[huntie]

(2017)

It's probably worth noting that this is only a runtime assembler, i.e. it does no parsing. It also doesn't support all of the addressing modes for the instructions that it does support. Nonetheless it does show that assemblers aren't that hard. Adding support for the various addressing modes and amd64 does complicate things but not too badly. Moving forward from this you'd probably want a better scheme for handling the various "extra" bytes (SIB, REX, etc.).

[deckard1]

I've written a (mostly) fully-featured x86 assembler.

There are two gotchas that most people starting out probably won't realize until it's too late.

1) Encoding instructions on x86 can be more challenging than they first appear. Understanding how this works and fits together will save you a lot of headaches.

2) Forward jumps are a pain. Any reasonable assembler will provide symbolic labels for jumps and other operations. Because x86 instructions are variable length, you can't simply count up the lines to the label and multiply by some fixed amount. It doesn't work that way. Instructions can range from one byte to 15 bytes. What this means to you is that you'll need to design a multi-pass assembler. You allocate a fixed amount of space for the jump instruction and come back later to "patch" that jump instruction with the correct byte offset. Labels simply become keys in a dictionary that record the byte offset into the assembled binary data.

Other things to make sure you understand: big/little endian, LSB/MSB, how two's-complement works, etc.

One surprising aspect of writing an assembler is that you learn to decode instructions by simply looking at a hex dump of the binary output. You almost feel like Neo in the Matrix, in more nerdy less awesome sort of way. You start thinking of assembler and macros as merely convenience of having to write out hex (or binary) of the instructions you want. And on top of that, you see C as convenience for having to do assembler.

[userbinator]

You allocate a fixed amount of space for the jump instruction and come back later to "patch" that jump instruction with the correct byte offset.

Also worth noting there are "short" and "near" jumps, which trade off between a shorter encoding and a longer range of destination; if the target is a forward reference, not yet known at the time the jump is assembled, then nearly all assemblers will use the long variant, because they don't know yet whether the destination will be close enough. One notable exception is fasm, which starts off with being "optimistic" about jump sizes, and then increases only the ones which didn't quite make it, repeatedly, until all the offsets are large enough. Here's a series of very detailed posts about that from its author:

https://board.flatassembler.net/topic.php?t=20249

[waterhouse]

Glancing at the posts, I have a question: do they defend against the pathological cases that require O(n) passes, and therefore at least naively have O(n^2) or worse behavior? Let's say that a jump can be encoded with either an 8-bit signed offset in 2 bytes, or a 32-bit signed offset in 5 bytes. (Supposedly x86 has a 16-bit offset version, but that uses the same opcode as the 32-bit offset, and seems unsupported in 64-bit mode.) Consider the following:

  L0: jmp M0
  L1: jmp M1
  L2: jmp M2
  ...
  L25: jmp M25
  M0: jmp N0
  M1: jmp N1
  ...
  M25: jmp N25
  N0: jmp O0
  ...
  ...
  Z0: jmp LAST
  Z1: jmp LAST
  ...
  Z25: jmp LAST
  [more than 128 bytes of unrelated stuff]
  LAST:

If all the jmps are encoded with 5 bytes, then the offset from e.g. L0 to M0 is 130 bytes, which is over the limit of 127 you can get from an 8-bit signed offset. The optimistic approach will discover in the first pass that all the Z jmps have to be 5 bytes. Pass 2, assuming it goes left to right, will see that everything up to Y24 still has offset ≤ 127 (even "Y24: jmp Z24" could be 127 bytes or 130 depending on how Y25's jump is encoded), but will find that "Y25: jmp Z25" has an offset of 130 and must be 5 bytes. Pass 3 will discover that Y24 needs to be 5 bytes too, and discover nothing else. Pass 4 will discover the sole fact that Y23 needs to be 5 bytes, and so on. If the total number of jmps here is n, then it will take close to n passes to fix up all the jmps. (If the passes go from right to left, or even alternate the direction or randomize... I think it's still possible to construct an example where pessimizing Y10 kicks Y20 over the edge, which kicks Y9 over the edge, which triggers pessimizing Y21, etc.)

This isn't a flaw unique to the optimistic approach, by the way—I could construct a similar version where all the L, M, etc. chains go from 0 to 62, and the "more than 128 bytes of unrelated stuff" becomes "0 bytes of unrelated stuff"; then, in the pessimistic approach, once the Zs have settled down, the subsequent passes will discover, one at a time, that Y62 only has a 126-byte offset and can be encoded with 2 bytes, then that Y61 can be similarly encoded, and so on.

I'm sure this doesn't actually happen in practice, unless you're compiling malicious code off the internet (in which case there are likely worse vulnerabilities inside the compiler)... but I'm curious if assemblers have nonetheless seriously addressed the problem.

One of the posts talks about NUMBER_OF_PASSES; this suggests, but I don't think the post explicitly states, that the defense mechanism is "arbitrarily limiting the number of passes". Do you know if fasm and others actually do this? (Looks like nasm has a command-line switch to turn off multi-pass branch offset optimization.)

[brandmeyer]

There's a standard technique for this that is as old as variable-length ISA's. Yes, you've identified the fundamental problem with the optimistic approach.

You do start with a pessimistic approach and assume jump distances that are the maximum length instructions. On each pass, you restrict yourself to reducing the lengths of instructions. Each pass shrinks only those instructions which currently reach their targets and recomputes only those jump distances. Yes, you can construct pathological cases where you only get to compact one insn at a time.

It doesn't matter. The goal of the asssembler is not to produce the optimal encoding, it is to produce a valid encoding. The program was valid at every step of the way. So after running N passes, you just stop and emit the program you got.

Oddly enough, its the fixed-length RISC ISA's that have a bigger problem with this. A RISC-V branch usually only has 12 bits of jump distance available. That's good enough most of the time, but 5% of the time (varying widely with the program) the target isn't in range. To handle those cases, you have to emit a longer-distance unconditional jump, and invert the logic of the parent branch to hop over the long jump instead of falling through to the 'else' label. Essentially, its a 64-bit encoding of a branch instruction with 20 bits of jump distance. Pretty sure the RISC-V assembler assumes the case of 12-bit distances in 32-bit instructions at first.

Eventually, there is a pass limit for optimistic assemblers as well. But when the limit is reached, instead of emitting a program that is less than optimal, you just return an error, because you didn't have a valid program that you could emit at all.

[elderK]

Thanks for posting this.

I've been wondering about how assemblers handle this for a very long time :)

[mehrdadn]

For multiple passes, is it ever possible that the next pass shrinks the size of the code? Off the top of my head I don't see if this is possible, but I've always wondered this since it would then suggest you could end up bouncing back and forth on every pass, unless explicitly avoid it somehow... can that happen?

[deckard1]

It could, depending on how you initially encode the forward jmp instruction. If you encode them all as 16-bit int and your assembler optimizes, then it could shrink the code to use 8-bit jmp variant, for example.

The way I got around this issue entirely was to encode all jumps as 16-bit variant. Then, during the 2nd pass I would check for overflow and throw an error and halt, if the jmp was no longer within range. I had a simple type declaration syntax for "byte", "word", and "dword" that you could use to coerce a label to a specific size (and thus, control the instruction variant you want the assembler to use).

So the assembler is actually a simple 2-pass assembler. The 2nd pass locks the code size and errors out if the 1st pass assumptions and types do not hold.

Otherwise, as userbinator mentioned, your problem becomes a backtracking/constraint problem. It's an interesting CS problem, but not so fun when all you want is a working assembler.

edit: Should point out that the initial jmp encoding may be dependent on operand mode (i.e. 16/32/64-bit instruction mode). It's been awhile and I can't remember all the details there, plus my assembler was a few years before 64-bit on the x86 became mainstream, so YMMV.

[mehrdadn]

Thanks!

[mhh__]

Never having written an assembler for a proper ISA, I was under the impression that assemblers for real CPUs are extremely simple until you start writing them.

[kabdib]

One of my hobbies in the 1980s was writing assemblers, because most of the existing commercial offerings were pretty bad. I started out writing my own utterly terrible (though fast) assemblers, and after several years and my fifth or sixth try I had one that was fast, very usable and that people liked a lot. It was shipped as a component of our company's devkits and wasn't "commercial" in the sense of a standalone product, but I still count it a commercial success.

Things that make assemblers useful -

- A real macro language. The C-preprocessor does not count.

- Support of the manufacturer's mnemonics and syntax. Unless the manufacturer's official syntax is "AT&T / Unix assembler syntax" then that doesn't count, either. Parsing addressing modes is often painful -- the 68000 was a bear that took several days to get right -- but telling your users "Oh, just use this alternate syntax . . . documented where? Umm..." is lots more difficult.

- Listings and cross-referencing. Maybe this was a function of the era when I was writing these, but all of the assemblers I wrote did not output a listing (addresses and generated bytes along with the program text) and my initial users were reluctant to use them. When I added listings to my last effort -- it took a day or two IIRC -- the tool suddenly became usable in their eyes.

- Speed. No reason these things can't crunch through a million lines a second.

(For my own game carts, I would print out a complete listing every couple of days because these were really helpful in debugging. At the end of a typical project I'd have a couple five-foot-high stacks of fanfold paper, which I'd have shredded. I'll point out that while my floppy-based copies of my games' sources have gone walkabout, I still have the most recent complete assembly listings in binders).

I've thought about reviving my hobby . . . but modern CPUs are much more complicated than the 68Ks and 8-bit wonders of decades ago, and life is too damned short to be writing assembly anyway.

[chrisseaton]

Assemblers are one of those things that's very simple for the simple cases, and then when you add more complex cases you start to think it's actually very hard until you go back and add the proper abstractions, which would have seemed needlessly complicated for the simple cases.

So people doing something trivial thinks they're easy (like this example), people trying to do a bit more think they're really hard, and people doing an entire assembler think they're easy again.

[mises]

I might try doing this as a learning project. Can you offer any advice as to what those abstractions are? I'd appreciate some advice to avoid the "going back" part.

[chrisseaton]

See this real-world assembler for example

https://github.com/oracle/graal/tree/master/compiler/src/org...

Then take a random instruction

https://github.com/oracle/graal/blob/master/compiler/src/org...

See how it has common methods used by other instructions simdPrefix and emitModRM? Those are the abstractions.

[univerio]

What are the proper abstractions necessary?

[chrisseaton]

Most simple cases of instructions seem straightforward to just go ahead and emit the bytes for. Then when you start to use more addressing modes you realise it gets to be a lot of code and it turns out there's a common pattern for everything.

Here's a concrete example from an industrial assembler.

Almost every simple instruction boils down this helper method which is parameterised by a bunch of flags and can then deal with all of them.

https://github.com/oracle/graal/blob/f3a3576493f87abdea1045d...

Even that looks complicated! But writing this per instruction (the simple case) would be insanely complicated.

[userbinator]

Ideally, the common fields and patterns noted by the manufacturer in the ISA description would be implemented, and then the remainder of the logic can be (relatively) simple table-lookup.

[huntie]

I wrote a runtime assembler for a small subset of amd64. I had basically zero experience with assembly and I was still able to write what I needed in half a day. The hard part is understanding how the extra bytes work, which I did by running small bits of assembly through the GNU assembler and then calling objdump on the output.

I think a RISC ISA might be more difficult to start with, and supporting things like AVX might be hard. I only needed support for some basic instructions though. The amd64 manual is actually pretty good. Overall, it was much easier than I expected it to be.

[munk-a]

I think RISC would actually be far easier, since there are far fewer irregular instructions (like you get with x86) and you could leverage the fixed instruction size to not have to worry about some things.

Honestly a low LOC Assembler is a bit of a :shrug: for me, since the compilation of assembly from a higher level language is where things are ridiculously complex, assemblers are basically just translation mappers, there are some weird things that happen in them, but the vast majority of weirdness is covered elsewhere. It is a neat experiment to write an assembler certainly, but I found it much more interesting in Uni trying to express complex high level concepts in assembly. I came to appreciate just how ridiculously helpful compilers are.