RISC OS London Show 2014

what on Earth are alignment exceptions?

Something to switch off for better compatibility.

Er, yes.

The way that “unaligned” memory reads and writes (ie. ones to memory addresses which are not multiples of four) happen changed on ARMv7 processors compared to the older chips, resulting in them being unlikely to load or save data in the way that the author of the software making use of them expected. In other words, any software using such “unaligned” transfers is very likely to be corrupting its data in some way when used on modern hardware; in the worst case, this could lead on to other areas of memory (eg. OS workspace, disc maps) being corrupted.

Because such corruption is undesirable, the new processors have two ways of handling such loads and stores. They can either carry on and hope for the best (alignment exceptions “off”), or they can raise an “exception” which RISC OS catches as an error, stopping the job completely (alignment exceptions “on”).

in the meanwhile why not switch them off?

Every time an application stops with the exceptions on, it’s tried to do something that would have corrupted memory and the system has caught it. Turning exceptions off may allow it to run, but it won’t stop it executing those “unaligned” loads and stores; this means that each time it would have errored, it’s very likely just corrupted its own data in some way and not told you. This may also lead to corruption of data belonging to the OS as well.

I can’t see any scenario where I’d want to run with the exceptions off, to be honest. If software needs them off to run, you probably don’t want to be running it on the machine anyway — unless you have no unsaved work and extensive backups of your disc/SD card contents.

Wow, thanks Steve – and now I know what that particular bit of jargon means, I’m almost shocked since it’s like… dirty.

The Pi is an ARM v7 and so can be told to replicate the earlier behaviour. Panda is later and cannot pretend to be an earlier ARM.

So for my PandaRo Pro it’s irrelevant – I don’t want to turn alignment exceptions off, and there’s no point anyway?

I can’t see any scenario where I’d want to run with the exceptions off, to be honest. If software needs them off to run, you probably don’t want to be running it on the machine anyway — unless you have no unsaved work and extensive backups of your disc/SD card contents.

I won’t be so extreme. Lots of software works only with AE off, without any problem. To be honest, most software that was not recompiled for ARMv7. Of course it can crash the system, then stop using the application…

My only advice when an application does not work, is to turn AE off and to test it. It’s better to have 2000 software than 400 :)

Firefox is a good example of application that works OK with AE off. There are hundreds of other examples. Of course new software should not rely on this feature.

Lots of software works only with AE off, without any problem. To be honest, most software that was not recompiled for ARMv7.

Yes – it seems the older C compiler seemed to have a habit of using unaligned accesses (not sure when or why).

Also, be aware that the software is loading data incorrectly. Whether or not this makes a difference is up to you to decide, however since one of my old apps caused the Wimp to give some strange errors, it may be that the application can be doing all sorts of weird things, like passing very invalid data in SWI calls, or corrupted wimp blocks, for example.

My only advice when an application does not work, is to turn AE off and to test it. It’s better to have 2000 software than 400 :)

As mentioned before, after you have backed up your data and with nothing else running…

I use “AE off” by default in Predesk (Jeffrey posted (?) a short basic tool in the past) since 2010 with my first Beagle C4. No bigger known problems with Beagle, -xM, Pandora, PandaA4 and -ES with the software I use.

I use “AE off” by default in Predesk (Jeffrey posted (?) a short basic tool in the past) since 2010 with my first Beagle C4. No bigger known problems.

Hmmm. Do you also like standing on high places, flying kites into thunderstorms?

How about “No bigger known problems”… … so far.

“Firefox is a good example of application that works OK with AE off.”
Not my experience here, sadly (RasPi, 5.21 [13-Sept-14]). I’ve tried Firefox RISC OS Release 8, Aug 2009, AKA version 2.0.0.20, on several occasions under ARMv5 compatibility mode and ARMv7 fast mode (AE off). It will load and open – for example – BBC Sport’s webpage, then crashes horribly, scattering disk errors liberally throughout my SD card. The same version would run, slowly and not fully stably, on my Iyo under 5.16.

then crashes horribly, scattering disk errors liberally throughout my SD card.

Utter speculation – but imagine some weird code is performing unaligned loads to determine an address of a buffer (might not be that weird, would “packed” structs behave like this?). The program will be wanting to write to, say, stash its data to a block of memory within its workspace, but thanks to the load behaving in an unexpected manner, the data pointer is somewhere in the dynamic area used by SDFS for the disc map.

It is a contrived example, but hopefully you’ll start to understand that the processor isn’t whinging because it is bored or wants fed or a stroking… it is trying to tell you something is amiss.

How about “No bigger known problems”…

Any times a program failed but not often and with no known data lost since I use this.
At this time no many programs need this (2010 the most).
I’m often “on the road”. I use RISC OS portable and online (Pandora, SchlepPi). ComCentre for Surfsticks need PPP. PPP need “AE off”. RemotePrinterFS need “AE off”. I can not print without this. Firefox I need to configure my network/router…
In privat I not really use a other OS. Only for “Video” I use Ubuntu Studio…

Any times a program failed but not often and with no known data lost since I use this

There is an English phrase: “You’re pushing your luck”
Essentially a warning that probability says things balance out eventually, so the longer you push your luck to the limit the more likely the other outcome.

There is a comparable German phrase. But I think is not the complete truth.
At the beginning I double all the things. I use my a9home and try the same with my Beagle. After a “long” time without problems, I switch near complete to the new hardware (xM, now PandaES). But in this time most of the programs I use are up to date and “AE on” compatible. When I use this programs with “AE off” no problems expected. The “handful” tools that need “AE off”, I use for so long time that no additional problems are expected. I also never heard that any User has data lost for this reason (AE off).
Nevertheless, everyone must decide for themselves. Possible that there are combinations that do not work. Use at your own risk.

“You’re pushing your luck”

I’m with Raik.
Only when I switch the AE to “on”, the computer (ARMiniX) moves to mutiny. Raik already mentioned RemotePrintersFS, but there are quite a few more, such as the dict module, whithout which I couldn’t do.

On the other hand, the unpredictable errors and stiffenings that I do encounter from time to time, may very well be attributed to the “AE off” decision after all. However, I never detected any data loss or disc damages. A Control-break or a reset always solves the problem.

So, we may still be pushing our luck, but apparently not to an extent that it leaves us.

One thing that’s worth mentioning is that the random crashes, etc. that some people experience when they have alignment exceptions turned off might not be due to software doing unaligned loads. There are plenty of other code sequences which can cause issues on ARMv6+ (more on ARMv7 than ARMv6). Unaligned loads/stores are the only feature we can turn off – other features like interworking on MOV PC,LR can’t be disabled, and you’ll only find out about them causing problems is after the fact. The CPU could switch to Thumb mode due to a dodgy MOV PC,LR and execute any number of garbage instructions (corrupting memory as it goes) before it finally hits an instruction which causes a crash. So I tend to view alignment exceptions as being an “early warning system” – if something crashes when they’re turned on then it indicates that it’s an old piece of software which could suffer from any number of compatibility issues, and it’s probably best to try and avoid using the software where possible.

This is also why I abandoned my idea of creating a module which would catch alignment exceptions and emulate the ARMv5 behaviour – a lot of the Iyonix-compatible software I was testing was actually crashing due to encountering other compatibility issues, and creating a piece of software which fixes alignment exceptions would only serve to expose peoples systems to the more serious issues which might happen later on during a program’s execution.

RISC OS Pi’s !Configure offers 3 CPU configurations: ARMv5 compatibility mode (emulate ‘rotated loads’ to support older software); ARMv7 strict mode (‘alignment exceptions’ on) Older software using ‘rotated loads’ will not run; ARMv7 fast mode (‘alignment exceptions’ off) New software may run faster, older software may crash or corrupt data.

ATM I am using the ARMv5 mode, as I have quite a bit of elderly software (Rhapsody, Eureka). You seem to be saying that emulation of ARMv5 behaviour is not desirable – or have I misunderstood? Or is it recommended to run in ARMv7 strict mode at all times, even when using elderly software?

As far as I know, running ARMv5 mode on the Pi is fine – compared to a real ARMv5 there aren’t any prescribed differences that I’m aware of that will cause compatibility issues. The problem however is that there might be some unprescribed differences. E.g. MOVS PC,LR is an instruction which has always been described as being unpredictable if executed in USR32 mode. Some software (I can’t remember what) that I’d been running on my Iyonix used that instruction, and it always ran fine. Chances are it had also been running fine on RiscPCs and other machines running 32bit OS’s (perhaps even the A9, which is also ARMv6). But when I tried running it on my BeagleBoard I found that it crashed horribly due to that instruction.

I’m yet to hear of any problems on the Pi caused by instructions like that, but since we’ve already shaken out pretty much all of the ARMv6/v7 compatibility issues from the OS sources the chances of me encountering a problem like that during my day-to-day work are pretty slim.

RISC OS Pi’s !Configure offers 3 CPU configurations: ARMv5 compatibility mode (emulate ‘rotated loads’ to support older software); ARMv7 strict mode (‘alignment exceptions’ on) Older software using ‘rotated loads’ will not run; […]

ARMv5 compatibility is okay, as the quirky older behaviour will behave as would be expected. We’re mainly referring to the likes of the OMAP which do not have a compatibility mode.

You seem to be saying that emulation of ARMv5 behaviour is not desirable – or have I misunderstood?

Emulating the older way of working is fine – and it is one of the nice things about the Pi that you have this choice.

What we are raising eyebrows over is turning off the alignment exceptions (in ARMv7 mode) in the hope that it will magically make the problems go away. It’s a fallacy – the problems are still there, you’re just not being told. That’s all.

Or is it recommended to run in ARMv7 strict mode at all times, even when using elderly software?

I’d say “suck it and see”; but given that elderly software written in C used the older compiler, there’s a good chance that you’ll either get weird errors (I think one of my programs caused the Wimp to say “Not a task” or something like that when it started) or you’ll get the backtrace gibberish.
At that point, at least, you can just shrug and go back to ARMv5 mode. :-)

Isn’t that just shrug and admire all the disk errors liberally scattered throughout my SD card?

FWIW I dislike the way AE are described simply as ‘On/Off’. It keeps confusing me as to what ‘On’ and ‘Off’ actually mean. Is it the exceptions or the errors I’m switching on? Then have to think it though again.

I’d prefer terms that explictly warn of which option is safe and which lets software run on risking problems.

e.g. Misalignments : ‘Trap and Warn (safe)’ versus ‘Ignore and allow (risky)’

But I agree that’s probably too long. So something like ‘Warn (safe)’ and ‘Ignore (risky)’ might be better.

Jim

Perhaps you shouldn’t use it if you don’t understand it.

Isn’t that just shrug and admire all the disk errors liberally scattered throughout my SD card?

Is it possible your card is the problem?
http://elinux.org/RPi_SD_cards#Working_.2F_Non-working_SD_cards
Such a list of cards are aviable for all the SoC-boards. Any cards are working better than other. I have a card is working fine with my Panda A4 but with my PandaES a lot of errors like broken map, data aborts and so on… Check with DiscKnight via cardreader, no errors found.

At the moment, my SD card doesn’t have a problem; I was referring to this post by George T. Greenfield on November 5th at 6:55pm…

I’ve tried Firefox RISC OS Release 8, Aug 2009, AKA version 2.0.0.20, on several occasions under ARMv5 compatibility mode and ARMv7 fast mode (AE off). It will load and open – for example – BBC Sport’s webpage, then crashes horribly, scattering disk errors liberally throughout my SD card.

Perhaps you shouldn’t use it if you don’t understand it.

Erm, you don’t say who you’re talking to or what “it” you have in mind.

For machines like ARMiniX we have to “use” this in the sense that we have to choose to either trap and warn or allow the machine to proceed regardless of risks. The difficulty I was pointing out is actually understanding what choice you’re making by an “On” or “Off” if you’re not already a helicopter-hat programmer. 8-]

FWIW I have my ARMiniX set to trap and warn. The problem is that the phrasings used are confusing unless you’ve thought this though. Not helpful for mere users who just want a coconut.

The basic problem is what “On” means when you’re offered the choice:

Alignment Exceptions On/Off

if you have no idea what “Alightment Exceptions” actually means. Particularly as it is easy to think of “Errors” rather than “Exceptions” so miss the point. That’s then aggrivated when someone gets warings without then seeing any consequent problems they can identify. “Oooh! Alignment Exceptions keep popping up! That’s a pest, I don’t think I want any of them. Better turn them off!”

Jim

if you have no idea what “Alightment Exceptions” actually means.

I think what he is saying is if you don’t understand what the setting means, leave it alone. ;-)