LDAPURSB

Load-Acquire RCpc Register Signed Byte (unscaled) calculates an address from a base register and an immediate offset, loads a signed byte from memory, sign-extends it, and writes it to a register.

The instruction has memory ordering semantics as described in Load-Acquire, Load-AcquirePC, and Store-Release, except that:

There is no ordering requirement, separate from the requirements of a Load-AcquirePC or a Store-Release, created by having a Store-Release followed by a Load-AcquirePC instruction.
The reading of a value written by a Store-Release by a Load-AcquirePC instruction by the same observer does not make the write of the Store-Release globally observed.

This difference in memory ordering is not described in the pseudocode.

For information about memory accesses, see Load/Store addressing modes.

Unscaled offset
(FEAT_LRCPC2)

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	0	1	1	0	0	1	1	x	0	imm9									0	0	Rn					Rt
size								opc

32-bit (opc == 11)

LDAPURSB <Wt>, [<Xn|SP>{, #<simm>}]

64-bit (opc == 10)

LDAPURSB <Xt>, [<Xn|SP>{, #<simm>}]

bits(64) offset = SignExtend(imm9, 64);

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 optional signed immediate byte offset, in the range -256 to 255, defaulting to 0 and encoded in the "imm9" 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 tagchecked = memop != MemOp_PREFETCH && (n != 31);

Operation

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

AccessDescriptor accdesc;
if memop == MemOp_LOAD then
    accdesc = CreateAccDescLDAcqPC(tagchecked);
elsif memop == MemOp_STORE then
    accdesc = CreateAccDescAcqRel(memop, tagchecked);

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

address = GenerateAddress(address, offset, accdesc);

case memop of
    when MemOp_STORE
        data = X[t, 8];
        Mem[address, 1, accdesc] = data;

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

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

Operational information

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