Apple II User Manual

Exit: The two-byte signed two's complement representation of the integer

portion is left in M1 (high-order byte) and M1+1 (low-order byte).  The

floating point values +24.63 and -61.2 are converted to the integers +24

and -61 respectively.  FP1 and E are altered but FP2, E, SIGN, and SCR are

not.  The A- and X-REGs are altered but the Y-REG is not.

Example: The floating point value +274 is in FP1 prior to calling FIX.

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

FP1: | $88 |  | $44 |  | $80 |  |  0  |   (+274)

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

       X1       M1

After calling FIX, M1 (high-order byte) and M1+1 (low-order byte) contain

the integer representation of +274 ($0112).

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

FP1: | $8E |  | $01 |  | $12 |  |  0  |

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

       X1       M1

Note: FP1 contains an unnormalized representation of +274 upon exit.

NORM Subroutine (address $F463)

Purpose: To normalize the value in FP1, thus insuring maximum precision.

Entry: A normalized or unnormalized value is in FP1.

Exit: The value in FP1 is normalized.  A zero mantissa will exit with X1=0

(2 exponent).  If the exponent on exit is -128 (X1=0) then the mantissa

(M1) is not necessarily normalized (with the two high-order mantissa bits

unequal).  E, FP2, SIGN, AND SCR are not distubed.  The A-REG is disturbed

but the X- and Y-REGs are not.  The carry is set.

Example: FP1 contains +12 in unnormalized form (as .0011 x 2 ).

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

FP1: | $86 |  | $0C |  |  0  |  |  0  |   (+12)

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

       x1       M1

Upon exit from NORM, FP1 contains +12 in normalized form (as 1.1 x 2 ).

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

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