LD2 (multiple structures)

Load multiple 2-element structures to two registers

This instruction loads multiple 2-element structures from memory and writes the result to the two SIMD&FP registers, with de-interleaving.

For an example of de-interleaving, see LD3 (multiple structures).

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: No offset

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		0	0	1	1	0	0	0	1	0	0	0	0	0	0	1	0	0	0
	Q								L							opcode				size		Rn					Rt

LD2 { <Vt>.<T>, <Vt2>.<T> }, [<Xn|SP>]

Decoding algorithm

if !IsFeatureImplemented(FEAT_AdvSIMD) then EndOfDecode(Decode_UNDEF);
constant integer t = UInt(Rt);
constant integer n = UInt(Rn);
constant integer m = integer UNKNOWN;
constant boolean wback = FALSE;
constant boolean nontemporal = FALSE;
constant boolean tagchecked = wback || n != 31;

Encoding: Post-index

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		0	0	1	1	0	0	1	1	0						1	0	0	0
	Q								L		Rm					opcode				size		Rn					Rt

Immediate offset (Rm == 11111)

LD2 { <Vt>.<T>, <Vt2>.<T> }, [<Xn|SP>], <imm>

Register offset (Rm != 11111)

LD2 { <Vt>.<T>, <Vt2>.<T> }, [<Xn|SP>], <Xm>

Decoding algorithm

if !IsFeatureImplemented(FEAT_AdvSIMD) then EndOfDecode(Decode_UNDEF);
constant integer t = UInt(Rt);
constant integer n = UInt(Rn);
constant integer m = UInt(Rm);
constant boolean wback = TRUE;
constant boolean nontemporal = FALSE;
constant boolean tagchecked = wback || n != 31;

Operation

constant integer datasize = 64 << UInt(Q);
constant integer esize = 8 << UInt(size);
constant integer elements = datasize DIV esize;

constant integer rpt = 1;
constant integer selem = 2;

// .1D format only permitted with LD1 & ST1
if size:Q == '110' && selem != 1 then EndOfDecode(Decode_UNDEF);

CheckFPAdvSIMDEnabled64();

bits(64) address;
bits(64) eaddr;
bits(64) offs;
bits(datasize) rval;
integer tt;
constant integer ebytes = esize DIV 8;

constant boolean privileged = PSTATE.EL != EL0;
constant AccessDescriptor accdesc = CreateAccDescASIMD(MemOp_LOAD, nontemporal, tagchecked,
                                                       privileged);
if n == 31 then
    CheckSPAlignment();
    address = SP[64];
else
    address = X[n, 64];

offs = Zeros(64);
for r = 0 to rpt-1
    for e = 0 to elements-1
        tt = (t + r) MOD 32;
        for s = 0 to selem-1
            rval = V[tt, datasize];
            eaddr = AddressIncrement(address, offs, accdesc);
            Elem[rval, e, esize] = Mem[eaddr, ebytes, accdesc];
            V[tt, datasize] = rval;
            offs = offs + ebytes;
            tt = (tt + 1) MOD 32;
if wback then
    if m != 31 then
        offs = X[m, 64];
    address = AddressAdd(address, offs, accdesc);
    if n == 31 then
        SP[64] = address;
    else
        X[n, 64] = address;

Explanations

<Vt>: Is the name of the first or only SIMD&FP register to be transferred, encoded in the "Rt" field.
<T>: <Vt2>: Is the name of the second SIMD&FP register to be transferred, encoded as "Rt" plus 1 modulo 32.
<Xn|SP>: Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the "Rn" field.
<imm>: <Xm>: Is the 64-bit name of the general-purpose post-index register, excluding XZR, encoded in the "Rm" 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.