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LDRSB (immediate)

Load Register Signed Byte (immediate) loads a byte from memory, sign-extends it to either 32 bits or 64 bits, and writes the result to a register. The address that is used for the load is calculated from a base register and an immediate offset. For information about memory accesses, see Load/Store addressing modes.

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

Post-index

313029282726252423222120191817161514131211109876543210
001110001x0imm901RnRt
sizeopc

32-bit (opc == 11)

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

64-bit (opc == 10)

LDRSB <Xt>, [<Xn|SP>], #<simm>

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

Pre-index

313029282726252423222120191817161514131211109876543210
001110001x0imm911RnRt
sizeopc

32-bit (opc == 11)

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

64-bit (opc == 10)

LDRSB <Xt>, [<Xn|SP>, #<simm>]!

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

Unsigned offset

313029282726252423222120191817161514131211109876543210
001110011ximm12RnRt
sizeopc

32-bit (opc == 11)

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

64-bit (opc == 10)

LDRSB <Xt>, [<Xn|SP>{, #<pimm>}]

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

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

Assembler Symbols

<Wt>

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

<Xt>

Is the 64-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.

Shared Decode

integer n = UInt(Rn);
integer t = UInt(Rt);
MemOp memop;
boolean signed;
integer regsize;

if opc<1> == '0' then
    // store or zero-extending load
    memop = if opc<0> == '1' then MemOp_LOAD else MemOp_STORE;
    regsize = 32;
    signed = FALSE;
else
    // sign-extending load
    memop = MemOp_LOAD;
    regsize = if opc<0> == '1' then 32 else 64;
    signed = TRUE;

boolean tag_checked = memop != MemOp_PREFETCH && (wback || n != 31);

Operation

if HaveMTEExt() then
    SetNotTagCheckedInstruction(!tag_checked);

bits(64) address;
bits(8) data;

boolean wb_unknown = FALSE;
boolean rt_unknown = FALSE;

if memop == MemOp_LOAD && wback && n == t && n != 31 then
    c = ConstrainUnpredictable(Unpredictable_WBOVERLAPLD);
    assert c IN {Constraint_WBSUPPRESS, Constraint_UNKNOWN, Constraint_UNDEF, Constraint_NOP};
    case c of
        when Constraint_WBSUPPRESS wback = FALSE;    // writeback is suppressed
        when Constraint_UNKNOWN    wb_unknown = TRUE;    // writeback is UNKNOWN
        when Constraint_UNDEF      UNDEFINED;
        when Constraint_NOP        EndOfInstruction();

if memop == MemOp_STORE && 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   UNDEFINED;
        when Constraint_NOP     EndOfInstruction();

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

if !postindex then
    address = address + offset;

case memop of
    when MemOp_STORE
        if rt_unknown then
            data = bits(8) UNKNOWN;
        else
            data = X[t];
        Mem[address, 1, AccType_NORMAL] = data;

    when MemOp_LOAD
        data = Mem[address, 1, AccType_NORMAL];
        if signed then
            X[t] = SignExtend(data, regsize);
        else
            X[t] = ZeroExtend(data, regsize);

    when MemOp_PREFETCH
        Prefetch(address, t<4:0>);

if wback then
    if wb_unknown then
        address = bits(64) UNKNOWN;
    elsif 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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