LDRSB (literal) -- AArch32

Load Register Signed Byte (literal) calculates an address from the PC value and an immediate offset, loads a byte from memory, sign-extends it to form a 32-bit word, and writes it to a register. For information about memory accesses see Memory accesses.

It has encodings from the following instruction sets: A32 ( A1 ) and T32 ( T1 ) .

if P == '0' && W == '1' then SEE "LDRSBT"; constant t = UInt(Rt); constant imm32 = ZeroExtend(imm4H:imm4L, 32); constant add = (U == '1'); constant wback = (P == '0') || (W == '1'); if t == 15 || wback then UNPREDICTABLE;

CONSTRAINED UNPREDICTABLE behavior

T1

LDRSB{<c>}{<q>} <Rt>, <label> // (Preferred syntax)

LDRSB{<c>}{<q>} <Rt>, [PC, #{+/-}<imm>] // (Alternative syntax)

if Rt == '1111' then SEE "PLI"; constant t = UInt(Rt); constant imm32 = ZeroExtend(imm12, 32); constant add = (U == '1'); // Armv8-A removes UNPREDICTABLE for R13

For more information about the constrained unpredictable behavior of this instruction, see Architectural Constraints on UNPREDICTABLE behaviors.

Assembler Symbols

<c>	See Standard assembler syntax fields.

<q>	See Standard assembler syntax fields.

<Rt>	Is the general-purpose register to be transferred, encoded in the "Rt" field.

<label>

For encoding A1: the label of the literal data item that is to be loaded into <Rt>. The assembler calculates the required value of the offset from the Align(PC, 4) value of the instruction to this label. Any value in the range -255 to 255 is permitted.

If the offset is zero or positive, imm32 is equal to the offset and add == TRUE, encoded as U == 1. If the offset is negative, imm32 is equal to minus the offset and add == FALSE, encoded as U == 0.

For encoding T1: the label of the literal data item that is to be loaded into <Rt>. The assembler calculates the required value of the offset from the Align(PC, 4) value of the instruction to this label. Permitted values of the offset are -4095 to 4095.

If the offset is zero or positive, imm32 is equal to the offset and add == TRUE, encoded as U == 1.

If the offset is negative, imm32 is equal to minus the offset and add == FALSE, encoded as U == 0.

+/-

Specifies the offset is added to or subtracted from the base register, defaulting to + if omitted and encoded in U:

U	+/-
0	-
1	+

<imm>	For encoding A1: is the 8-bit unsigned immediate byte offset, in the range 0 to 255, defaulting to 0 if omitted, and encoded in the "imm4H:imm4L" field.
	For encoding T1: is a 12-bit unsigned immediate byte offset, in the range 0 to 4095, encoded in the "imm12" field.

The alternative syntax permits the addition or subtraction of the offset and the immediate offset to be specified separately, including permitting a subtraction of 0 that cannot be specified using the normal syntax. For more information, see Use of labels in UAL instruction syntax.

Operation

if ConditionPassed() then EncodingSpecificOperations(); constant base = Align(PC32,4); constant address = if add then (base + imm32) else (base - imm32); R[t] = SignExtend(MemU[address,1], 32);

Operational information

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

Internal version only: isa v01_31, pseudocode v2024-03_rel ; Build timestamp: 2024-03-25T10:05

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
!= 1111				0	0	0	P	U	1	W	1	1	1	1	1	Rt				imm4H				1	1	0	1	imm4L
cond

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
1	1	1	1	1	0	0	1	U	0	0	1	1	1	1	1	!= 1111				imm12
																Rt

LDRSB (literal)

A1

A1 (!(P == 0 && W == 1))

CONSTRAINED UNPREDICTABLE behavior

T1

T1

Assembler Symbols

Operation

Operational information