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LDR (immediate)

Load Register (immediate) loads a word or doubleword from memory and writes it 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. The Unsigned offset variant scales the immediate offset value by the size of the value accessed before adding it to the base register value.

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

Post-index

313029282726252423222120191817161514131211109876543210
1x111000010imm901RnRt
sizeopc

32-bit (size == 10)

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

64-bit (size == 11)

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

boolean wback = TRUE;
boolean postindex = TRUE;
integer scale = UInt(size);
bits(64) offset = SignExtend(imm9, 64);

Pre-index

313029282726252423222120191817161514131211109876543210
1x111000010imm911RnRt
sizeopc

32-bit (size == 10)

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

64-bit (size == 11)

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

boolean wback = TRUE;
boolean postindex = FALSE;
integer scale = UInt(size);
bits(64) offset = SignExtend(imm9, 64);

Unsigned offset

313029282726252423222120191817161514131211109876543210
1x11100101imm12RnRt
sizeopc

32-bit (size == 10)

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

64-bit (size == 11)

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

boolean wback = FALSE;
boolean postindex = FALSE;
integer scale = UInt(size);
bits(64) offset = LSL(ZeroExtend(imm12, 64), scale);

For information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors, and particularly LDR (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>

For the 32-bit variant: is the optional positive immediate byte offset, a multiple of 4 in the range 0 to 16380, defaulting to 0 and encoded in the "imm12" field as <pimm>/4.

For the 64-bit variant: is the optional positive immediate byte offset, a multiple of 8 in the range 0 to 32760, defaulting to 0 and encoded in the "imm12" field as <pimm>/8.

Shared Decode

integer n = UInt(Rn);
integer t = UInt(Rt);
integer regsize;

regsize = if size == '11' then 64 else 32;
integer datasize = 8 << scale;
boolean tag_checked = wback || n != 31;

boolean wb_unknown = FALSE;

if 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();

Operation

if HaveMTEExt() then
    SetTagCheckedInstruction(tag_checked);

bits(64) address;
bits(datasize) data;

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

if !postindex then
    address = address + offset;

data = Mem[address, datasize DIV 8, AccType_NORMAL];
X[t] = ZeroExtend(data, regsize);

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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