Encoding:

Format:

MULQ_S.W rd, rs, rt

Purpose:

Multiply Fractional Words to Same Size Product with Saturation

Multiply two Q31 fractional format word values to create a fractional Q31 result, with saturation.

Description:

rd = sat32(rs31..0 * rt31..0)

The Q31 fractional format words in registers rs and rt are multiplied together to create a 64-bit fractional format intermediate result. The intermediate result is left-shifted by one bit position, and then truncated by discarding the 32 least-significant bits to create a Q31 fractional format result. This result is then written to destination register rd.

If both input multiplicands are equal to -1 (0x80000000 hexadecimal), the product is clipped to the maximum positive

Q31 fractional format value (0x7FFFFFFF hexadecimal), and written to the destination register.

To stay compliant with the base architecture, this instruction leaves the base HI/LO pair (accumulator ac0) UNPREDICTABLE after the operation completes. The other DSP Module accumulators, ac1, ac2, and ac3, are unchanged.

This instruction, on an overflow or underflow of the operation, writes a 1 to bit 21 in the DSPControl register in the

ouflag field.

Restrictions:

No data-dependent exceptions are possible.

The operands must be a value in the specified format. If they are not, the results are UNPREDICTABLE and the values of the operand vectors become UNPREDICTABLE.

Operation:

ValidateAccessToDSP2Resources()
if ( GPR[rs]31..0 = 0x80000000 ) and ( GPR[rt]31..0 = 0x80000000 ) then
   temp63..0 = 0x7FFFFFFF00000000
   DSPControlouflag:21 = 1
else 
   temp63..0 = ( GPR[rs]31..0 * GPR[rt]31..0 ) << 1
endif
GPR[rd]31..0 = temp63..32
HI[0]31..0 = UNPREDICTABLE
LO[0]31..0 = UNPREDICTABLE

Exceptions:

Reserved Instruction, DSP Disabled

Programming Notes:

The base MIPS32 architecture states that after a GPR-targeting multiply instruction such as MUL, the contents of registers HI and LO are UNPREDICTABLE. To maintain compliance with the base architecture, this multiply instruction, MULQ_S.W, has the same requirement. Software must save and restore the ac0 register if the previous value in the ac0 register is needed following the MULQ_S.W instruction.

Note that the requirement on HI and LO does not apply to the new accumulator registers ac1, ac2, and ac3; as a result, the values in these accumulators need not be saved.