Apple II User Manual

Note: M1 sign bit is unchanged.

RTLOG subroutine (address $F480)

Purpose: To shift the 6-byte field MANT1 and E one bit to the right (toward

the least significant bit).  The 6502 carry bit is shifted into the

high-order M1 bit.  This is useful in correcting binary sum overflows.

Entry: A normalized or unnormalized floating point value is in FP1.  The

carry must be cleared or set by the user since it is shifted Into the sign

bit of M1.

Exit: Same as RTAR except that the sign of M1 is not preserved (it is set

to the value of the carry bit on entry)

Caution: Same as RTAR.

Example: Prior to calling RTLOG, FP1 contains the normalized value -12 and

the carry is clear.

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

FP1: | $83 |  | $A0 |  |  0  |  |  0  |   (-12)

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

       X1       M1

After calling RTLOG, M1 is shifted one bit to the right and the sign bit is

clear.  X1 is incremented by 1.

      _____    _____    _____    _____

     |     |  |     |  |     |  |     |

FP1: | $84 |  | $50 |  |  0  |  |  0  |   (+20)

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

       X1       M1

Note: The bit shifted off the end of MANT1 is rotated into the high-order

bit of the 3-byte extension E.  The 3-byte E field is also shifted one bit

to the right.

RTLOG1 subroutine (address $F484)

Purpose: To shift MANT1 and E right one bit without adjusting X1.  This is

used by the multiply loop.  The carry is shifted into the sign bit of

MANT1.

Entry: M1 and E contain a 6-byte unsigned field.  E is the 3-byte low-order

extension of MANT1.

Exit: Same as RTLOG except that X1 is not altered and an overflow exit

cannot occur.