Assembly Primer Part 4 — Hello World — ARM

On to Assembly Primer — Part 4. This is where we start writing a small assembly program for the platform. In this case, I don’t know the language and I don’t know the ABI. Learning these from scratch ranges from interesting to tedious :)

Regarding the language (available instructions, mnemonics and assembly syntax): I’m using the ARM Architecture Reference Manual as my reference for the architecture (odd, I know). It’s very long and the documentation for each instruction is extensive — which is good because there are a lot of instructions, and many of them do a lot of things at once.

Regarding the ABI (particularly things like argument passing, return values and system calls): there’s the Procedure Call Standard for the ARM Architecture, and there are a few other references I’ve found, such as the Debian ARM EABI Port wiki page.

“EABI is the new “Embedded” ABI by ARM ltd. EABI is actually a family of ABI’s and one of the “subABIs” is GNU EABI, for Linux.”

– from Debian ARM EABI Port

System Calls

To perform a system call using the GNU EABI:

  • put the system call number in r7
  • put the arguments in r0-r6 (64bit arguments must be aligned to an even numbered register i.e. in r0+r1, r2+r3, or r4+r5)
  • issue the Supervisor Call instruction with a zero operand — svc #0

(Supervisor Call was previously named Software Interruptswi)

Just Exit

Based on the above, it’s not difficult to reimplement JustExit.s (original) for ARM.

.text

.globl _start

_start:
        mov r7, #1
        mov r0, #0
        svc #0

mov here is Move (Immediate) which puts the #-prefixed literal into the named register.

Hello World

Likewise, the conversion of HelloWorldProgram.s (original) is not difficult:

.data 

HelloWorldString:
      .ascii "Hello World\n"

.text 

.globl _start 

_start:
      # Load all the arguments for write () 

      mov r7, #4
      mov r0, #1
      ldr r1,=HelloWorldString
      mov r2, #12
      svc #0

      # Need to exit the program 

      mov r7, #1
      mov r0, #0
      svc #0

This includes the load register pseudo-instruction, ldr — the compiler stores the address of HelloWorldString into the literal pool, a portion of memory located in the program text, and the 32bit address is loaded from the literal pool (more details).

When compiling a similar C program with -mcpu=cortex-a8, I notice that the compiler generates Move (immediate) and Move Topmovw and movt — instructions to load the address directly from the instruction stream, which is presumably more efficient on that architecture.

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3 thoughts on “Assembly Primer Part 4 — Hello World — ARM”

  1. Thank you very much. Only problem is to me that where I can get ‘as’- and ‘ld’-commands to N900.

  2. I use a cross-toolchain on an x86 desktop machine, which include suitable ARM as and ld. I’ve not tried to do the assembling or linking on the N900 itself – though it can be done (I recall seeing instructions somewhere in the forums on maemo.org)

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