Encoding:

P32A

001000

rt

shift

ac

11

111

001

111

111

6

5

5

2

2

3

3

3

3

Format: 

EXTR_S.H rt, ac, shift

DSP

Extract Halfword Value From Accumulator to GPR With Right Shift and Saturate

Purpose:

Extract Halfword Value From Accumulator to GPR With Right Shift and Saturate

Extract a halfword value from a 64-bit accumulator to a GPR with right shift and saturation.

Description: 

rt = sat16(ac >> shift)

The value in the 64-bit accumulator ac is shifted right by shift bits with sign extension (arithmetic shift right). The 64bit value is then saturated to 16-bits, sign extended to 32 bits, and written to the destination register rt. The shift argument is provided in the instruction.

The value of ac can range from 0 to 3. When ac=0, this refers to the original HI/LO register pair of the MIPS32 architecture. After the execution of this instruction, ac remains unmodified.

This instruction sets bit 23 of the DSPControl register in the ouflag field if the operation results in saturation.

Restrictions:

No data-dependent exceptions are possible.

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

Operation: 

ValidateAccessToDSPResources()
temp63..0 = shiftShortAccRightArithmetic( ac, shift )
if ( temp63..0 > 0x0000000000007FFF ) then
   temp31..0 = 0x00007FFF
   DSPControlouflag:23 = 1
else if ( temp63..0 < 0xFFFFFFFFFFFF8000 ) then
   temp31..0 = 0xFFFF8000
   DSPControlouflag:23 = 1
endif
GPR[rt]31..0 = temp31..0
function shiftShortAccRightArithmetic( ac1..0, shift4..0 )
   sign = HI[ac]31
   if ( shift = 0 ) then
      temp63..0 = HI[ac]31..0 || LO[ac]31..0
   else
      temp63..0 = signshift || (( HI[ac]31..0 || LO[ac]31..0 ) >> shift )
   endif
   if ( sign != temp31 ) then
      DSPControlouflag:23 = 1
   endif
   return temp63..0
endfunction shiftShortAccRightArithmetic

Exceptions:

Reserved Instruction, DSP Disabled