Motorola 24-Bit Digital Signal Processor Signed Multiply and Round, Operation Assembler Syntax

INSTRUCTION DESCRIPTIONS

	MPYR	Signed Multiply and Round		MPYR
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

:

	Before Execution
Y0
	$654321

;square and negate Y0, update R3

	After Execution
Y0
		$654321

B

$00:000000:000000

B

$FF:AFE3ED:000000

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).

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