PowerPC Assembly Language 35
Josh Aas writes: "I've been looking for a way to learn PowerPC assembly language for a while now. My search for books only led to extremely out-of-date publications, and the whole ordeal was generally frustrating. I was amazed at the lack of documentation. Even Motorola and IBM's documentation resources (on the web) were lacking anything of use to me. However, it turns out that Apple provides a pretty good free tutorial on the subject. It's tailored for coding in Mac OS X, but I imagine it would be just as useful in any PowerPC environment. For some reason it includes instructions for the Intel architecture. Perhaps this has to do with the fact that Darwin runs on x86 as well."
RISC (Score:1)
Its RISC, so is there a point in programming at such a low level? I'd think that compilers these days make all but the most extremely obscure optimizations pointless. And that privilege might be reserved for the people who designed the architecture.
Re:RISC (Score:1)
Not everyone writes application software using Visual Basic and ActiveX. For example, what about the people who write the compilers? Anyway, optimization is not a "solved problem" as most people think it is, and hand-coded assembler is still undoubtedly superior in some cases.
Ask me why you need to know assembly after you've written a JVM (without a JIT even) that needs to achieve respectable performance. You'd have to be a better man than I to do that without peeking at the assembly code your compiler generates.
Re:RISC (Score:1)
I've done some of this. It's really hard to make code so tight that you get better than a 5% speed improvement.
There is some value, however, when doing intricate floating-point code, especially when you can get control over the rounding conventions. It's also good for encryption code.
there is plenty of documentation (Score:1)
PPC Altivec assembly information (Score:3, Informative)
Is available at here [altivec.org]
--jeffThe real use of assembly (Score:5, Informative)
The real use of learning assembly is to validate your compiler output. It's rare to want to code much from the ground up in assembly anymore, save hardware-banging code, but knowing assembly still makes all the difference.
Yes, most compilers will perform 1,001 optimizations, but the final bit of optimization you can eck out demands that you profile, examine the hot points, and replace them with assembly or restructure your C with a good idea of the code you really want it to generate.
You'll still find things that the compiler engineers haven't thought of, or were lazy about implementing, or operations which implement unneccessary logic/precision.
For example, sometimes it's best to hoist an expensive calculation -into- a loop because it fits where the processor otherwise stalls, but most compilers take the opposite approach. It's tough to force a C compiler to do this, because most want to hoist code -out- of loops when possible, assuming it's cheapest.
Dependency on non-native signedness, float format or type sizes can mean that shifts, transcendental math, type conversion, etc is being done in software when a single opcode is what you expected.
Understanding the code will tell you if something unusual is happening, such as non-native sized bools or less than optimal variables being turned into register variables.
Some operations are simply not available to the C-only programmer. Subsequent lookups to sin/cos operations can be combined into a single opcode, but I've yet to see it done by any compiler. It's only available in assembly.
The PowerPC offers a rough inverse/division approximation which is faster than a real divide, and is good enough for operations only requiring low precision or an approximation... also completely inaccessible to C code.
Knowing a variable is guaranteed to be within a certain range or of a fixed set of values at a certain point can let you get away with all kinds of murderous assumptions which it's impossible to express in C.
That said, I haven't seen a single PPC assembler reference that was half as good as just looking at the code. Look at code, look at a lot of code, and past that, just look at system implementors' documentation. x86 through Pentium III aside, most current assembler books are just fluff, are wrong about half the pipelining, omit a million useful optimzations and don't cover the real story at all. It's really gotten to the point where the only real way to learn is by doing and doing and doing.
Re:The real use of assembly (Score:1)
Assembly shows you how your CPU does stuff at a much more basic level (e.g. what does the CPU really do with pointers? how does it do a for loop? How are signed numbers different from unsigned? etc). IMHO, every programmer should have taken at least one assembly course in their teachings. Even if they never program in assembly again, the indirect knowledge gained is very valuable.
Re: A whole different reason (Score:2)
I think its very unlikely nowadays someone would write a computer application in assembly laungage. It would take way to long. But in the embedded world there are tons of reasons to know assembly. At the very least, everyone should know how to read it....
Re:The real use of assembly (Score:1)
In the related field of Built-In Test software (Self-Test, Operational Ready Tests, On-Board Diagnostics, whatever you want to call them), there is also a high dependency on knowing exactly what the processor (and bridges) are doing and what their exact timing is. So that is another area that demands assembly knowledge. Especially if you're writing tests for the processor itself.
Not to mention, anyone working on compilers or operating systems in general will need the assembly and architectural knowledge.
PowerPC Documentation from Motorola (Score:4, Informative)
Re:PowerPC Documentation from Motorola (Score:2, Interesting)
If you just want a few lines of assembly, write a small C function that does roughtly what you want, disassemble it and 'adopt' the generated assembly into your own piece.
One very useful notation in codewarrior is that you can specify the register to use for parameters, it helps a lot when doing those things:
long blah(long p1 : __r10, long p2 : __r11) : __r4 {..... }
For patching stuff, I've found that using CodeWarrior assembly functions is extremely useful too. As well as for getting the hex value for opcodes from the disassembled output, saves a trip to the reference book.
Re:PowerPC Documentation from Motorola (Score:1)
The little green book is indispensible, along with the User's Manual for the specific processor.
Another great book from IBM is the PowerPC Compiler Writer's Guide [ibm.com]. It is available in pdf format on line, or IBM will ship you a copy.
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A few good links (Score:2, Informative)
Technical Library [ibm.com]
PowerPC FAQ [motorola.com]
Lightsoft: Beginners Guide to PowerPC Assembly Language [lightsoft.co.uk]
March 95 - Balance of Power: Introducing PowerPC Assembly Language [mactech.com]
PowerPc Architecture [mactech.com]
PowerPC Library [mot.com]
The Metroworks Code Warrior documentation also has some helpful stuff. I found a copy online a while ago, but it's gone now.
x86 instructions (Score:3, Funny)
I thought the Power PC was x86 compatible.
Re:x86 instructions (Score:1)
Re:x86 instructions (Score:1)
PPC has M68k emulation right on the die, at least the older chips did. That's probably what you're thinking of. This allowed MacOS to slowly migrate to PPC native code. M68k is CISC architecturre. x86 is a CISC architecture. PPC is a RISC architecture.
<aside>
It's good to see Intel's flagship CPU running a VLIW instruction set, with x86 emulation on die.
We've been dragging arround the legacy x86 instruction set for a few decades now. x86 instructions are pretty compact, but they have a serious lack of registers. There's a lot of trickery going on in the Athlon XP and P4 in order to overcome the instruction set's weaknesses. Less trickery = less power consumption and/or better cost/performance. Personally, I'd like to see CPUs with only 2 or 3 ALUs per "CPU" and "SMP on a chip" since it's hard to average more than 2.5 issues per clock cycle. 90% of the time you've got a few ALUs that you've paid for just sitting there. Unfortunately, most of the bachmarks and apps out there are a single threaded single process, so you wouldn't see much of a performance increase until people started writing multithreaded apps. Of course, most server apps use a pool of processes to handle requests, or fork off a new process for each new request. Servers and multi-user machines would imediately see an increase in performance. VLIW is designed to really cut down on idle ALUs by making multiple issues explicit in the instruction set and moving all of the trickery into the compiler. VLIW is kind of like a RISC revival. It's about getting back to simpler chips, made possible by better compilers and higher appearent memory bandwidth.</aside>
Re:x86 instructions (Score:1)
Re:x86 instructions (Score:1)
You'll have to excuse my uncalled for amount of obvious thinking... but did some supreme power ever forbid adding more registers or smthn? It would be entirely possible (and most likely pretty easy tho I am not an engineer) to just add 8 or so new registers while maintaining backwards compatibility... ?
You will also have to excuse my cynicism... I just wait patiently for the day for someone to give me a good reason why x86 needs to be replaced. Not an easily solved half assed excuse like lack of registers.
Re:x86 instructions (Score:2, Interesting)
It's just a really klugy instruction set. Floating point was added after the fact. IIRC, you have to multiply something in EAX by something pointed to by ESX or some such. This means you end up doing lots of needless loads and stores and loads and stores if, for instance, you're dealing with even a small system of polynomial equations.
Your modern x86 CPUs can do "register aliasing" to try and overcome some of this register thrashing and to allow more parallelism, but aliasing circitry takes up realistate.
Allowing self-modifying code is also a pain in the ass (PPC simply does not allow it) if you're going to try and run the instruction set non-natively (such as on the P4 core). Self-modifying code will thrash the P4's "translation cache". Memory pages have three permissions (read, write, execute) just like Unix files. However, several of the eight combinations are not allowed by the instruction set. This causes some minor security problems. (I forget which modes cannot be set. You can argue that some of them are useless as the x86 architects did, but there are some non-obvious situations where you might want some of the stranger modes.)
People also complain that the x86 has too many addressing modes. IMHO, they should design instruction sets to be simple and allow for simple emulation. They know that they'll be emulating it on a more advanced core later down the line or shipping with an emulator in the BIOS or some such, so they might as well make their lives easier. IBM had the right idea with OS/390. Basically they designed the hardware and low level system such that the user never saw any code running on bare metal. The "OS" that the user saw was running in something like VMWare. Linux for S/390 runs in an OS/390 partition (basically a virtual machine) and never on the bare CPU. This has lots of advantages for security, stability, modularity, isolation, and maintainability, as well as ease of migration on to the next generation.
There are reasons Intel is migrating from x86 on the high-end CPUs. Don't get me wrong, I'm a big AMD fan, but I think sledgehammer is going in the wrong direction. I woould much prefer that AMD made a PPC, SPARC, ARM, or MIPS instruction set cpu with on-die x86 emulation. We've been doing x86 emulation on a RISC core since the original Pentium chip. It wouldn't take much realistate to expose the RISC or VLIW instruction set as well.
More simply (Score:2, Interesting)
RISK instruction sets assume you don't mind having slightly larger code segmets if it means that most of your instructions execute in one clock cycle and you can up your clock rate (this is why the pentium family uses RISC cores). It also assumes you don't want to pay in transistors (and power and heat) for rarely used instructions. It also assumes you have compilers that a good enough so that you don't pay too high a penalty for not having those scan opcodes, having to work with only a couple of addressing modes, etc.
VLIW takes this one step further. You don't mind increasing your code size because it means that your compiler has lots of time (comparatively) to figure out how to do multiple simultaneous issues in order to maximize your performance per transistor by explicity putting each issue into the verly long instruction. This assumes that you have very smart compilers. Unfortunately, it also means that your instructions are big. If clock multipliers continue to climb, you may see VLIW instruction sets, but decompressors on the die in order to increase the appearent memory bandwidth.
Note that this progression in instruction sets represents a progressive moving of certain aspects of the CPU somplexity from the CPU core into the compiler. This allows the limited die realestate to be used for more ALUs, more Cache, smarter branch prediction, "SMP on a chip", etc. while continuing to allow the price of CPUs to fall.
You can do on-chip emulation of the legacy insstruction sets (like on the P4 and even the Itanium), but it's baggage you have to drag arround even when running your native code. Software emulation of the legacy hardware seems more appropriate. If we all switched to some VILW CPUs now, in another 3 years (being pessimistic), Bochs (or another emulator) running on those CPUs would run the legacy code as fast as today's machines, and would run native code much faster.
Re:x86 instructions (Score:1)
The only thing which is on the die is switchable big or little endian format.
There was never a on the die x86 or 68k processor.
However, the later software emulations of 68k on PowerPC outperformed the original 68040 MACs.
Regards,
angel'o'sphere
Re:x86 instructions (Score:2)
Like horses and donkeys, but they invariably produce a sterile hybrid called the Transmeta.
(slightly offtopic) - Excellent assembly book (Score:1)
Just an added use of assembly---it's pretty essential in programming the older consoles, although probably not these days
Free docs from Motorola (Score:1)
Just a word of warning, though -- the docs AREN'T tutorial-style. If you can pick up PPC assembler based on your previous knowledge of another architecture, then these documents should benefit you nicely.
greg