Arm® A64 Instruction Set Architecture: Future Architecture Technologies in the A architecture profile | STP: Store Pair of Registers.

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STP

Store Pair of Registers calculates an address from a base register value and an immediate offset, and stores two 32-bit words or two 64-bit doublewords to the calculated address, from two registers. For information about memory accesses, see Load/Store addressing modes.

It has encodings from 3 classes: Post-index , Pre-index and Signed offset

Post-index

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
x	0	1	0	1	0	0	0	1	0	imm7							Rt2					Rn					Rt
opc									L

32-bit (opc == 00)

STP <Wt1>, <Wt2>, [<Xn|SP>], #<imm>

64-bit (opc == 10)

STP <Xt1>, <Xt2>, [<Xn|SP>], #<imm>

boolean wback = TRUE;
boolean postindex = TRUE;

Pre-index

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
x	0	1	0	1	0	0	1	1	0	imm7							Rt2					Rn					Rt
opc									L

32-bit (opc == 00)

STP <Wt1>, <Wt2>, [<Xn|SP>, #<imm>]!

64-bit (opc == 10)

STP <Xt1>, <Xt2>, [<Xn|SP>, #<imm>]!

boolean wback = TRUE;
boolean postindex = FALSE;

Signed offset

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
x	0	1	0	1	0	0	1	0	0	imm7							Rt2					Rn					Rt
opc									L

32-bit (opc == 00)

STP <Wt1>, <Wt2>, [<Xn|SP>{, #<imm>}]

64-bit (opc == 10)

STP <Xt1>, <Xt2>, [<Xn|SP>{, #<imm>}]

boolean wback = FALSE;
boolean postindex = FALSE;

For information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors, and particularly STP.

Assembler Symbols

<Wt1>

Is the 32-bit name of the first general-purpose register to be transferred, encoded in the "Rt" field.

<Wt2>

Is the 32-bit name of the second general-purpose register to be transferred, encoded in the "Rt2" field.

<Xt1>

Is the 64-bit name of the first general-purpose register to be transferred, encoded in the "Rt" field.

<Xt2>

Is the 64-bit name of the second general-purpose register to be transferred, encoded in the "Rt2" field.

<Xn|SP>

Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the "Rn" field.

<imm>	For the 32-bit post-index and 32-bit pre-index variant: is the signed immediate byte offset, a multiple of 4 in the range -256 to 252, encoded in the "imm7" field as <imm>/4.
	For the 32-bit signed offset variant: is the optional signed immediate byte offset, a multiple of 4 in the range -256 to 252, defaulting to 0 and encoded in the "imm7" field as <imm>/4.
	For the 64-bit post-index and 64-bit pre-index variant: is the signed immediate byte offset, a multiple of 8 in the range -512 to 504, encoded in the "imm7" field as <imm>/8.
	For the 64-bit signed offset variant: is the optional signed immediate byte offset, a multiple of 8 in the range -512 to 504, defaulting to 0 and encoded in the "imm7" field as <imm>/8.

Shared Decode

integer n = UInt(Rn);
integer t = UInt(Rt);
integer t2 = UInt(Rt2);
if L:opc<0> == '01' || opc == '11' then UNDEFINED;
integer scale = 2 + UInt(opc<1>);
integer datasize = 8 << scale;
bits(64) offset = LSL(SignExtend(imm7, 64), scale);
boolean tag_checked = wback || n != 31;

boolean rt_unknown = FALSE;

if wback && (t == n || t2 == n) && n != 31 then
    Constraint c = ConstrainUnpredictable(Unpredictable_WBOVERLAPST);
    assert c IN {Constraint_NONE, Constraint_UNKNOWN, Constraint_UNDEF, Constraint_NOP};
    case c of
        when Constraint_NONE    rt_unknown = FALSE;    // value stored is pre-writeback
        when Constraint_UNKNOWN rt_unknown = TRUE;    // value stored is UNKNOWN
        when Constraint_UNDEF   UNDEFINED;
        when Constraint_NOP     EndOfInstruction();

Operation

bits(64) address;
bits(datasize) data1;
bits(datasize) data2;
constant integer dbytes = datasize DIV 8;

if HaveMTEExt() then
    SetTagCheckedInstruction(tag_checked);

if n == 31 then
    CheckSPAlignment();
    address = SP[];
else
    address = X[n];

if !postindex then
    address = address + offset;

if rt_unknown && t == n then
    data1 = bits(datasize) UNKNOWN;
else
    data1 = X[t];
if rt_unknown && t2 == n then
    data2 = bits(datasize) UNKNOWN;
else
    data2 = X[t2];
Mem[address, dbytes, AccType_NORMAL] = data1;
Mem[address+dbytes, dbytes, AccType_NORMAL] = data2;

if wback then
    if postindex then
        address = address + offset;
    if n == 31 then
        SP[] = address;
    else
        X[n] = address;

Operational information

If PSTATE.DIT is 1, the timing of this instruction is insensitive to the value of the data being loaded or stored.

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