STRB (immediate)

Store register byte (immediate)

This instruction stores the least significant byte of a 32-bit register to memory. The address that is used for the store is calculated from a base register and an immediate offset. For information about addressing modes, see Load/Store addressing modes.

Encoding: 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
0	0	1	1	1	0	0	0	0	0	0										0	1
size					VR			opc			imm9											Rn					Rt

STRB <Wt>, [<Xn|SP>], #<simm>

Decoding algorithm

constant boolean wback = TRUE;
constant boolean postindex = TRUE;
constant bits(64) offset = SignExtend(imm9, 64);

Encoding: 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
0	0	1	1	1	0	0	0	0	0	0										1	1
size					VR			opc			imm9											Rn					Rt

STRB <Wt>, [<Xn|SP>, #<simm>]!

Decoding algorithm

constant boolean wback = TRUE;
constant boolean postindex = FALSE;
constant bits(64) offset = SignExtend(imm9, 64);

Encoding: Unsigned 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
0	0	1	1	1	0	0	1	0	0
size					VR			opc		imm12												Rn					Rt

STRB <Wt>, [<Xn|SP>{, #<pimm>}]

Decoding algorithm

constant boolean wback = FALSE;
constant boolean postindex = FALSE;
constant bits(64) offset = LSL(ZeroExtend(imm12, 64), 0);

Operation

constant integer t = UInt(Rt);
constant integer n = UInt(Rn);
constant boolean nontemporal = FALSE;
constant boolean tagchecked = wback || n != 31;

Constraint c;
boolean rt_unknown = FALSE;
if wback && n == t && n != 31 then
    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 original value
        when Constraint_UNKNOWN rt_unknown = TRUE;     // Value stored is UNKNOWN
        when Constraint_UNDEF   EndOfDecode(Decode_UNDEF);
        when Constraint_NOP     EndOfDecode(Decode_NOP);

bits(64) address;

constant boolean privileged = PSTATE.EL != EL0;
constant AccessDescriptor accdesc = CreateAccDescGPR(MemOp_STORE, nontemporal, privileged,
                                                     tagchecked);

if n == 31 then
    CheckSPAlignment();
    address = SP[64];
else
    address = X[n, 64];

if !postindex then
    address = AddressAdd(address, offset, accdesc);

bits(8) data;
if rt_unknown then
    data = bits(8) UNKNOWN;
else
    data = X[t, 8];

Mem[address, 1, accdesc] = data;

if wback then
    if postindex then
        address = AddressAdd(address, offset, accdesc);
    if n == 31 then
        SP[64] = address;
    else
        X[n, 64] = address;

Explanations

<Wt>: Is the 32-bit name of the general-purpose register to be transferred, encoded in the "Rt" field.
<Xn|SP>: Is the 64-bit name of the general-purpose base register or stack pointer, encoded in the "Rn" field.
<simm>: Is the signed immediate byte offset, in the range -256 to 255, encoded in the "imm9" field.
<pimm>: Is the optional positive immediate byte offset, in the range 0 to 4095, defaulting to 0 and encoded in the "imm12" 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.