LD2B (scalar plus immediate)

Contiguous load two-byte structures to two vectors (immediate index)

Contiguous load two-byte structures, each to the same element number in two vector registers from the memory address generated by a 64-bit scalar base and an immediate index which is a multiple of 2 in the range -16 to 14 that is multiplied by the vector's in-memory size, irrespective of predication,

Each predicate element applies to the same element number in each of the two vector registers, or equivalently to the two consecutive bytes in memory which make up each structure. Inactive elements will not cause a read from Device memory or signal a fault, and the corresponding element is set to zero in each of the two destination vector registers.

Encoding: SVE

Variants: FEAT_SVE || FEAT_SME (FEAT_SVE || FEAT_SME)

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
1	0	1	0	0	1	0	0	0	0	1	0					1	1	1
							msz		opc			imm4							Pg			Rn					Zt

LD2B { <Zt1>.B, <Zt2>.B }, <Pg>/Z, [<Xn|SP>{, #<imm>, MUL VL}]

Decoding algorithm

if !IsFeatureImplemented(FEAT_SVE) && !IsFeatureImplemented(FEAT_SME) then
    EndOfDecode(Decode_UNDEF);
constant integer t = UInt(Zt);
constant integer n = UInt(Rn);
constant integer g = UInt(Pg);
constant integer esize = 8;
constant integer offset = SInt(imm4);
constant integer nreg = 2;

Operation

CheckSVEEnabled();
constant integer VL = CurrentVL;
constant integer PL = VL DIV 8;
constant integer elements = VL DIV esize;
bits(64) base;
constant bits(PL) mask = P[g, PL];
bits(64) addr;
constant integer mbytes = esize DIV 8;
array [0..1] of bits(VL) values;
constant boolean contiguous = TRUE;
constant boolean nontemporal = FALSE;
constant boolean tagchecked = n != 31;
constant AccessDescriptor accdesc = CreateAccDescSVE(MemOp_LOAD, nontemporal, contiguous,
                                                     tagchecked);

if !AnyActiveElement(mask, esize) then
    if n == 31 && ConstrainUnpredictableBool(Unpredictable_CHECKSPNONEACTIVE) then
        CheckSPAlignment();
else
    if n == 31 then CheckSPAlignment();

base = if n == 31 then SP[64] else X[n, 64];
addr = AddressAdd(base, offset * elements * nreg * mbytes, accdesc);

for e = 0 to elements-1
    for r = 0 to nreg-1
        if ActivePredicateElement(mask, e, esize) then
            Elem[values[r], e, esize] = Mem[addr, mbytes, accdesc];
        else
            Elem[values[r], e, esize] = Zeros(esize);
        addr = AddressIncrement(addr, mbytes, accdesc);

for r = 0 to nreg-1
    Z[(t+r) MOD 32, VL] = values[r];

Explanations

<Zt1>: Is the name of the first scalable vector register to be transferred, encoded in the "Zt" field.
<Zt2>: Is the name of the second scalable vector register to be transferred, encoded as "Zt" plus 1 modulo 32.
<Pg>: Is the name of the governing scalable predicate register P0-P7, encoded in the "Pg" field.
<Xn|SP>: Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the "Rn" field.
<imm>: Is the optional signed immediate vector offset, a multiple of 2 in the range -16 to 14, defaulting to 0, encoded in the "imm4" field.

Operational Notes

If PSTATE.DIT is 1, the timing of this instruction is insensitive to the value of the data being loaded or stored when its governing predicate register contains the same value for each execution.