Load addresses to a register using ADR
The ADR instruction loads 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 PC.
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 and encoding:
Any value that can be produced by rotating an 8-bit value right by any even number of bits within a 32-bit word. The range is relative to the PC.
- 32-bit Thumb encoding
±4095 bytes to a byte, halfword, or word-aligned address.
- 16-bit Thumb encoding
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
This example shows ARM code that implements a jump table. Here, the ADR instruction loads the address of the jump table.
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 arguments 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 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 this example, 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:
Result = argument2 + argument3.
Result = argument2 – argument3.
The jump table is implemented with the following instructions and assembler directives:
Is an assembler directive. You use it to give a value to a symbol. In this example, 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.
Declares one or more words of store. In this example, each DCD stores the address of a routine that handles a particular clause of the jump table.
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.