STRB (register)

Store register byte (register)

This instruction calculates an address from a base register value and an offset register value, and stores a byte from a 32-bit register to the calculated address. For information about addressing modes, see Load/Store addressing modes.

The instruction uses an offset addressing mode, that calculates the address used for the memory access from a base register value and an offset register value. The offset can be optionally shifted and extended.

Encoding: 32-bit

313029282726252423222120191817161514131211109876543210
0011100000110
sizeVRopcRmoptionSRnRt

Extended register (option != 011)

STRB <Wt>, [<Xn|SP>, (<Wm>|<Xm>), <extend> {<amount>}]

Shifted register (option == 011)

STRB <Wt>, [<Xn|SP>, <Xm>{, LSL <amount>}]

Decoding algorithm

if option<1> == '0' then EndOfDecode(Decode_UNDEF);             // sub-word index
constant ExtendType extend_type = DecodeRegExtend(option);
constant integer shift = 0;

Operation

constant integer t = UInt(Rt);
constant integer n = UInt(Rn);
constant integer m = UInt(Rm);
constant boolean nontemporal = FALSE;
constant boolean tagchecked = TRUE;
constant bits(64) offset = ExtendReg(m, extend_type, shift, 64);
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];

address = AddressAdd(address, offset, accdesc);

Mem[address, 1, accdesc] = X[t, 8];

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.
<Wm>: When option<0> is set to 0, is the 32-bit name of the general-purpose index register, encoded in the "Rm" field.
<Xm>: When option<0> is set to 1, is the 64-bit name of the general-purpose index register, encoded in the "Rm" field.
<extend>: <amount>: Is the index shift amount, it must be #0, encoded in "S" as 0 if omitted, or as 1 if present.

Operational Notes

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