INSTRUCTION DESCRIPTIONS
A - 232 INSTRUCTION SET DETAILS
MOTOROLA
Operation: Assembler Syntax:
±
S1
∗
S2+r
➞
D (parallel move) MPYR (
±
)S1,S2,D (parallel move)
±
S1
∗
S2+r
➞
D (parallel move) MPYR (
±
)S2,S1,D (parallel move)
±
(S1
∗
2
-n
)+r
➞
D (
no
parallel move) MPYR (
±
)S,#n,D (
no
parallel move)
Description:
Multiply the two signed 24-bit source operands S1 and S2 (
or
the signed
24-bit source operand S by the positive 24-bit immediate operand 2
-n
), round the result
using convergent rounding, and store it in the specified 56-bit destination accumulator D.
The “–” sign option is used to negate the product prior to rounding. The default sign
option is “+”. The contribution of the LS bits of the result is rounded into the upper portion
of the destination accumulator (A1 or B1) by adding a constant to the LS bits of the lower
portion of the accumulator (A0 or B0). The value of the constant added is determined by
the scaling mode bits S0 and S1 in the status register. Once the rounding has been com-
pleted, the LS bits of the destination accumulator D (A0 or B0) are loaded with zeros to
maintain an unbiased accumulator value which may be reused by the next instruction.
The upper portion of the accumulator (A1 or B1) contains the rounded result which may
be read out to the data buses. Refer to the RND instruction for more complete informa-
tion on the convergent rounding process.
Example 1:
:
MPYR –Y0,Y0,B (R3)–N3 ;square and negate Y0, update R3
:
Explanation of Example 1:
Prior to execution, the 24-bit Y0 register contains the value
$654321 (0.791111112), and the 56-bit B accumulator contains the value
$00:000000:000000 (0.0). The execution of the MPYR –Y0,Y0,B instruction squares the
24-bit signed value in the Y0 register, negates the resulting 48-bit product, rounds the
result into B1, and zeros B0 (–Y0
∗
Y0=–0.625856790961748 approximately=
$FF:AFE3EC:B76B7E, which is rounded to the value $FF:AFE3ED:000000=
–0.625856757164002=B).
MPYR Signed Multiply and Round MPYR
Before Execution After Execution
Y0 $654321
B B
$00:000000:000000 $FF:AFE3ED:000000
$654321
Y0