Introduction

At the time of writing, I work for an AV company. I saw that they only had expertise in X86 assembly language, and yet most people I knew accessed the internet from ARM-based telephones and tablets. I had a philosophical preference for RISC architectures, so decided to be proactive and learn more about these ARM chips.

I discuss assembly language programming on the ARM processor (SheevaPlug and Raspberry Pi). ARM processors are found in lots of other places - most current phones and many tablets contain one.

Both of my computers run Debian GNU/Linux.

Here I use the GNU assembler, gas, the GNU C compiler, gcc, the GNU linker, ld and the GNU debugger, gdb.

I referred to GNU-ARM-Assy-Quick-Ref.pdf and More Assembler Directives. The instruction set is defined here

I can recommend Thinkingeek.com as a good source of tutorials for ARM assembly programming.

For the first examples, I followed those supplied with the excellent X86 assembly language book,Professional Assembly Language by Richard Blum.

The code is at git@github.com:bobblestiltskin/professional_assembly_language.git if you want to play with it.

Getting Started

Familiarity with gdb is useful. e.g.

• Use b _start to set a break point at the label _start:
• Use run to run to the break point.
• s to step through from the break point
• inspect registers or abbreviate to i r
• x/4d &data to eXamine 4 decimal items of data

This page explains the basic usage well of gdb.

Communicating with Linux

We can invoke a system call to the operating system, exit, which allows us to pass back one parameter, as the exit code of the program.

.global _start
_start:
mov     r0, #42             @ the value in r0 is returned as the exit status of the process
mov     r7, #1              @ set r7 to 1 - the syscall for exit
                            @ calls listed in /usr/include/asm/unistd.h
swi     0                   @ then invoke the syscall from linux

bob@poland:~/src/asm$ /usr/bin/as -gstabs -o syscall.o syscall.s
bob@poland:~/src/asm$ /usr/bin/ld -o syscall syscall.o
bob@poland:~/src/asm$ ./syscall 
bob@poland:~/src/asm$ echo $?
42
bob@poland:~/src/asm$