ARM VERSION 1.2 manual Implementing a jump table with ADR

Writing ARM and Thumb Assembly Language

Implementing a jump table with ADR

Example 2-7 on page 2-33 shows ARM code that implements a jump table. It is supplied as jump.s in the examples\asm subdirectory of ADS. Refer to Code examples on page 2-2 for instructions on how to assemble, link, and execute the example.

The ADR pseudo-instruction loads the address of the jump table.

In the example, the function arithfunc takes three arguments and returns a result in r0. The first argument determines which operation is 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 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.
DCD	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.
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 program
		counter.

2-32 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B