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Annotation of 41BSD/libc/gen/ldexp.s, revision 1.1.1.1

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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