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LDR (register offset)

Load with register offset, pre-indexed register offset, or post-indexed register offset.

Syntax

LDR{type}{cond} Rt, [Rn, ±Rm {, shift}] ; register offset

LDR{type}{cond} Rt, [Rn, ±Rm {, shift}]! ; pre-indexed ; A32 only

LDR{type}{cond} Rt, [Rn], ±Rm {, shift} ; post-indexed ; A32 only

LDRD{cond} Rt, Rt2, [Rn, ±Rm] ; register offset, doubleword ; A32 only

LDRD{cond} Rt, Rt2, [Rn, ±Rm]! ; pre-indexed, doubleword ; A32 only

LDRD{cond} Rt, Rt2, [Rn], ±Rm ; post-indexed, doubleword ; A32 only

where:

type

can be any one of:

B
unsigned Byte (Zero extend to 32 bits on loads.)
SB
signed Byte (LDR only. Sign extend to 32 bits.)
H
unsigned Halfword (Zero extend to 32 bits on loads.)
SH
signed Halfword (LDR only. Sign extend to 32 bits.)
-
omitted, for Word.
cond
is an optional condition code.
Rt
is the register to load.
Rn
is the register on which the memory address is based.
Rm
is a register containing a value to be used as the offset. -Rm is not permitted in T32 code.
shift
is an optional shift.
Rt2
is the additional register to load for doubleword operations.

Not all options are available in every instruction set and architecture.

Offset register and shift options

The following table shows the ranges of offsets and availability of these instructions:

Table 13-12 Options and architectures, LDR (register offsets)

Instruction ±Rm a shift    
A32, word or byte b ±Rm LSL #0-31 LSR #1-32  
    ASR #1-32 ROR #1-31 RRX
A32, signed byte, halfword, or signed halfword ±Rm Not available
A32, doubleword ±Rm Not available
T32 32-bit encoding, word, halfword, signed halfword, byte, or signed byte b +Rm LSL #0-3    
T32 16-bit encoding, all except doubleword c +Rm Not available

Register restrictions

In the pre-index and post-index forms, Rn must be different from Rt.

Doubleword register restrictions

For A32 instructions:

  • Rt must be an even-numbered register.
  • Rt must not be LR.
  • Arm strongly recommends that you do not use R12 for Rt.
  • Rt2 must be R(t + 1).
  • Rm must be different from Rt and Rt2 in LDRD instructions.
  • Rn must be different from Rt2 in the pre-index and post-index forms.

Use of PC

In A32 instructions you can use PC for Rt in LDR word instructions, and you can use PC for Rn in LDR instructions with register offset syntax (that is the forms that do not writeback to the Rn).

Other uses of PC are not permitted in A32 instructions.

In T32 instructions you can use PC for Rt in LDR word instructions. Other uses of PC in these T32 instructions are not permitted.

Use of SP

You can use SP for Rn.

In A32 code, you can use SP for Rt in word instructions. You can use SP for Rt in non-word A32 instructions but this is deprecated.

You can use SP for Rm in A32 instructions but this is deprecated.

In T32 code, you can use SP for Rt in word instructions only. All other use of SP for Rt in these instructions are not permitted in T32 code.

Use of SP for Rm is not permitted in T32 state.

a 

Where ±Rm is shown, you can use –Rm, +Rm, or Rm. Where +Rm is shown, you cannot use –Rm.

b 

For word loads, Rt can be the PC. A load to the PC causes a branch to the address loaded. In Arm®v4, bits[1:0] of the address loaded must be 0b00. In Armv5T and above, bits[1:0] must not be 0b10, and if bit[0] is 1, execution continues in T32 state, otherwise execution continues in A32 state.

c 

Rt, Rn, and Rm must all be in the range R0-R7.

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