URSQRTE

Unsigned reciprocal square root estimate

This instruction reads each vector element from the source SIMD&FP register, calculates an approximate inverse square root for each value, places the result into a vector, and writes the vector to the destination SIMD&FP register. All the values in this instruction are unsigned integer values.

Depending on the settings in the CPACR_EL1, CPTR_EL2, and CPTR_EL3 registers, and the current Security state and Exception level, an attempt to execute the instruction might be trapped.

Encoding: Vector

Variants: FEAT_AdvSIMD (ARMv8.0)

31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0		1	0	1	1	1	0	1		1	0	0	0	0	1	1	1	0	0	1	0
	Q	U							sz						opcode							Rn					Rd

URSQRTE <Vd>.<T>, <Vn>.<T>

Decoding algorithm

if !IsFeatureImplemented(FEAT_AdvSIMD) then EndOfDecode(Decode_UNDEF);
constant integer d = UInt(Rd);
constant integer n = UInt(Rn);

if sz == '1' then EndOfDecode(Decode_UNDEF);
constant integer esize = 32;
constant integer datasize = 64 << UInt(Q);
constant integer elements = datasize DIV esize;

Operation

CheckFPAdvSIMDEnabled64();
constant bits(datasize) operand = V[n, datasize];
bits(datasize) result;
bits(32) element;

for e = 0 to elements-1
    element = Elem[operand, e, 32];
    Elem[result, e, 32] = UnsignedRSqrtEstimate(element);

V[d, datasize] = result;

Explanations

<Vd>: Is the name of the SIMD&FP destination register, encoded in the "Rd" field.
<T>: <Vn>: Is the name of the SIMD&FP source register, encoded in the "Rn" field.