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flat assembler > Non-x86 architectures > [Arm] Has the Linux interface spec changed?

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AnewUser



Joined: 02 Jun 2012
Posts: 7
Hi all, it's my first post here and I hope it's in the right place.
As regards writing ELF code for the ARM, I was under the impression that to
use Linux System calls, one populates the first 6 parameters (if all applicable)
in regs R1 to R6 then the call number in R7 then followed by a SWI 0.
However I see with FasmArm this is different (printing hello world example)

Code:
 ; All the format stuff etc I've left out for this example.

mov r0,0  ; I thought the handle for STDOUT was 1 not 0
add r1,pc,hello-$-8  ; OK understand
mov r2,hello-len ; OK understand
swi 0x900004 ; why not swi 0, is it because the call table lives at 0x900000?
    

Can anyone elaborate as I'm now confused.
Thanks.
Post 02 Jun 2012, 12:29
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16587
Location: Earth 2.0 beta
The example you show was tested using an older version of Linux used on the Gumstix SOM by one of the users here. It is entirely possible that the call structure has changed in later versions of Linux.

If you have an example that is updated to run on the current Linux version then please feel free to post it here.
Post 02 Jun 2012, 12:33
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AnewUser



Joined: 02 Jun 2012
Posts: 7
Hi revolution
The example I used is straight from the examples of FasmARM ver 1.26
Post 02 Jun 2012, 12:36
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16587
Location: Earth 2.0 beta
I don't have an updated example for current Linux versions. If you know that the interface has been changed then you are welcome to submit an update.
Post 02 Jun 2012, 12:38
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AnewUser



Joined: 02 Jun 2012
Posts: 7
I don't know if it has been changed, I'm just trying to confirm if it will
work as I don't have an ARM based machine running Linux at my disposal.
Post 02 Jun 2012, 12:40
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16587
Location: Earth 2.0 beta
AnewUser wrote:
... I don't have an ARM based machine running Linux at my disposal.
Me either.

AFAIK it was working previously. But I can't verify if it still works, or if it did work.
Post 02 Jun 2012, 12:41
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AnewUser



Joined: 02 Jun 2012
Posts: 7
No problem, thanks.
You mentioned that the example was from Gumstix SOM, any idea
what kernel version it was using?

From what I can gather, the system calls are pretty standard across kernel
version.
I'm just intrigued by the 0 instead of a 1 for STDOUT.
As regards the swi 0x900004, it can very well be as with ARM, it boots to
0x0 then the bootloader starts to populate any kernel parameters from 0x100
and the actual kernel starting at 0x8000, so it could well be that the
system call vector table starts at 0x900000 and since sys_write is the fouth call
it stands to reason it would be at 0x900004.


Last edited by AnewUser on 02 Jun 2012, 12:51; edited 1 time in total
Post 02 Jun 2012, 12:44
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16587
Location: Earth 2.0 beta
Post 02 Jun 2012, 12:48
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16587
Location: Earth 2.0 beta
AnewUser wrote:
... so it could well be that the
system call vector table starts at 0x900000 and since sys_write is the fouth call
it stands to reason it would be at 0x900004.
SWI does not work that way. The number is arbitrary and is for software use only. It doesn't affect any hardware call destination.
Post 02 Jun 2012, 12:54
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AnewUser



Joined: 02 Jun 2012
Posts: 7
Interesting, are you then saying that swi 800004 would achieve the same result?
Or have I missed something?
Post 02 Jun 2012, 13:01
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16587
Location: Earth 2.0 beta
AnewUser wrote:
Interesting, are you then saying that swi 800004 would achieve the same result?
Or have I missed something?
You have to match the numbers with what the kernel is expecting. The hardware call destination won't change, but it is possible that the Linux kernel has changed over time to use a different numbering scheme.
Post 02 Jun 2012, 13:03
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AnewUser



Joined: 02 Jun 2012
Posts: 7
OK, I think the confusion may be coming from us using different terminology.
The addresses and offsets I refered to are software addresses as we are
"talking" to the OS kernel and not a peripheral.
Not trying to be argumentative, just trying to clear my beginners confusion.
Post 02 Jun 2012, 13:15
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 16587
Location: Earth 2.0 beta
It is up to the kernel if it wants to jump directly to the SWI parameter passed by the user. Although, to me that would seem like an extremely bad idea. More likely it will have some sort of lookup table to decide what to do. Anyhow, I suppose the Linux source code will give the ultimate answer to your question.
Post 02 Jun 2012, 13:50
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AnewUser



Joined: 02 Jun 2012
Posts: 7
I agree.
I'm going to load QEMU on my PC and get a Linux ARM distro running on it
then compile one of the armelf examples that came with FasmARM and see
what the result is.
Thanks for your input.
Post 02 Jun 2012, 14:08
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cwpjr



Joined: 03 Jul 2012
Posts: 45
Post your travels if you would!
Post 03 Jul 2012, 06:14
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