[an error occurred while processing this directive]

HP OpenVMS Systems Documentation

Content starts here

HP OpenVMS RTL Library (LIB$) Manual


Previous Contents Index

For 2-byte opcodes, the escape code (for example, hex FD) is in the low-order byte. You must use this argument to examine the opcode instead of reading the bytes pointed to by instr-PC. This is because if a debugger breakpoint has been set on the instruction, only opcode contains the original instruction.

instr-PC


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

Value of the PC for the instruction that caused the fault. The instr-PC argument is the address of a longword that contains the PC value.

Note the difference between this value and the contents of the registers array element that corresponds to the PC. R15 of the registers array element contains the address of the byte after the instruction that caused the fault.

PSL


OpenVMS usage: longword_unsigned
type: longword (unsigned)
access: modify
mechanism: by reference

Processor status longword (PSL) at the time of the fault. The PSL argument is the address of a longword that contains this PSL. Your user action routine may modify this PSL within the restrictions of the VAX architecture.

registers


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

Contents of registers R0 through R15 (PC) at the time of the fault but after operand addressing-mode processing. This includes any autoincrements or autodecrements. The registers argument is the address of this 16-longword array. Each longword of the registers array contains the contents of one register.

Your user action routine may modify these values. If it does, the new values will be reflected when instruction execution continues.

To modify vector registers, execute a vector instruction. Executing a vector instruction in the handler modifies the state of the vector processor. The state of the vector processor is not restored when the handler returns. This has the effect of altering the state when the execution continues.

R15 denotes the sixteenth longword in the registers array, which corresponds to the PC. R15 contains the address of the next byte after the current instruction. Unless this value is modified by your user action routine, instruction execution will resume at that address. An exception is for the CASEB, CASEW, and CASEL instructions; R15 contains the address of the first displacement word. For these instructions, your user action routine must modify R15 to point to the next instruction to execute.

Upon instruction completion, registers R0-R15 are restored from this array. However, if signal-procedure is used to cause a fault or if instruction restart is specified by returning LIB$_RESTART, original-registers is used instead.

operand-count


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

Number of operands in the instruction currently being decoded. The operand-count is the address of a longword that contains this number.

operand-types


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

Array of longwords, one element for each operand, that contains the type codes for the associated operand. The operand-types argument is the address of this array.

The operand type codes are further defined in the section called Instruction Operand Definition Codes.

read-operand-locations


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

Array of longwords, one element for each operand, that contains the addresses of the operands to be read. The read-operand-locations argument is the address of this array.

The address given in the array may not be the actual address of the operand if the operand is not a memory location. If the operand is a register, the address indicates a copy of the register values at the time of operand evaluation. If the operand access type is ADDRESS or FIELD and the operand is not a register, the address is the address of the item. If the operand access type is FIELD and the operand is a register, the address refers to the appropriate element in the registers array. If the operand access type is BRANCH, the address is the destination PC of the branch. For WRITE access operands, the address value is zero.

write-operand-locations


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

Array of longwords, one element for each operand, that contains the addresses of operands that are to be written. The write-operand-locations argument is the address of this array. If the operand access type is not MODIFY, WRITE, ADDRESS, or FIELD, the pointer value is zero.

signal-arguments


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

Signal arguments list of the original exception, as passed from OpenVMS to your condition handler and then to LIB$DECODE_FAULT. The signal-arguments argument is the address of an array of longwords that contains these signal arguments.

signal-procedure


OpenVMS usage: procedure
type: procedure value
access: call without stack unwinding
mechanism: by reference

Entry mask of a routine that your user action routine must call if it wants to report an exception for the instruction that faulted. The signal-procedure argument is the address of this entry mask.

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.

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-

        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

      


Previous Next Contents Index