Load addresses to a register using ADR

ARM Compiler toolchain v4.1 for µVision Using the Assembler

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Load addresses to a register using ADR

The ADR instruction enables you to generate an address, within a certain range, without performing a data load. ADR accepts a PC-relative expression, that is, a label with an optional offset where the address of the label is relative to the current PC.

Note

The label used with ADR must be within the same code section. The assembler faults references to labels that are out of range in the same section.

The available range of addresses for the ADR instruction depends on the instruction set:

ARM: ±255 bytes to a byte or halfword-aligned address.
±1020 bytes to a word-aligned address.
32-bit Thumb: ±4095 bytes to a byte, halfword, or word-aligned address.
16-bit Thumb: 0 to 1020 bytes. label must be word-aligned. You can use the ALIGN directive to ensure this.

Example of a jump table implementation with ADR

Example 10 shows ARM code that implements a jump table. Here, the ADR instruction loads the address of the jump table.

Example 10. Implementing a jump table (ARM)

        AREA    Jump, CODE, READONLY     ; Name this block of code
        ARM                              ; Following code is ARM code
num     EQU     2                        ; Number of entries in jump table
        ENTRY                            ; Mark first instruction to execute
start                                    ; First instruction to call
        MOV     r0, #0                   ; Set up the three parameters
        MOV     r1, #3
        MOV     r2, #2
        BL      arithfunc                ; Call the function
stop
        MOV     r0, #0x18                ; angel_SWIreason_ReportException
        LDR     r1, =0x20026             ; ADP_Stopped_ApplicationExit
        SVC     #0x123456                ; ARM semihosting (formerly SWI)
arithfunc                                ; Label the function
        CMP     r0, #num                 ; Treat function code as unsigned integer
        BXHS    lr                       ; If code is >= num then simply return
        ADR     r3, JumpTable            ; Load address of jump table
        LDR     pc, [r3,r0,LSL#2]        ; Jump to the appropriate routine
JumpTable
        DCD     DoAdd
        DCD     DoSub
DoAdd
        ADD     r0, r1, r2               ; Operation 0
        BX      lr                       ; Return
DoSub
        SUB     r0, r1, r2               ; Operation 1
        BX      lr                       ; Return
        END                              ; Mark the end of this file

In Example 10, the function arithfunc takes three arguments and returns a result in R0. The first argument determines the operation to be carried out on the second and third arguments:

argument1=0: Result = argument2 + argument3.
argument1=1: Result = argument2 - argument3.

The jump table is implemented with the following instructions and assembler directives:

EQU

Is an assembler directive. It is used to give a value to a symbol. In Example 10 it assigns the value 2 to num. When num is used elsewhere in the code, the value 2 is substituted. Using EQU in this way is similar to using #define to define a constant in C.

DCD

Declares one or more words of store. In Example 10 each DCD stores the address of a routine that handles a particular clause of the jump table.

LDR

The LDR PC,[R3,R0,LSL#2] instruction loads the address of the required clause of the jump table into the PC. It:

multiplies the clause number in R0 by 4 to give a word offset
adds the result to the address of the jump table
loads the contents of the combined address into the PC.