Texas Instruments manual ±41. MSP50P614/MSP50C614 Computational Modes

MSP50P614/MSP50C614 Computational Modes

Table 4±41. MSP50P614/MSP50C614 Computational Modes

Computational	Setting	Resetting	Function
Mode	Instruction	Instruction
Sign extension	SXM	RXM	STAT.XM = 1 produces sign extension on data as it is
			passed into accumulators. This mode copies the 16th bit of
			the data in the multiplier/multiplicand to the 17th bit. This
			causes signed multiplication of two signed numbers.
			STAT.XM = 0 suppresses sign extension.
Unsigned	none	none	STAT.UM = 1 causes unsigned multiplication where the mul-
			tiplier assumes its arguments as unsigned value. MOVU
			instruction can be used to enable this mode. STAT.UM = 0
			disables unsigned multiplication.
Overflow	SOVM	ROVM	STAT.OM = 1 initiates overflow mode. Overflows cause the
			accumulator to acquired the most positive or most negative
			value. In the case of string values, only the MSB 16 bits are
			modified. The remaining bits in the string are unchanged.
			STAT.OM = 0 normal overflow operation and the
			accumulator content is unchanged if any overflow occurs.
			Affects OF bit of STAT in case of overflow.
Fractional	SFM	RFM	STAT.FM = 1 enables fractional multiplication shift mode.
			The multiplier is shifted left 1 bit to produce a 17 bit operand.
			This mode is used on signed binary fractions and does not
			require the user to left shift as it would have been required if
			the FM bit was not set. STAT.FM = 1 turns off fractional
			mode.

Sign Extension Mode: Sign extension mode can be enabled/disabled by setting/resetting the XM bit of STAT. When in sign extension mode, a multiply operation will copy the 16th bit of the multiplier/multiplicand to the 17th bit. When multiplied, this will give a 17 x 17 bit multiplication producing 34 bit result where the upper two bits (33rd and 34th bits) are the sign bits and discarded by the processor. Sign extension is also applicable in string mode. Sign extension mode is the recommended mode to use for signed number multiplication.

Example 4.6.1	SXM
	MOV A0,		0x8000
	MOV	MR,	0x8000

MULTPL A0, A0

This example illustrates the sign extension mode during multiplication. Here, two negative number 0x8000 are multiplied with 0x8000 to obtain a positive number 0x40000000. If the signs were not extended, we would have obtained 0xC0000000, a negative number.

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