = LIB$EXTV (15,1,%VAL(READ_OPS(1))) IF (.NOT. OVERFLOW) THEN IF (SMALLER(OP_SIZE,%VAL(READ_OPS(1)), 1 %VAL(READ_OPS(2)))) 2 RESULT_NEGATIVE = .NOT. RESULT_NEGATIVE END IF GO TO 9000 C+ C 2000 - SUBF2, SUBF3, SUBD2, SUBD3, SUBG2, SUBG3, SUBH2, SUBH3 C C Result's sign is the opposite of that of the first operand, C unless this is an underflow, in which case the magnitudes of C the values may change the sign. C- 2000 RESULT_NEGATIVE = .NOT. LIB$EXTV (15,1,%VAL(READ_OPS(1))) IF (.NOT. OVERFLOW) THEN IF (SMALLER(OP_SIZE,%VAL(READ_OPS(1)), 1 %VAL(READ_OPS(2)))) 2 RESULT_NEGATIVE = .NOT. RESULT_NEGATIVE END IF GO TO 9000 C+ C 3000 - MULF2, MULF3, MULD2, MULD3, MULG2, MULG3, MULH2, MULH3, C DIVF2, DIVF3, DIVD2, DIVD3, DIVG2, DIVG3, DIVH2, DIVH3, C C If the signs of the first two operands are the same, then the C result's sign is positive, if they are not it is negative. C- 3000 SIGN1 = LIB$EXTV (15,1,%VAL(READ_OPS(1))) SIGN2 = LIB$EXTV (15,1,%VAL(READ_OPS(2))) RESULT_NEGATIVE = SIGN1 .XOR. SIGN2 GOTO 9000 C+ C 4000 - ACBF, ACBD, ACBG, ACBH C C The result's sign is the same as that of the second operand C (addend), unless this is underflow, in which case the C magnitudes of the addend and index may change the sign. C We must also determine if the branch is to be taken. C- 4000 SIGN2 = LIB$EXTV (15,1,%VAL(READ_OPS(2))) RESULT_NEGATIVE = SIGN2 IF (.NOT. OVERFLOW) THEN IF (SMALLER(OP_SIZE,%VAL(READ_OPS(2)), 1 %VAL(READ_OPS(3)))) 2 RESULT_NEGATIVE = .NOT. RESULT_NEGATIVE END IF C+ C If this is overflow, then the branch is not taken, since the C result is always going to be greater or equal in magnitude C to the limit, and will be the correct sign. If underflow, C the branch is ALMOST always taken. The only case where the C branch might not be taken is when the result is exactly C equal to the limit. For this example, we are going to ignore C this exceptional case. C- IF (.NOT. OVERFLOW) 1 REGISTERS(15) = READ_OPS(4) ! Branch destination GO TO 9000 C+ C 5000 - CVTDF, CVTGF, CVTHF, CVTHD, CVTHG C C Result's sign is the same as that of the first operand. C- 5000 RESULT_NEGATIVE = LIB$EXTV (15,1,%VAL(READ_OPS(1))) GO TO 9000 C+ C 6000 - EMODF, EMODD, EMODG, EMODH C C If the signs of the first and third operands are the same, then the C result's sign is positive, else it is negative. C- 6000 SIGN1 = LIB$EXTV (15,1,%VAL(READ_OPS(1))) SIGN2 = LIB$EXTV (15,1,%VAL(READ_OPS(3))) RESULT_NEGATIVE = SIGN1 .XOR. SIGN2 GOTO 9000 C+ C All code paths merge here to store the result value. We also C set the PSL appropriately. First, determine which operand is C the result. C- 9000 RESULT_OP = OP_COUNT IF (INST_CLASS .EQ. 4) 1 RESULT_OP = RESULT_OP - 1 ! ACBx C+ C Select result based on datatype and exception type. C- IF (OVERFLOW) THEN CALL LIB$MOVC3 (OP_SIZE,BIGGEST,%VAL(WRITE_OPS(RESULT_OP))) ELSE GO TO (9100,9200,9300,9400), OP_DTYPE-(LIB$K_DCFTYP_F-1) C+ C Should never get here. Resignal original exception. C- FIXUP_ACTION = SS$_RESIGNAL RETURN C+ C 9100 - F_floating result C- 9100 CALL LIB$MOVC3 (OP_SIZE,SMALL_F,%VAL(WRITE_OPS(RESULT_OP))) GOTO 9500 C+ C 9200 - D_floating result C- 9200 CALL LIB$MOVC3 (OP_SIZE,SMALL_D,%VAL(WRITE_OPS(RESULT_OP))) GOTO 9500 C+ C 9300 - G_floating result C- 9300 CALL LIB$MOVC3 (OP_SIZE,SMALL_G,%VAL(WRITE_OPS(RESULT_OP))) GOTO 9500 C+ C 9400 - H_floating result C- 9400 CALL LIB$MOVC3 (OP_SIZE,SMALL_H,%VAL(WRITE_OPS(RESULT_OP))) GOTO 9500 9500 END IF C+ C Modify the PSL to reflect the stored result. If the result was C negative, set the N bit. Clear the V (overflow) and Z (zero) bits. C If the instruction was an ACBx, leave the C (carry) bit unchanged, C otherwise clear it. C- IF (RESULT_NEGATIVE) THEN PSL = IBSET (PSL,PSL$V_N) ! Set N bit ELSE PSL = IBCLR (PSL,PSL$V_N) ! Clear N bit END IF PSL = IBCLR (PSL,PSL$V_V) ! Clear V bit PSL = IBCLR (PSL,PSL$V_Z) ! Clear Z bit IF (INST_CLASS .NE. 4) 1 PSL = IBCLR (PSL,PSL$V_C) ! Clear C bit if not ACBx C+ C Set the sign of result. C- IF (RESULT_NEGATIVE) 1 CALL LIB$INSV (1,15,1,%VAL(WRITE_OPS(RESULT_OP))) C+ C Fixup is complete. Return to LIB$DECODE_FAULT. C- FIXUP_ACTION = SS$_CONTINUE RETURN END C+ C Function which compares two floating values. It returns .TRUE. if C the first argument is smaller in magnitude than the second. C- LOGICAL4 FUNCTION SMALLER(NBYTES,VAL1,VAL2) INTEGER4 NBYTES ! Number of bytes in values INTEGER2 VAL1(),VAL2() ! Floating values to compare INTEGER*4 WORDA,WORDB SMALLER = .TRUE. ! Initially return true C+ C Zero extend to a longword for unsigned compares. C Compare first word without sign bit. C- WORDA = IBCLR(ZEXT(VAL1(1)),15) WORDB = IBCLR(ZEXT(VAL2(1)),15) IF (WORDA .LT. WORDB) RETURN DO I=2,NBYTES/2 WORDA = ZEXT(VAL1(I)) WORDB = ZEXT(VAL2(I)) IF (WORDA .LT. WORDB) RETURN END DO SMALLER = .FALSE. ! VAL1 not smaller than VAL2 RETURN END

 
C+ 
C  Example condition handler and user-action routine using 
C  LIB$DECODE_FAULT.  This example demonstrates the use of 
C  most of the features of LIB$DECODE_FAULT.  Its purpose 
C  is to handle floating underflow and overflow faults, 
C  replacing the result of the instruction with the correctly 
C  signed smallest possible value for underflows, or greatest 
C  possible value for overflows. 
C 
C  For simplicity, faults involving the POLYx instructions are 
C  not handled. 
C 
C*** 
C  FIXUP_RESULT is the condition handler enabled by the program 
C  desiring the fixup of overflows and underflows. 
C*** 
C- 
 
        INTEGER*4 FUNCTION FIXUP_RESULT(SIGARGS, MECHARGS) 
 
        IMPLICIT NONE 
        INCLUDE '($SSDEF)'              ! SS$_ symbols 
        INCLUDE '($LIBDCFDEF)'          ! LIB$DECODE_FAULT symbols 
        INTEGER*4 SIGARGS(1:*)          ! Signal arguments list 
        INTEGER*4 MECHARGS(1:*)         ! Mechanism arguments list 
 
C+ 
C This is a sample redefinition of MULH3 instruction. 
C- 
 
        BYTE OPTABLE(8) /'FD'X,'65'X,           ! MULH3 opcode 
        1                LIB$K_DCFOPR_RH,       ! Read H_floating 
        2                LIB$K_DCFOPR_RH,       ! Read H_floating 
        3                LIB$K_DCFOPR_WH,       ! Write H_floating 
        4                LIB$K_DCFOPR_END,      ! End of operands 
        5                'FF'X,'FF'X/           ! End of instructions 
 
        INTEGER*4 LIB$DECODE_FAULT      ! External function 
        EXTERNAL FIXUP_ACTION   ! Action routine to do the fixup 
 
 
C+ 
C       Determine if the exception is one we want to handle. 
C- 
 
 
        IF ((SIGARGS(2) .EQ. SS$_FLTOVF_F) .OR. 
        1   (SIGARGS(2) .EQ. SS$_FLTUND_F)) THEN 
 
C+ 
C         We think we can handle the fault.  Call 
C         LIB$DECODE_FAULT and pass it the signal arguments and 
C         the address of our action routine and opcode table. 
C- 
 
          FIXUP_RESULT = LIB$DECODE_FAULT (SIGARGS, 
        1   MECHARGS, %DESCR(FIXUP_ACTION),, OPTABLE) 
 
          RETURN 
        END IF 
 
C+ 
C       We can only get here if we couldn't handle the fault. 
C       Resignal the exception. 
C- 
 
        FIXUP_RESULT = SS$_RESIGNAL 
        RETURN 
        END 
 
C+ 
C  User action routine to handle the fault. 
C- 
 
        INTEGER*4 FUNCTION FIXUP_ACTION (OPCODE,INSTR_PC,PSL, 
        1                                REGISTERS,OP_COUNT, 
        2                                OP_TYPES,READ_OPS, 
        3                                WRITE_OPS,SIGARGS, 
        4                                SIGNAL_ROUT,CONTEXT, 
        5                                USER_ARG,ORIG_REGS) 
 
        IMPLICIT NONE 
        INCLUDE '($SSDEF)'              ! SS$_ definitions 
        INCLUDE '($PSLDEF)'             ! PSL$ definitions 
        INCLUDE '($LIBDCFDEF)'          ! LIB$DECODE_FAULT 
                                        ! definitions 
 
        INTEGER*4 OPCODE                ! Instruction opcode 
        INTEGER*4 INSTR_PC              ! PC of this instruction 
        INTEGER*4 PSL                   ! Processor status 
                                        ! longword 
        INTEGER*4 REGISTERS(0:15)       ! R0-R15 contents 
        INTEGER*4 OP_COUNT              ! Number of operands 
        INTEGER*4 OP_TYPES(1:*)         ! Types of operands 
        INTEGER*4 READ_OPS(1:*)         ! Addresses of read operands 
        INTEGER*4 WRITE_OPS(1:*)        ! Addresses of write operands 
        INTEGER*4 SIGARGS(1:*)          ! Signal argument list 
        INTEGER*4 SIGNAL_ROUT           ! Signal routine address 
        INTEGER*4 CONTEXT               ! Signal routine context 
        INTEGER*4 USER_ARG              ! User argument value 
        INTEGER*4 ORIG_REGS(0:15)       ! Original registers 
 
 
C+ 
C  Declare and initialize table of class codes for each of the 
C  "real" opcodes.  We'll index into this by the first byte of 
C  one-byte opcodes, the second byte of two-byte opcodes.  The 
C  class codes will be used in a computed GOTO (CASE).  The 
C  codes are: 
C               0 - Unsupported 
C               1 - ADD 
C               2 - SUB 
C               3 - MUL,DIV 
C               4 - ACB 
C               5 - CVT 
C               6 - EMOD 
C 
C  The class mainly determines how we compute the sign of the 
C  result, except for ACB. 
C- 
 
        BYTE INST_CLASS_TABLE(0:255) 
        DATA INST_CLASS_TABLE / 
        1       48*0,                                   ! 00-2F 
        2       0,0,0,5,0,0,0,0,0,0,0,0,0,0,0,0,        ! 30-3F 
        3       1,1,2,2,3,3,3,3,0,0,0,0,0,0,0,4,        ! 40-4F 
        4       0,0,0,0,6,0,0,0,0,0,0,0,0,0,0,0,        ! 50-5F 
        5       1,1,2,2,3,3,3,3,0,0,0,0,0,0,0,4,        ! 60-6F 
        6       0,0,0,0,6,0,5,0,0,0,0,0,0,0,0,0,        ! 70-7F 
        7       112*0,                                  ! 80-EF 
        8       0,0,0,0,0,0,5,5,0,0,0,0,0,0,0,0/        ! F0-FF 
 
C+ 
C  Table of operand sizes in 8-bit bytes, indexed by the 
C  datatype code contained in the OP_TYPES array.  Only floating 
C  types matter. 
C- 
 
        BYTE OP_SIZES(9) /0,0,0,0,0,4,8,8,16/ 
 
        INTEGER*4 LIB$EXTV              ! External function 
        INTEGER*4 RESULT_NEGATIVE       ! -1 if result negative, 
                                        ! 0 if positive 
        INTEGER*4 SIGN1,SIGN2,SIGN3     ! Signs of operands 
        INTEGER*4 INST_BYTE             ! Current opcode byte 
        INTEGER*4 INST_CLASS            ! Class of instruction 
                                        ! from table 
        INTEGER*4 OP_DTYPE              ! Datatype of operand 
        INTEGER*4 OP_SIZE               ! Size of operand in 
                                        ! 8-bit bytes 
        INTEGER*4 RESULT_OP             ! Position of result 
                                        ! in WRITE_OPS array 
        LOGICAL*4 OVERFLOW              ! TRUE if SS$_FLTOVF_F 
        LOGICAL*4 SMALLER               ! Function which 
                                        ! compares operands 
        PARAMETER ESCD = '0FD'X         ! First byte of G,H instructions 
 
        INTEGER*2 SMALL_F(2)            ! Smallest F_floating 
        DATA SMALL_F /'0080'X,0/ 
        INTEGER*2 SMALL_D(4)            ! Smallest D_floating 
        DATA SMALL_D /'0080'X,0,0,0/ 
        INTEGER*2 SMALL_G(4)            ! Smallest G_floating 
 
        DATA SMALL_G /'0010'X,0,0,0/ 
        INTEGER*2 SMALL_H(8)            ! Smallest H_floating 
        DATA SMALL_H /'0001'X,0,0,0,0,0,0,0/ 
        INTEGER*2 BIGGEST(8)            ! Biggest value (all datatypes) 
        DATA BIGGEST /'7FFF'X,7*'FFFF'X/ 
 
        INTEGER*4 SIGNAL_ARRAY(2)       ! Array for signalling new 
                                        ! exception 
C+ 
C 
C    NOTE:  Because the operands arrays contain the locations of 
C           the operands, rather than the operands themselves, 
C           we must call a routine using the %VAL function to 
C           "fool" the called routine into considering the 
C           contents of an operands array element as the address 
C           of an item.  This would not be necessary in a 
C           language that understood the concept of pointer 
C           variables, such as PASCAL. 
C 
C 
C  If FPD is set in the PSL, signal SS$_ROPRAND (reserved operand). In 
C  reality this shouldn't happen since none of the instructions we 
C  handle can set FPD, but do it as an example. 
C- 
 
        IF (BTEST(PSL,PSL$V_FPD)) THEN 
          SIGNAL_ARRAY(1) = 1           ! Count of signal arguments 
          SIGNAL_ARRAY(2) = SS$_ROPRAND ! Error status value 
          CALL SIGNAL_ROUT ( 
        1       1,                      ! Fault flag - signal as fault 
        2       SIGNAL_ARRAY,           ! Signal arguments array 
        3       CONTEXT)                ! Context as passed to us 
                                        ! Call will never return 
          END IF 
 
C+ 
C  Set OVERFLOW according to the exception type.  We assume that 
C  the only alternatives are SS$_FLTOVF_F and SS$_FLTUND_F. 
C- 
 
        OVERFLOW = (SIGARGS(2) .EQ. SS$_FLTOVF_F) 
 
C+ 
C  Determine the datatype of the instruction by that of its 
C  second operand, since that is always the type of the 
C  destination. 
C- 
 
        OP_DTYPE = IBITS(OP_TYPES(2),LIB$V_DCFTYP,LIB$S_DCFTYP) 
 
C+ 
C  Get the size of the datatype in words. 
C- 
 
        OP_SIZE = OP_SIZES (OP_DTYPE) 
 
C+ 
C  Determine the class of instruction and dispatch to the 
C  appropriate routine. 
C- 
 
 
        INST_BYTE = IBITS(OPCODE,0,8)   ! Get first byte 
        IF (INST_BYTE .EQ. ESCD) INST_BYTE = IBITS(OPCODE,8,8) 
        INST_CLASS = INST_CLASS_TABLE(INST_BYTE) 
        GO TO (1000,2000,3000,4000,5000,6000),INST_CLASS 
 
C+ 
C  If we get here, the instruction's entry in the 
C  INST_CLASS_TABLE is zero. This might happen if the instruction was 
C  a POLYx, or was some other unsupported instruction.  Resignal the 
C  original exception. 
C- 
 
        FIXUP_ACTION = SS$_RESIGNAL     ! Resignal condition to next handler 
        RETURN                          ! Return to LIB$DECODE_FAULT 
 
 
C+ 
C  1000 - ADDF2, ADDF3, ADDD2, ADDD3, ADDG2, ADDG3, ADDH2, ADDH3 
C 
C  Result's sign is the same as that of the first operand, 
C  unless this is an underflow, in which case the magnitudes of 
C  the values may change the sign. 
C- 
 
1000    RESULT_NEGATIVE = LIB$EXTV (15,1,%VAL(READ_OPS(1))) 
        IF (.NOT. OVERFLOW) THEN 
          IF (SMALLER(OP_SIZE,%VAL(READ_OPS(1)), 
        1                     %VAL(READ_OPS(2)))) 
        2   RESULT_NEGATIVE = .NOT. RESULT_NEGATIVE 
          END IF 
        GO TO 9000 
 
C+ 
C  2000 - SUBF2, SUBF3, SUBD2, SUBD3, SUBG2, SUBG3, SUBH2, SUBH3 
C 
C  Result's sign is the opposite of that of the first operand, 
C  unless this is an underflow, in which case the magnitudes of 
C  the values may change the sign. 
C- 
 
2000    RESULT_NEGATIVE = .NOT. LIB$EXTV (15,1,%VAL(READ_OPS(1))) 
        IF (.NOT. OVERFLOW) THEN 
          IF (SMALLER(OP_SIZE,%VAL(READ_OPS(1)), 
        1                     %VAL(READ_OPS(2)))) 
        2   RESULT_NEGATIVE = .NOT. RESULT_NEGATIVE 
          END IF 
        GO TO 9000 
 
C+ 
C  3000 - MULF2, MULF3, MULD2, MULD3, MULG2, MULG3, MULH2, MULH3, 
C         DIVF2, DIVF3, DIVD2, DIVD3, DIVG2, DIVG3, DIVH2, DIVH3, 
C 
C  If the signs of the first two operands are the same, then the 
C  result's sign is positive, if they are not it is negative. 
C- 
 
3000    SIGN1 = LIB$EXTV (15,1,%VAL(READ_OPS(1))) 
        SIGN2 = LIB$EXTV (15,1,%VAL(READ_OPS(2))) 
        RESULT_NEGATIVE = SIGN1 .XOR. SIGN2 
 
        GOTO 9000 
 
C+ 
C  4000 - ACBF, ACBD, ACBG, ACBH 
C 
C  The result's sign is the same as that of the second operand 
C  (addend), unless this is underflow, in which case the 
C  magnitudes of the addend and index may change the sign. 
C  We must also determine if the branch is to be taken. 
C- 
 
4000    SIGN2 = LIB$EXTV (15,1,%VAL(READ_OPS(2))) 
        RESULT_NEGATIVE = SIGN2 
        IF (.NOT. OVERFLOW) THEN 
          IF (SMALLER(OP_SIZE,%VAL(READ_OPS(2)), 
        1                     %VAL(READ_OPS(3)))) 
        2   RESULT_NEGATIVE = .NOT. RESULT_NEGATIVE 
          END IF 
 
C+ 
C  If this is overflow, then the branch is not taken, since the 
C  result is always going to be greater or equal in magnitude 
C  to the limit, and will be the correct sign.  If underflow, 
C  the branch is ALMOST always taken.  The only case where the 
C  branch might not be taken is when the result is exactly 
C  equal to the limit.  For this example, we are going to ignore 
C  this exceptional case. 
C- 
 
        IF (.NOT. OVERFLOW) 
        1  REGISTERS(15) = READ_OPS(4)  ! Branch destination 
        GO TO 9000 
 
C+ 
C  5000 - CVTDF, CVTGF, CVTHF, CVTHD, CVTHG 
C 
C  Result's sign is the same as that of the first operand. 
C- 
 
5000    RESULT_NEGATIVE = LIB$EXTV (15,1,%VAL(READ_OPS(1))) 
        GO TO 9000 
 
C+ 
C  6000 - EMODF, EMODD, EMODG, EMODH 
C 
C  If the signs of the first and third operands are the same, then the 
C  result's sign is positive, else it is negative. 
C- 
 
6000    SIGN1 = LIB$EXTV (15,1,%VAL(READ_OPS(1))) 
        SIGN2 = LIB$EXTV (15,1,%VAL(READ_OPS(3))) 
        RESULT_NEGATIVE = SIGN1 .XOR. SIGN2 
        GOTO 9000 
 
C+ 
C  All code paths merge here to store the result value.  We also 
C  set the PSL appropriately.  First, determine which operand is 
C  the result. 
C- 
 
 
9000    RESULT_OP = OP_COUNT 
        IF (INST_CLASS .EQ. 4) 
        1  RESULT_OP = RESULT_OP - 1    ! ACBx 
 
C+ 
C       Select result based on datatype and exception type. 
C- 
 
        IF (OVERFLOW) THEN 
          CALL LIB$MOVC3 (OP_SIZE,BIGGEST,%VAL(WRITE_OPS(RESULT_OP))) 
        ELSE 
          GO TO (9100,9200,9300,9400), OP_DTYPE-(LIB$K_DCFTYP_F-1) 
 
C+ 
C         Should never get here.  Resignal original exception. 
C- 
 
          FIXUP_ACTION = SS$_RESIGNAL 
          RETURN 
 
C+ 
C  9100 - F_floating result 
C- 
 
9100      CALL LIB$MOVC3 (OP_SIZE,SMALL_F,%VAL(WRITE_OPS(RESULT_OP))) 
          GOTO 9500 
 
C+ 
C  9200 - D_floating result 
C- 
 
9200      CALL LIB$MOVC3 (OP_SIZE,SMALL_D,%VAL(WRITE_OPS(RESULT_OP))) 
          GOTO 9500 
 
C+ 
C  9300 - G_floating result 
C- 
 
9300      CALL LIB$MOVC3 (OP_SIZE,SMALL_G,%VAL(WRITE_OPS(RESULT_OP))) 
          GOTO 9500 
 
C+ 
C  9400 - H_floating result 
C- 
 
9400      CALL LIB$MOVC3 (OP_SIZE,SMALL_H,%VAL(WRITE_OPS(RESULT_OP))) 
          GOTO 9500 
 
9500    END IF 
 
C+ 
C  Modify the PSL to reflect the stored result.  If the result was 
C  negative, set the N bit.  Clear the V (overflow) and Z (zero) bits. 
C  If the instruction was an ACBx, leave the C (carry) bit unchanged, 
C  otherwise clear it. 
C- 
 
        IF (RESULT_NEGATIVE) THEN 
          PSL = IBSET (PSL,PSL$V_N)     ! Set N bit 
        ELSE 
 
          PSL = IBCLR (PSL,PSL$V_N)     ! Clear N bit 
        END IF 
        PSL = IBCLR (PSL,PSL$V_V)       ! Clear V bit 
        PSL = IBCLR (PSL,PSL$V_Z)       ! Clear Z bit 
        IF (INST_CLASS .NE. 4) 
        1  PSL = IBCLR (PSL,PSL$V_C)    ! Clear C bit if not ACBx 
 
C+ 
C  Set the sign of result. 
C- 
 
        IF (RESULT_NEGATIVE) 
        1  CALL LIB$INSV (1,15,1,%VAL(WRITE_OPS(RESULT_OP))) 
C+ 
C  Fixup is complete.  Return to LIB$DECODE_FAULT. 
C- 
 
        FIXUP_ACTION = SS$_CONTINUE 
        RETURN 
        END 
 
C+ 
C Function which compares two floating values.  It returns .TRUE. if 
C the first argument is smaller in magnitude than the second. 
C- 
 
        LOGICAL*4 FUNCTION SMALLER(NBYTES,VAL1,VAL2) 
 
        INTEGER*4 NBYTES                ! Number of bytes in values 
        INTEGER*2 VAL1(*),VAL2(*)       ! Floating values to compare 
        INTEGER*4 WORDA,WORDB 
 
        SMALLER = .TRUE.                ! Initially return true 
 
C+ 
C       Zero extend to a longword for unsigned compares. 
C       Compare first word without sign bit. 
C- 
 
        WORDA = IBCLR(ZEXT(VAL1(1)),15) 
        WORDB = IBCLR(ZEXT(VAL2(1)),15) 
        IF (WORDA .LT. WORDB) RETURN 
 
        DO I=2,NBYTES/2 
        WORDA = ZEXT(VAL1(I)) 
        WORDB = ZEXT(VAL2(I)) 
        IF (WORDA .LT. WORDB) RETURN 
        END DO 
 
        SMALLER = .FALSE.       ! VAL1 not smaller than VAL2 
        RETURN 
        END

LIB$DEC_OVER

The Enable or Disable Decimal Overflow Detection routine enables or disables decimal overflow detection for the calling routine activation. The previous decimal overflow setting is returned.

Note
No support for arguments passed by 64-bit address reference or for use of 64-bit descriptors, if applicable, is planned for this routine.

This routine is available on OpenVMS Alpha systems in translated form and is applicable to translated VAX images only.

Format

LIB$DEC_OVER new-setting

RETURNS

OpenVMS usage: longword_unsigned

type: longword integer (unsigned)

access: write only

mechanism: by value

The old decimal overflow enable setting (the previous contents of SF$W_PSW[PSW$V_DV] in the caller's frame).

Argument

new-setting

OpenVMS usage: longword_unsigned

type: longword (unsigned)

access: read only

mechanism: by reference

New decimal overflow enable setting. The new-setting argument is the address of an unsigned longword that contains the new decimal overflow enable setting. Bit 0 set to 1 means enable; bit 0 set to 0 means disable.

Description

The caller's stack frame is modified by this routine.
A call to LIB$DEC_OVER affects only the current routine activation and does not affect any of its callers or any routines that it may call. However, the setting does remain in effect for any routines that are subsequently entered through a JSB entry point.

Example


DECOVF: ROUTINE OPTIONS (MAIN); DECLARE LIB$DEC_OVER ENTRY (FIXED BINARY (7)) /* Address of byte for /* enable/disable /* setting / RETURNS (FIXED BINARY (31)); / Old setting / DECLARE DISABLE FIXED BINARY (7) INITIAL (0) STATIC READONLY; DECLARE RESULT FIXED BINARY (31); DECLARE (A,B) FIXED DECIMAL (4,2); ON FIXEDOVERFLOW PUT SKIP LIST ('Overflow'); RESULT = LIB$DEC_OVER (DISABLE); / Disable recognition of decimal /* overflow in this block / A = 99.99; B = A + 2; PUT SKIP LIST ('In MAIN'); BEGIN; B = A + 2; PUT LIST ('In BEGIN block'); CALL Q; Q: ROUTINE; B = A + 2; PUT LIST ('In Q'); END Q; END / Begin */; END DECOVF;

DECOVF: ROUTINE OPTIONS (MAIN); 
 
DECLARE LIB$DEC_OVER ENTRY (FIXED BINARY (7))   /* Address of byte for 
                                                /*  enable/disable 
                                                /*  setting             */ 
        RETURNS (FIXED BINARY (31));            /* Old setting          */ 
 
DECLARE DISABLE FIXED BINARY (7) INITIAL (0) STATIC READONLY; 
DECLARE RESULT FIXED BINARY (31); 
DECLARE (A,B) FIXED DECIMAL (4,2); 
 
ON FIXEDOVERFLOW PUT SKIP LIST ('Overflow'); 
 
RESULT = LIB$DEC_OVER (DISABLE);        /* Disable recognition of decimal 
                                        /* overflow in this block       */ 
A = 99.99; 
B = A + 2; 
PUT SKIP LIST ('In MAIN'); 
        BEGIN; 
        B = A + 2; 
        PUT LIST ('In BEGIN block'); 
        CALL Q; 
                Q: ROUTINE; 
                B = A + 2; 
                PUT LIST ('In Q'); 
                END Q; 
        END /* Begin */; 
END DECOVF;

This PL/I program shows how to use LIB$DEC_OVER to enable or disable the detection of decimal overflow. Note that in PL/I, disabling decimal overflow using this routine causes the condition to be disabled only in the current block; descendent blocks will enable the condition unless this routine is called in each block.

Contents

Index

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OpenVMS usage:	mask_longword
type:	longword (unsigned)
access:	read only
mechanism:	by reference

OpenVMS usage:	arg_list
type:	longword (unsigned)
access:	read only
mechanism:	by reference, array reference

OpenVMS usage:	longword_unsigned
type:	longword integer (unsigned)
access:	write only
mechanism:	by value