ADR instruction enables you to generate an
address, within a certain range, without performing a data load.
a PC-relative expression, that is, a label with an optional offset
where the address of the label is relative to the current PC.
The label used with
ADR must be within the same
armasm faults references to labels that
are out of range in the same section.
The available range of addresses for the
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 T32 encoding
±4095 bytes to a byte, halfword, or word-aligned address.
- 16-bit T32 encoding
0 to 1020 bytes.
must be word-aligned. You can use the
ALIGNdirective to ensure this.
Example 11 shows A32
code that implements a jump table. Here, the
loads the address of the jump table.
AREA Jump, CODE, READONLY ; Name this block of code ARM ; Following code is A32 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 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 Example 11,
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 Example 11 it assigns the value 2 to
is used elsewhere in the code, the value 2 is substituted. Using
EQUin this way is similar to using
#defineto define a constant in C.
Declares one or more words of store. In Example 11 each
DCDstores the address of a routine that handles a particular clause of the jump table.
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
R0by 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.