Writing ARM and Thumb Assembly Language
2-32 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B
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.
