INSTRUCTION DESCRIPTIONS
A - 258 INSTRUCTION SET DETAILS MOTOROLA
Operation: Assembler Syntax:
D+r ➞ D (parallel move) RND D (parallel move)
Description: Round the 56-bit value in the specified destination operand D and store the
result in the MSP portion of the destination accumulator (A1 or B1). This instruction uses
a convergent rounding technique. The contribution of the LS bits of the result (A0 and
B0) is rounded into the upper portion of the result (A1 or B1) by adding a rounding con-
stant to the LS bits of the result. The MSP portion of the destination accumulator con-
tains the rounded result which may be read out to the data buses.
The value of the rounding constant added is determined by the scaling mode bits S0 and
S1 in the system status register (SR). A “1” is added in the rounding position as shown
below:
Normal or “standard’’ rounding consists of adding a rounding constant to a given
number of LS bits of a value to produce a rounded result. The rounding constant
depends on the scaling mode being used as previously shown. Unfortunately, when
using a twos-complement data representation, this process introduces a positive bias in
the statistical distribution of the roundoff error.
RND Round Accumulator RND
Rounding Rounding Constant
S1 S0 Scaling Mode Position 55 - 25 24 23 22 21 - 0
0 0 No Scaling 23 0. . . .0 0 1 0 0. . . .0
0 1 Scale Down 24 0. . . .0 1 0 0 0. . . .0
1 0 Scale Up 22 0. . . .0 0 0 1 0. . . .0