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Annotation of 40BSD/libc/gen/ldexp.s, revision 1.1

1.1     ! root        1: #
        !             2: #      double ldexp (value, exp)
        !             3: #              double value;
        !             4: #              int exp;
        !             5: #
        !             6: #      Ldexp returns value*2**exp, if that result is in range.
        !             7: #      If underflow occurs, it returns zero.  If overflow occurs,
        !             8: #      it returns a value of appropriate sign and largest
        !             9: #      possible magnitude.  In case of either overflow or underflow,
        !            10: #      the external int "errno" is set to ERANGE.  Note that errno is
        !            11: #      not modified if no error occurs, so if you intend to test it
        !            12: #      after you use ldexp, you had better set it to something
        !            13: #      other than ERANGE first (zero is a reasonable value to use).
        !            14: #
        !            15: #      Constants
        !            16: #
        !            17:        .set    erange,34       # error number for range error
        !            18: 
        !            19:        .data
        !            20:        .globl  _errno          # error flag
        !            21: 
        !            22: huge:  .word   0x7fff          # The largest number that can
        !            23:        .word   0xffff          #   be represented in a long floating
        !            24:        .word   0xffff          #   number.  This is given in hex in order
        !            25:        .word   0xffff          #   to avoid floating conversions
        !            26: #
        !            27: #      Entry point
        !            28: #
        !            29:        .text
        !            30:        .globl  _ldexp
        !            31: _ldexp:        .word   0x0000          # We use r2, but do not save it
        !            32: 
        !            33:        movd    4(ap),r0        # Fetch "value"
        !            34: 
        !            35:        extzv   $7,$8,r0,r2     # r2 := biased exponent
        !            36:        jeql    ld1             # If it's zero, we're done
        !            37: 
        !            38:        addl2   12(ap),r2       # r2 := new biased exponent
        !            39:        jleq    under           # if it's <= 0, we have an underflow
        !            40:        cmpl    r2,$256         # Otherwise check if it's too big
        !            41:        jgeq    over            # jump if overflow
        !            42: #
        !            43: #      Construct the result and return
        !            44: #
        !            45:        insv    r2,$7,$8,r0     # Put the exponent back in the result
        !            46: ld1:   ret
        !            47: #
        !            48: #      Underflow
        !            49: #
        !            50: under: clrd    r0              # Result is zero
        !            51:        jbr     err             # Join general error code
        !            52: #
        !            53: #      Overflow
        !            54: #
        !            55: over:  movd    huge,r0         # Largest possible floating magnitude
        !            56:        jbc     $15,4(ap),err   # Jump if argument was positive
        !            57:        mnegd   r0,r0           # If arg < 0, make result negative
        !            58: 
        !            59: err:   movl    $erange,_errno  # Indicate range error
        !            60:        ret

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