How to write assembly code for the iPhone

http://www.shervinemami.info/armAssembly.html#howto

by frr149 • 14 years ago

48 25 14 years ago

shervinemami.info

z3phyr• 14 years ago

I ve heard that ARM assembly is easier to grasp than x86..

arthulia• 14 years ago

http://www-cs-faculty.stanford.edu/~eroberts/courses/soco/pr...

That is probably because there are fewer instructions to memorize.

sparky• 14 years ago

ARM is still a bit icky, but nearly anything this side of a VAX mainframe will be easier to grasp than x86.

If anyone's looking to learn, MIPS is really easy to pick up. There's a good simulator (SPIM[1]) with terminal and GUI interfaces and lots of tutorials, as it's used in a lot of undergrad computer architecture courses.

[1] http://spimsimulator.sourceforge.net/

pjmlp2• 14 years ago

As I grew up with x86 I beg to differ.

chj5• 14 years ago

Writing assembler for iPhone is a pain in the ass. I don't know which version its gas is based on, but it is definitely a very very old one. The most important feature missing is to create a macro that takes arguments. A most recent change is that some guy in Apple decided to switch conditional instructions like ldrneb to ldrbne, ruining my existing code base (> 10K lines). Now I need to remember what's the right way for iPhone, and what's for Android.

Btw: there is zero support from Apple, so your only chance is to find this kind of blog posts, and they are very few. Bookmark it now if you may need it someday.

__float• 14 years ago

10K lines? Can you elaborate a little on what this is used for? I am having trouble seeing why Obj-C (it is just C after all) can't be used for the majority.

chj5• 14 years ago

C is sufficient for most apps, however, what I am working on happens to be in the rare few --- x86 emulator(http://www.aemula.net). We need to make good use of every register and the overhead of function call is considered unacceptable.

brigade2• 14 years ago

There is documentation of the OS X assembler [1] that generally applies on iOS as well. In particular, you can have macros with up to 10 arguments, numbered $0 to $9. (actually this shown in the article...)

I'm pretty sure ldrneb was never a valid instruction; gnu as tended to be more flexible than ARM's specification of the assembly language. Some of this flexibility used to be allowed by ARM (maybe), the rest was depreciated with UAL and llvm only supports ARM's UAL syntax. You can get this behaviour from GNU as via .syntax unified (which you really ought to be using, since it makes it really easy to support assembling to both ARM and Thumb-2)

[1] https://developer.apple.com/library/mac/#documentation/Devel...

chj5• 14 years ago

You're my hero. Thanks a lot.

Unfortunately this works different from the assembler from linux (Android), so I need to keep two different macros. No matter what, this is still better than nothing!

saurik3• 14 years ago

Apple (NextSTEP, really) forked their version of gas so long ago that it predates the formation of "GNU binutils". While Apple has occasionally gone back and merged updates for some files, as far as I've been able to tell, most of the code has been stagnant since ~1990. The version of GAS they are using is pretty much 1.38. :(

(One of the most irritating things I had to do while supporting all of the initial iPhone jailbreak efforts was to reimplement a ton of gas macro features from later versions of the assembler so I could compile the "normal programs" that people expected to work on this system.)

chj5• 14 years ago

Did you write a translator or fix the assembler itself?

stephencanon5• 14 years ago

The current Apple devtools default to using the (much more modern) llvm-mc assembler, not gas. Patches and bug reports are always welcome!

panic• 14 years ago

Your C compiler can also generate NEON instructions if you use NEON intrinsics:

http://gcc.gnu.org/onlinedocs/gcc/ARM-NEON-Intrinsics.html

http://blog.llvm.org/2010/04/arm-advanced-simd-neon-intrinsi...

ksec• 14 years ago

Interesting that After all these years Compiler still haven't catch up with self written Assembly speed. Or is there some LLVM optimization not used yet?

stephencanon5• 14 years ago

Compilers and assembly programmers serve two different roles.

- A compiler is expected to produce reasonably fast code in microseconds.

- An assembly programmer is expected to produce the fastest code possible in hours or days or weeks.

Given hours or days, and development work focused on such a task, compilers would be able to compete with good assembly programmers; it simply hasn't been the focus of much compiler development, because most users don't want to wait three days for their code to compile.

What's remarkable is that compilers are able to fairly consistently beat assembly programmers of average skill, while delivering results thousands of times faster.

How to write assembly code for the iPhone

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