Apple II 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.