LDTRB
Load Register Byte (unprivileged) loads a byte from memory, zero-extends it, 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.
Memory accesses made by the instruction behave as if the instruction was executed at EL0 if the Effective value of PSTATE.UAO is 0 and either:
- The instruction is executed at EL1.
- The instruction is executed at EL2 when the Effective value of HCR_EL2.{E2H, TGE} is {1, 1}.
Otherwise, the memory access operates with the restrictions determined by the Exception level at which the instruction is executed. For information about memory accesses, see Load/Store addressing modes.
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 | 1 | 0 | imm9 | 1 | 0 | Rn | Rt | ||||||||||||||||
size | opc |
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. |
<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); unpriv_at_el1 = PSTATE.EL == EL1 && !(EL2Enabled() && HaveNVExt() && HCR_EL2.<NV,NV1> == '11'); unpriv_at_el2 = PSTATE.EL == EL2 && HaveVirtHostExt() && HCR_EL2.<E2H,TGE> == '11'; user_access_override = HaveUAOExt() && PSTATE.UAO == '1'; if !user_access_override && (unpriv_at_el1 || unpriv_at_el2) then acctype = AccType_UNPRIV; else acctype = AccType_NORMAL; boolean tag_checked = n != 31;
Operation
if HaveMTEExt() then SetNotTagCheckedInstruction(!tag_checked); bits(64) address; bits(8) data; if n == 31 then CheckSPAlignment(); address = SP[]; else address = X[n]; address = address + offset; data = Mem[address, 1, acctype]; X[t] = ZeroExtend(data, 32);
Operational information
If PSTATE.DIT is 1, the timing of this instruction is insensitive to the value of the data being loaded or stored.