ARM VERSION 1.2

Writing ARM and Thumb Assembly Language

2.7.2 Loading addresses with LDR Rd, = label

The

LDR Rd,=

pseudo-instruction can load any 32-bit constant into a register. See

Loading with LDR Rd, =const on page2-27. It also accepts program-relative

expressions such as labels, and labels with offsets.

The assembler c onverts an

LDR r0,=label

pseudo-instruction by:

•Placing the address of

label

in a literal pool (a portion of memory embedded in

the code to hold constant values).

•Generating a program-relative

LDR

instruction that reads the address from the

literal pool, for example:

LDR rn [pc, #offset to literal pool]

; load register n with one word

; from the address [pc + offset]

You must ensure that there is a literal pool within range. Refer to Placing literal

pools on page2-28 for more information.

Unlike the

ADR

and

ADRL

pseudo-instructions, you can use

LDR

with labels that are outside

the current section. If the label is outside the current section, the assembler places a

relocation directive in the object code when the source file is assembled. The relocation

directive instructs the linker to resolve the address at link time. The address remains

valid wherever the linker places the section containing the

LDR

and the literal pool.

Example2-9 shows how this works. It is supplied as

ldrlabel.s

in the

examples\asm

subdirectory of the ADS. Refer to Code examples on page2-2 for instructions on how

to assemble, link, and execute th e example.

The instructions list ed in the comments are the ARM instr uctions that are ge nerated by

the assembler.

Example2-9

AREA LDRlabel, CODE,READONLY

ENTRY ; Mark first instruction to execute

start

BL func1 ; Branch to first subroutine

BL func2 ; Branch to second subroutine

stop MOV r0, #0x18 ; angel_SWIreason_ReportException

LDR r1, =0x20026 ; ADP_Stopped_ApplicationExit

SWI 0x123456 ; ARM semihosting SWI

func1

LDR r0, =start ; => LDR R0,[PC, #offset into