Apple II User Manual

exit used in the event of a non-positive log argument.  OVFLW

(1E3B) is the error exit for overflow occuring during calculation

of e to some power.  OVFL (1FE4) is the error exit for

overflow in all of the floating point routines.  There is no

trap for underflow; in such cases, the result is set to 0.0.

All routines are called and exited in a uniform manner:

The arguments(s) are placed in the specified floating point

storage locations (for specifics, see the documentation preceeding

each routine in the listing), then a JSR is used to

enter the desired routine.  Upon normal completion, the

called routine is exited via a subroutine return instruction (RTS).

Note:  The preceeding documentation was written by the Editor, based

on phone conversations with Roy and studying the listing.  There is a

high probability that it is correct.  However, since it was not written

nor reviewed by the authors of these routines, the preceeding

documentation may contain errors in concept or in detail.

                                                          -- JCW, Jr.

    In the Exponent:

    00 Represents -128

          ...

    7F Represents -1

    80 Represents 0

    81 Represents +1

          ...

    FF Represents +127

  Exponent    Two's Complement Mantissa

  SEEEEEEE  SM.MMMMMM  MMMMMMMM  MMMMMMMM

     n         n+1       n+2       n+3

                *           JULY 5, 1976
                *     BASIC FLOATING POINT ROUTINES

                *       FOR 6502 MICROPROCESSOR
                *       BY R. RANKIN AND S. WOZNIAK

                *
                *     CONSISTING OF:

                *        NATURAL LOG
                *        COMMON LOG

                *        EXPONENTIAL (E**X)
                *        FLOAT      FIX

                *        FADD       FSUB
                *        FMUL       FDIV

                *
                *

                *     FLOATING POINT REPRESENTATION (4-BYTES)
                *                    EXPONENT BYTE 1

                *                    MANTISSA BYTES 2-4
                *

                *     MANTISSA:    TWO'S COMPLIMENT REPRESENTATION WITH SIGN IN
                *       MSB OF HIGH-ORDER BYTE.  MANTISSA IS NORMALIZED WITH AN

                *       ASSUMED DECIMAL POINT BETWEEN BITS 5 AND 6 OF THE HIGH-ORDER
                *       BYTE.  THUS THE MANTISSA IS IN THE RANGE 1. TO 2. EXCEPT