Apple II User Manual

FADD subroutine (address $F46E)

Purpose: To add two numbers in floating point form.

Entry: The two addends are in FP1 and FP2 respectively.  For maximum

precision, both should be normalized.

Uses: SWPALGN, ADD, NORM, RTLOG.

Exit: The normalized sum is left in FP1.  FP2 contains the addend of

greatest magnitude.  E is altered but sign is not.  The A-REG is altered

and the X-REG is cleared.  The sum mantissa is truncated to 24 bits.

Caution: Overflow may result if the sum is less that -2^128 or greater than

+2^128-1.  If so, a jump to location $3F5 is executed leaving 0 in X1, and

twice the proper sum in the mantissa M1.  The sign bit is left in the

carry, 0 for positive, 1 for negative.

      _____    __________

     |     |  |          |

FP1: |  0  |  | X.YYY... |

     |_____|  |__________|

       X1       M1

(For carry=0, true sum=+X.YYY x 2^128)

Example: Prior to calling FADD, FP1 contains +12 and FP2 contains -5.

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

FP1: | $83 |  | $60 |  |  0  |  |  0  |   (+12)

     |_____|  |_____|  |_____|  |_____|

       X1       M1

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

FP2: | $82 |  | $B0 |  |  0  |  |  0  |   (-5)

     |_____|  |_____|  |_____|  |_____|

       X2       M2

After calling FADD, FP1 contains +7 (FP2 contains +12).

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

FP1  | $82 |  | $70 |  |  0  |  |  0  |   (+7)

     |_____|  |_____|  |_____|  |_____|

       X1       M1