Compiling ARM .s for iPhone on Mac / Windows - ios

Question

I am new to this and here is what I want to do -
Create simple programs - loops, counters etc. using ARMv7 assembly
I want to be able to compile them on the Mac / Win / Linux for running on the iPhone
I have a jailbroken iPhone so I can upload the file there, sign it with ldid and run it
Can someone please point me to how I can do this with freely available tools?
Thanks!

Answer 1

I'm left wondering what you're trying to achieve here - is it to learn developing in ARM assembler? Do you want to write iOS applications?
No sane person writes complete applications in assembler these days - they use high level languages - and in a few restricted cases optimize in assembler. This is a very specialist and useful skill to have.
Using a complete C program as a surrogate host is good way to start. Create yourself a simple Hello world program in C which calls an (almost) empty function.
You can (mostly) get this to work using XCode (you need install the optional command-line tools). All but the final linking stage for ARM works using clang. This is obviously MacOSX only.
A better alternative for this kind of experimentation is an ARM Linux system where you're not fighting against the locked down environment of iOS. The Raspberry Pi is perfect for the job. You'll need a cross-compiling toolchain for ARMv7 - of which there are plenty. If using Ubuntu, there are pre-built packages readily available.
Main.c
#include<stdio.h>
extern void func();
int main()
{
printf("Hello World\n");
func();
}
and func.c
#include <stdio.h>
void func()
{
printf("In func()\n");
}
Compile both for your host environment and run it to see it works:
gcc main.c func.c
`./a.out'
Now compile for your target environment. The precise name of the cross-compiling tools varies depending what you installed (mine is arm-angstrom-linux-gnueabi-gcc)
arm-angstrom-linux-gnueabi-gcc main.c func.c
Copy to your target, and prove it works.
Now you can start to write some assembler. Get gcc to produce ARM assembler for our victim file func.c - this results in a file func.s
arm-angstrom-linux-gnueabi-gcc func.c -s
:
.cpu arm7tdmi-s
.fpu softvfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 2
.eabi_attribute 30, 6
.eabi_attribute 18, 4
.file "func.c"
.section .rodata
.align 2
.LC0:
.ascii "In func()\000"
.text
.align 2
.global func
.type func, %function
func:
# Function supports interworking.
# args = 0, pretend = 0, frame = 0
# frame_needed = 1, uses_anonymous_args = 0
stmfd sp!, {fp, lr}
add fp, sp, #4
ldr r0, .L2
bl puts
sub sp, fp, #4
ldmfd sp!, {fp, lr}
bx lr
.L3:
.align 2
.L2:
.word .LC0
.size func, .-func
.ident "GCC: (GNU) 4.5.4 20120305 (prerelease)"
.section .note.GNU-stack,"",%progbits
You can see here that between label func: and .L3 is the business end of func() - and it's almost all function prologue and epilogue. You'll want to check out the ARM Procedure Call Standard to understand what these are and for guidance on which registers to use.
Once you've done your edits, compile the whole thing again with GCC
arm-angstrom-linux-gnueabi-gcc main.c func.s
...and test it.