‘Hello World!’ in ARM assembly

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‘Hello World!’ in ARM assembly

14. January 2012 22:29

Over the last few weeks, in an effort to port a small C library to the platform, I’ve been doing a fair bit of tinkering around with theAndroid NDK.  The NDK is
primarily intended to allow Android developers to write performance-critical portions of their apps in native C or C++, which interface with the Android Java API through JNI.  As the C library in question required porting some x86 SIMD assembly, I figured
it would be helpful for me to get to know the bare bones of the ARM architecture.  As a means to this end, we can use the NDK’s cross-compiler as a standalone tool to write a simple ‘Hello World!’ console “app” in ARM assembly.  As Android is effectively Linux
under the hood, we can apply our x86 Linux assembly programming skills to the ARM platform.

First things first: ‘Hello World!’ in x86 assembly

Briefly, the method involves invoking system calls by talking directly to the underlying Linux kernel.  An example of how to do this in x86 assembly is givenhere
As ARM uses a different ABI to x86 (registers are named different things, for a start), this code needs a tiny bit of modification.

Firstly, notice that system calls are identified numerically – in the x86 example, #1 refers to exit() and #4 refers to write().  To invoke a system call, we put its identifier in the EAX register, pass (up to 6) arguments in EBX, ECX, EDX, ESI, EDI, EBP,
respectively, and interrupt 0x80 is generated.  This is described in further detailhere.  In contrast, the ARM ‘EABI’ calling convention uses a different
method which is described vaguely inthese patch notes.  We can glean that, on ARM, the system call identifier is put in register R7, arguments
are passed in R0-R6 (respecting “EABI arrangement” where appropriate, i.e. 64-bit arguments), and the kernel is called with the ‘SWI 0’ instruction. 

Secondly, as they are not guaranteed to be the same on each platform, we must look up the system call identifiers for exit() and write().  For this we refer to the Linux kernel source – $LINUX_SOURCE_ROOT/arch/arm/include/asm/unistd.h, specifically.  As
it turns out, these two system calls do have the same identifiers on both x86 and ARM platforms.


So our assembly code, in GAS syntax, looks very much like (see inline comments for details):


    .ascii      "Hello, ARM!n"
len = . - msg


.globl _start
    /* syscall write(int fd, const void *buf, size_t count) */
    mov     %r0, $1     /* fd -> stdout */
    ldr     %r1, =msg   /* buf -> msg */
    ldr     %r2, =len   /* count -> len(msg) */
    mov     %r7, $4     /* write is syscall #4 */
    swi     $0          /* invoke syscall */

    /* syscall exit(int status) */
    mov     %r0, $0     /* status -> 0 */
    mov     %r7, $1     /* exit is syscall #1 */
    swi     $0          /* invoke syscall */


Save the above as hello.S and run it through the GNU cross-assembler provided with the NDK.  I will assume that you have the prebuilt NDK toolchain directory in your PATH (in my case here, /Users/peterdn/android-ndk/toolchains/arm-linux-androideabi-4.4.3/prebuilt/darwin-x86/bin):

arm-linux-androideabi-as -o hello.o hello.S
arm-linux-androideabi-ld -s -o hello hello.o

Deploying to Android

For many, the easiest way to test the above binary is by deploying it to an Android device with an ARM processor.  This also means we can take advantage of the insanely useful ‘adb’ tool.  If you happen to be running bog-standard Linux on an ARM device,
the binary should still run, providing your kernel supports the newer EABI (I believe 2.6.15 and above).

To deploy and test on Android, simply run:

adb push hello /data/local/tmp/hello
adb shell /data/local/tmp/hello

It is also possible to run the binary locally on your device using the Android Terminal Emulator, as below:

Android Terminal Emulator screenshot



another version:

        .syntax unified

        @ --------------------------------
    .global main
        @ Stack the return address (lr) in addition to a dummy register (ip) to
        @ keep the stack 8-byte aligned.
        push    {ip, lr}

        @ Load the argument and perform the call. This is like 'printf("...")' in C.
        ldr     r0, =message
        bl      printf

        @ Exit from 'main'. This is like 'return 0' in C.
        mov     r0, #0    @ Return 0.

        @ Pop the dummy ip to reverse our alignment fix, and pop the original lr
        @ value directly into pc — the Program Counter — to return.
        pop     {ip, pc}

        @ --------------------------------
        @ Data for the printf calls. The GNU assembler's ".asciz" directive
        @ automatically adds a NULL character termination.
        .asciz "Hello, world.n"  

The GNU assembler uses a different line-comment delimiter for each platform. On ARM, it is
@. The GNU assembler also allows the use of C-style multi-line comments (such as "/* ... */").