I just had a discussion with a colleague about what RISC-V compliance actually means. We discussed the following topics in detail:
As far as I understood the idea, a processor is RISC-V compliant as long as it implements a RISC-V base instruction set and optionally one or more of the standard extensions. Entirely, not just partly. One might even define and implement own instructions (as brownfield or greenfield extensions) as long as they do not touch the base instruction set or any of the standard extensions. Guaranteeing this, the machine code generated by any RISC-V compliant compiler would run on my machine. That's the whole point of it, right?
The RISC-V ISA does not intend delayed branches. My understanding is that the definition whether branches are delayed or not is already part of the ISA and not a matter of implementation. Is this correct?
Assume that one wants to use RISC-V with delayed branches. Whether this is a good idea or not, let's just focus on the compliance question. In my opinion it were no longer RISC-V compliant to define and implement some of the existing branch/jump instructions of the base instruction set as delayed branches. The compilation of a RISC-V compliant compiler would no longer work on such a machine. One would be free to define own delayed branch instructions instead. Of course, as with any self-written extension, it cannot be expected that an arbitrary compiler would use such an instruction. Am I right?
According to the RISC-V specification, "the program counter pc holds the address of the current instruction." My interpretation of this sentence is that any jump/branch instruction refers to the address at which it is stored. Again, independent of the implementation. Example: Assume an implementation where the jump/branch instruction is executed a few cycles after it has been fetched. This would mean that PC has potentially increased already. It is therefore the implementation's task to somehow store the address of the jump/branch instruction. It is not the compiler's task to know about this delay and compensate for it by modifying the immediate that is to be added to the PC. Am I summarizing this correctly?
So, in a nutshell the short version of my questions:
Does RISC-V compliance mean that base integer instruction set and standard extension must neither be changed nor stripped?
Is the information whether a branch is delayed or not already part of the ISA?
Is the PC of RISC-V considered agnostic to any pipeline delay?
I consider an ISA in general to be agnostic to any implementation specifics. The counter-argument to what I claim is that one would have to tell the compiler about implementation specifics (delayed branches, PC behavior etc.) and that this could still be considered compliant with the ISA.
I am not an expert, but have implemented a few cores during the last 20 years. The key concept in your three part-questions are completeness and user visibility. To claim completeness means, in my opinion, that no part of a standard can be changed, nor stripped. However, it is a rare standard indeed if it has no dubious points and sections that may be interpreted differently by different people. In the specific case of RISC-V I would like to point to an aid to indicate compliance, if you have not seen it already.
It would be good to have some real experts answering this question.
Does RISC-V compliance mean that base integer instruction set and standard extension must neither be changed nor stripped?
I have the same understanding as you. It does not make sense to claim a behaviour as defined in a standard, and then not honouring that standard.
Is the information whether a branch is delayed or not already part of the ISA?
Again I concur with you. Delayed branches is an exposed feature for users of a processor. Hence an ISA must specify the eventual existence of such branches, indeed, from page 15 of riscv-spec-v2.2.pdf:
"Control transfer instructions in RV32I do not have architecturally visible delay slots."
Notice the wording, as long as your implementation does not expose any delay slot to a user, you can do as you want. And with a non-standard extension you are perfectly free to design instructions that have delay slots, you may even put RV32I instructions in those slots.
Is the PC of RISC-V considered agnostic to any pipeline delay?
Yes.
Well, they bring (should bring at least) great increase in performance, isn’t it?
So, I haven’t seen any Linux kernel sources, but ‘d love to ask: are they used somehow? (In this case – there must be some special “code-cap” for system that has no such instructions?)
The SSE and MMX instruction sets have limited value outside of audio/video and gaming work. You might find a few explicit uses in dark corners of the kernel, but I wouldn't count on it. The answer in the general case is "no, they are not used", nor are they used in most non-kernel/userspace applications.
The kernel does sometimes optionally use certain x86 instructions that are specific to certain CPUs (e.g. present on some AMD or Intel models but not all, nor vice-versa), such as syscall, but these are different from the SIMD instruction sets you're referring to, and are not part of some wider set of similarly-themed extensions.
After Mark's answer, I went looking. The only place I could easily identify them being used is in the RAID 6 library (which also has support for AltiVec, which is the PowerPC SIMD instruction set).
(Be wary just grepping the tree, there are a lot of spots where the kernel "knows" about SSE/MMX to support user-space applications, but isn't actually using it. Also a couple cases of unfortunate variable names that have absolutely nothing to do with SSE, e.g. in the SCTP implementation.)
There are severe restrictions on using vector registers and floating point registers in kernel code. See e.g. chapter 6.3 of "Calling conventions for different C++ compilers and operating systems". http://www.agner.org/optimize/#manuals
They are used in the kernel for a few things, such as
Software RAID
Encryption (possibly)
However, I believe it always checks their presence first.
"cpu simd instructions use FPU"
erm, no, not as I understand it. They're in part a modern and (much) more efficient replacement for FPU instructions, but a large part of the SIMD instruction set deals with integer operations.
I've never looked very hard at it, but I suppose (ok, hope) that SIMD code generated by a recent gcc version will not clobber any registers or state.
I've heard the theory. Address Space Location Randomization takes libraries and loads them at randomized locations in the virtual address space, so that in case a hacker finds a hole in your program, he doesn't have a pre-known address to execute a return-to-libc attack against, for example. But after thinking about it for a few seconds, it doesn't make any sense as a defensive measure.
Let's say that our hypothetical TargetLib (libc or anything else the hacker is looking for) is loaded at a randomized address instead of a deterministic one. Now the hacker doesn't know ahead of time where TargetLib and the routines inside it are, but neither does the application code. It needs to have some sort of lookup table somewhere in the binary in order to find the routines inside of TargetLib, and that has to be at a deterministic location. (Or at a random location, pointed to by something else. You can add as many indirections as you want, but eventually you have to start at a known location.)
This means that instead of pointing his attack code at the known location of TargetLib, all the hacker needs to do is point his attack code at the application's lookup table's entry for TargetLib and dereference the pointer to the target routine, and the attack proceeds unimpeded.
Is there something about the way ASLR works that I don't understand? Because as described, I don't see how it's anything more than a speed bump, providing the image of security but no actual substance. Am I missing something?
I believe that this is effective because it changes the base address of the shared library. Recall that imported functions from a shared library are patched into your executable image when it is loaded, and therefore there is no table per se, just specific addresses pointing at data and code scattered throughout the program's code.
It raises the bar for an effective attack because it makes a simple buffer overrun (where the return address on the stack can be set) into one where the overrun must contain the code to determine the correct location and then jmp to it. Presumably this just makes it harder.
Virtually all DLLs in Windows are compiled for a base address that they will likely not run at and will be moved anyway, but the core Windows ones tend to have their base address optimized so that the relocation is not needed.
I don't know if get you question correctly but I'll explain when ASLR is effective and when not.
Let's say that we have app.exe and TargetLib.dll.
app.exe is using(linked to) TargetLib.dll.
To make the explanation simple, let's assume that the virtual address space only has these 2 modules.
If both are ALSR enabled, app.exe's base address is unknown. It may resolve some function call addresses when it is loaded but an attacker knows neither where the function is nor where the resolved variables are. The same thing happens when TargetLib.dll is loaded.
Even though app.exe has a lookup table, an attacker does not know where the table is.
Since an attacker cannot tell what is the content of specific address he must attack the application without using any fixed address information. It is usually harder if he uses usual attacking method like stack overflow, heap overflow, use-after-free...
On the other hand, if app.exe is NOT ASLR enabled, it is much easier for an attacker to exploit the application. Because there may be a function call to a interesting API at specific address in app.exe and the attacker can use the address as a target address to jump. (Attacking an application usually starts from jumping to arbitrary address.).
Supplementation:
You may already understand it but I want to make one thing clear.
When an attacker exploit an application by vulnerability like memory corruption he is usually forced to usefixed address jump instruction. They cannot use relative address jump instruction to exploit. This is the reason why ALSR is really effective to such exploits.
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How do emulators work? When I see NES/SNES or C64 emulators, it astounds me.
Do you have to emulate the processor of those machines by interpreting its particular assembly instructions? What else goes into it? How are they typically designed?
Can you give any advice for someone interested in writing an emulator (particularly a game system)?
Emulation is a multi-faceted area. Here are the basic ideas and functional components. I'm going to break it into pieces and then fill in the details via edits. Many of the things I'm going to describe will require knowledge of the inner workings of processors -- assembly knowledge is necessary. If I'm a bit too vague on certain things, please ask questions so I can continue to improve this answer.
Basic idea:
Emulation works by handling the behavior of the processor and the individual components. You build each individual piece of the system and then connect the pieces much like wires do in hardware.
Processor emulation:
There are three ways of handling processor emulation:
Interpretation
Dynamic recompilation
Static recompilation
With all of these paths, you have the same overall goal: execute a piece of code to modify processor state and interact with 'hardware'. Processor state is a conglomeration of the processor registers, interrupt handlers, etc for a given processor target. For the 6502, you'd have a number of 8-bit integers representing registers: A, X, Y, P, and S; you'd also have a 16-bit PC register.
With interpretation, you start at the IP (instruction pointer -- also called PC, program counter) and read the instruction from memory. Your code parses this instruction and uses this information to alter processor state as specified by your processor. The core problem with interpretation is that it's very slow; each time you handle a given instruction, you have to decode it and perform the requisite operation.
With dynamic recompilation, you iterate over the code much like interpretation, but instead of just executing opcodes, you build up a list of operations. Once you reach a branch instruction, you compile this list of operations to machine code for your host platform, then you cache this compiled code and execute it. Then when you hit a given instruction group again, you only have to execute the code from the cache. (BTW, most people don't actually make a list of instructions but compile them to machine code on the fly -- this makes it more difficult to optimize, but that's out of the scope of this answer, unless enough people are interested)
With static recompilation, you do the same as in dynamic recompilation, but you follow branches. You end up building a chunk of code that represents all of the code in the program, which can then be executed with no further interference. This would be a great mechanism if it weren't for the following problems:
Code that isn't in the program to begin with (e.g. compressed, encrypted, generated/modified at runtime, etc) won't be recompiled, so it won't run
It's been proven that finding all the code in a given binary is equivalent to the Halting problem
These combine to make static recompilation completely infeasible in 99% of cases. For more information, Michael Steil has done some great research into static recompilation -- the best I've seen.
The other side to processor emulation is the way in which you interact with hardware. This really has two sides:
Processor timing
Interrupt handling
Processor timing:
Certain platforms -- especially older consoles like the NES, SNES, etc -- require your emulator to have strict timing to be completely compatible. With the NES, you have the PPU (pixel processing unit) which requires that the CPU put pixels into its memory at precise moments. If you use interpretation, you can easily count cycles and emulate proper timing; with dynamic/static recompilation, things are a /lot/ more complex.
Interrupt handling:
Interrupts are the primary mechanism that the CPU communicates with hardware. Generally, your hardware components will tell the CPU what interrupts it cares about. This is pretty straightforward -- when your code throws a given interrupt, you look at the interrupt handler table and call the proper callback.
Hardware emulation:
There are two sides to emulating a given hardware device:
Emulating the functionality of the device
Emulating the actual device interfaces
Take the case of a hard-drive. The functionality is emulated by creating the backing storage, read/write/format routines, etc. This part is generally very straightforward.
The actual interface of the device is a bit more complex. This is generally some combination of memory mapped registers (e.g. parts of memory that the device watches for changes to do signaling) and interrupts. For a hard-drive, you may have a memory mapped area where you place read commands, writes, etc, then read this data back.
I'd go into more detail, but there are a million ways you can go with it. If you have any specific questions here, feel free to ask and I'll add the info.
Resources:
I think I've given a pretty good intro here, but there are a ton of additional areas. I'm more than happy to help with any questions; I've been very vague in most of this simply due to the immense complexity.
Obligatory Wikipedia links:
Emulator
Dynamic recompilation
General emulation resources:
Zophar -- This is where I got my start with emulation, first downloading emulators and eventually plundering their immense archives of documentation. This is the absolute best resource you can possibly have.
NGEmu -- Not many direct resources, but their forums are unbeatable.
RomHacking.net -- The documents section contains resources regarding machine architecture for popular consoles
Emulator projects to reference:
IronBabel -- This is an emulation platform for .NET, written in Nemerle and recompiles code to C# on the fly. Disclaimer: This is my project, so pardon the shameless plug.
BSnes -- An awesome SNES emulator with the goal of cycle-perfect accuracy.
MAME -- The arcade emulator. Great reference.
6502asm.com -- This is a JavaScript 6502 emulator with a cool little forum.
dynarec'd 6502asm -- This is a little hack I did over a day or two. I took the existing emulator from 6502asm.com and changed it to dynamically recompile the code to JavaScript for massive speed increases.
Processor recompilation references:
The research into static recompilation done by Michael Steil (referenced above) culminated in this paper and you can find source and such here.
Addendum:
It's been well over a year since this answer was submitted and with all the attention it's been getting, I figured it's time to update some things.
Perhaps the most exciting thing in emulation right now is libcpu, started by the aforementioned Michael Steil. It's a library intended to support a large number of CPU cores, which use LLVM for recompilation (static and dynamic!). It's got huge potential, and I think it'll do great things for emulation.
emu-docs has also been brought to my attention, which houses a great repository of system documentation, which is very useful for emulation purposes. I haven't spent much time there, but it looks like they have a lot of great resources.
I'm glad this post has been helpful, and I'm hoping I can get off my arse and finish up my book on the subject by the end of the year/early next year.
A guy named Victor Moya del Barrio wrote his thesis on this topic. A lot of good information on 152 pages. You can download the PDF here.
If you don't want to register with scribd, you can google for the PDF title, "Study of the techniques for emulation programming". There are a couple of different sources for the PDF.
Emulation may seem daunting but is actually quite easier than simulating.
Any processor typically has a well-written specification that describes states, interactions, etc.
If you did not care about performance at all, then you could easily emulate most older processors using very elegant object oriented programs. For example, an X86 processor would need something to maintain the state of registers (easy), something to maintain the state of memory (easy), and something that would take each incoming command and apply it to the current state of the machine. If you really wanted accuracy, you would also emulate memory translations, caching, etc., but that is doable.
In fact, many microchip and CPU manufacturers test programs against an emulator of the chip and then against the chip itself, which helps them find out if there are issues in the specifications of the chip, or in the actual implementation of the chip in hardware. For example, it is possible to write a chip specification that would result in deadlocks, and when a deadline occurs in the hardware it's important to see if it could be reproduced in the specification since that indicates a greater problem than something in the chip implementation.
Of course, emulators for video games usually care about performance so they don't use naive implementations, and they also include code that interfaces with the host system's OS, for example to use drawing and sound.
Considering the very slow performance of old video games (NES/SNES, etc.), emulation is quite easy on modern systems. In fact, it's even more amazing that you could just download a set of every SNES game ever or any Atari 2600 game ever, considering that when these systems were popular having free access to every cartridge would have been a dream come true.
I know that this question is a bit old, but I would like to add something to the discussion. Most of the answers here center around emulators interpreting the machine instructions of the systems they emulate.
However, there is a very well-known exception to this called "UltraHLE" (WIKIpedia article). UltraHLE, one of the most famous emulators ever created, emulated commercial Nintendo 64 games (with decent performance on home computers) at a time when it was widely considered impossible to do so. As a matter of fact, Nintendo was still producing new titles for the Nintendo 64 when UltraHLE was created!
For the first time, I saw articles about emulators in print magazines where before, I had only seen them discussed on the web.
The concept of UltraHLE was to make possible the impossible by emulating C library calls instead of machine level calls.
Something worth taking a look at is Imran Nazar's attempt at writing a Gameboy emulator in JavaScript.
Having created my own emulator of the BBC Microcomputer of the 80s (type VBeeb into Google), there are a number of things to know.
You're not emulating the real thing as such, that would be a replica. Instead, you're emulating State. A good example is a calculator, the real thing has buttons, screen, case etc. But to emulate a calculator you only need to emulate whether buttons are up or down, which segments of LCD are on, etc. Basically, a set of numbers representing all the possible combinations of things that can change in a calculator.
You only need the interface of the emulator to appear and behave like the real thing. The more convincing this is the closer the emulation is. What goes on behind the scenes can be anything you like. But, for ease of writing an emulator, there is a mental mapping that happens between the real system, i.e. chips, displays, keyboards, circuit boards, and the abstract computer code.
To emulate a computer system, it's easiest to break it up into smaller chunks and emulate those chunks individually. Then string the whole lot together for the finished product. Much like a set of black boxes with inputs and outputs, which lends itself beautifully to object oriented programming. You can further subdivide these chunks to make life easier.
Practically speaking, you're generally looking to write for speed and fidelity of emulation. This is because software on the target system will (may) run more slowly than the original hardware on the source system. That may constrain the choice of programming language, compilers, target system etc.
Further to that you have to circumscribe what you're prepared to emulate, for example its not necessary to emulate the voltage state of transistors in a microprocessor, but its probably necessary to emulate the state of the register set of the microprocessor.
Generally speaking the smaller the level of detail of emulation, the more fidelity you'll get to the original system.
Finally, information for older systems may be incomplete or non-existent. So getting hold of original equipment is essential, or at least prising apart another good emulator that someone else has written!
Yes, you have to interpret the whole binary machine code mess "by hand". Not only that, most of the time you also have to simulate some exotic hardware that doesn't have an equivalent on the target machine.
The simple approach is to interpret the instructions one-by-one. That works well, but it's slow. A faster approach is recompilation - translating the source machine code to target machine code. This is more complicated, as most instructions will not map one-to-one. Instead you will have to make elaborate work-arounds that involve additional code. But in the end it's much faster. Most modern emulators do this.
When you develop an emulator you are interpreting the processor assembly that the system is working on (Z80, 8080, PS CPU, etc.).
You also need to emulate all peripherals that the system has (video output, controller).
You should start writing emulators for the simpe systems like the good old Game Boy (that use a Z80 processor, am I not not mistaking) OR for C64.
Emulator are very hard to create since there are many hacks (as in unusual
effects), timing issues, etc that you need to simulate.
For an example of this, see http://queue.acm.org/detail.cfm?id=1755886.
That will also show you why you ‘need’ a multi-GHz CPU for emulating a 1MHz one.
Also check out Darek Mihocka's Emulators.com for great advice on instruction-level optimization for JITs, and many other goodies on building efficient emulators.
I've never done anything so fancy as to emulate a game console but I did take a course once where the assignment was to write an emulator for the machine described in Andrew Tanenbaums Structured Computer Organization. That was fun an gave me a lot of aha moments. You might want to pick that book up before diving in to writing a real emulator.
Advice on emulating a real system or your own thing?
I can say that emulators work by emulating the ENTIRE hardware. Maybe not down to the circuit (as moving bits around like the HW would do. Moving the byte is the end result so copying the byte is fine). Emulator are very hard to create since there are many hacks (as in unusual effects), timing issues, etc that you need to simulate. If one (input) piece is wrong the entire system can do down or at best have a bug/glitch.
The Shared Source Device Emulator contains buildable source code to a PocketPC/Smartphone emulator (Requires Visual Studio, runs on Windows). I worked on V1 and V2 of the binary release.
It tackles many emulation issues:
- efficient address translation from guest virtual to guest physical to host virtual
- JIT compilation of guest code
- simulation of peripheral devices such as network adapters, touchscreen and audio
- UI integration, for host keyboard and mouse
- save/restore of state, for simulation of resume from low-power mode
To add the answer provided by #Cody Brocious
In the context of virtualization where you are emulating a new system(CPU , I/O etc ) to a virtual machine we can see the following categories of emulators.
Interpretation: bochs is an example of interpreter , it is a x86 PC emulator,it takes each instruction from guest system translates it in another set of instruction( of the host ISA) to produce the intended effect.Yes it is very slow , it doesn't cache anything so every instruction goes through the same cycle.
Dynamic emalator: Qemu is a dynamic emulator. It does on the fly translation of guest instruction also caches results.The best part is that executes as many instructions as possible directly on the host system so that emulation is faster. Also as mentioned by Cody, it divides the code into blocks ( 1 single flow of execution).
Static emulator: As far I know there are no static emulator that can be helpful in virtualization.
How I would start emulation.
1.Get books based around low level programming, you'll need it for the "pretend" operating system of the Nintendo...game boy...
2.Get books on emulation specifically, and maybe os development. (you won't be making an os, but the closest to it.
3.look at some open source emulators, especially ones of the system you want to make an emulator for.
4.copy snippets of the more complex code into your IDE/compliler. This will save you writing out long code. This is what I do for os development, use a district of linux
I wrote an article about emulating the Chip-8 system in JavaScript.
It's a great place to start as the system isn't very complicated, but you still learn how opcodes, the stack, registers, etc work.
I will be writing a longer guide soon for the NES.