Apple II User Manual

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

       X1       M1

NORM1 subroutine (address $F455)

Purpose: To normalize a floating point value in FP1 when it is known the

exponent is not -128 (X1=0) upon entry.

Entry: An unnormalized number is in FP1.  The exponent byte should not be 0

for normal use.

Exit: The normalized value is in FP1.  E, FP2, SIGN, and SCR are not not

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

ADD Subroutine (address $F425)

Purpose: To add the two mantissas (M1 and M2) as 3-byte integers.

Entry: Two mantissas are in M1 (through M1+2) and M2 (through M2+2).  They

should be aligned, that is with identical exponents, for use in the FADD

and FSUB subroutines.

Exit: the 24-bit integer sum is in M1 (high-order byte in M1, low-order

byte in M1+2).  FP2, X1, E, SIGN and SCR are not disturbed.  The A-REG

contains the high-order byte of the sum, the X-REG contains $FF and the

Y-REG is not altered.  The carry is the '25th' sum bit.

Example: FP1 contains +5 and FP2 contains +7 prior to calling ADD.

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

FP1: | $82 |  | $50 |  |  0  |  |  0  |   (+5)

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

       X1       M1

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

FP2: | $82 |  | $70 |  |  0  |  |  0  |   (+7)

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

Upon exit, M1 contains the overflow value for +12.  Note that the sign bit

is incorrect.  This is taken care of with a call to the right shift

routine.

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

FP:  | $82 |  | $C0 |  |  0  |  |  0  |   (+12)

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