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diff --git a/draft/Readme.md b/draft/Readme.md new file mode 100644 index 00000000..ba35aad5 --- /dev/null +++ b/draft/Readme.md @@ -0,0 +1,242 @@ +_(Draft of a new iteration of the project's documentation.)_ + +# Mu: a human-scale computer + +Mu is a minimal-dependency hobbyist computing stack (everything above the +processor and OS kernel). + +Mu is not designed to operate in large clusters providing services for +millions of people. Mu is designed for _you_, to run one computer. (Or a few.) +Running the code you want to run, and nothing else. + + ```sh + $ git clone https://github.com/akkartik/mu + $ cd mu + $ ./subx # requires C++ and Linux + ``` + +[](https://travis-ci.org/akkartik/mu) + +There's a minimal number of layers of abstraction, every layer depends +strictly on lower layers, and all levels have thorough automated tests, from +machine code up. + +## Goals + +In priority order: + +* [Reward curiosity.](http://akkartik.name/about) + * Easy to build, easy to run. [Minimal dependencies](https://news.ycombinator.com/item?id=16882140#16882555), + so that installation is always painless. + * All design decisions comprehensible to a single individual. (On demand.) + * All design decisions comprehensible without needing to talk to anyone. + (I always love talking to you, but I try hard to make myself redundant.) + * [A globally comprehensible _codebase_ rather than locally clean code.](http://akkartik.name/post/readable-bad) + * Clear error messages over expressive syntax. +* Safe. + * Thorough test coverage. If you break something you should immediately see + an error message. If you can manually test for something you should be + able to write an automated test for it. + * Memory leaks over memory corruption. +* Teach the computer bottom-up. + +## Non-goals + +* Efficiency. Clear programs over fast programs. +* Portability. Runs on any computer as long as it's x86. +* Compatibility. The goal is to get off mainstream stacks, not to perpetuate + them. Sometimes the right long-term solution is to [bump the major version number](http://akkartik.name/post/versioning). +* Syntax. Mu code is meant to be comprehended by [running, not just reading](http://akkartik.name/post/comprehension). + +## What works so far + +Mu contains a type-safe, memory-safe and testable language where most statements +map directly to a single CPU instruction. This language is built entirely in a +notation called SubX for a subset of the x86 instruction set. The language is +designed to be easy to implement in glorified machine code. + +(Some features will require multiple instructions for a statement: local +variable definitions, array indexing with bounds checking, dereferencing heap +allocations while protecting against freed memory.) + +### SubX + +Here's a quick rundown of SubX's capabilities from the outside. For more +details on the internal experience of the SubX notation itself, see [SubX.md](SubX.md). + +You can generate tiny zero-dependency ELF binaries with it. + + ```sh + $ ./ntranslate init.linux examples/ex1.subx -o examples/ex1 + $ ./examples/ex1 + $ echo $? + 42 + ``` + +You can run the generated binaries on an interpreter/VM for better error +messages. + + ```sh + $ ./subx run examples/ex1 # on Linux or BSD or Mac + $ echo $? + 42 + ``` + +Emulated runs can generate a trace that permits [time-travel debugging](https://github.com/akkartik/mu/blob/master/browse_trace/Readme.md). + + ```sh + $ ./subx --debug translate init.linux examples/factorial.subx -o examples/factorial + saving address->label information to 'labels' + saving address->source information to 'source_lines' + + $ ./subx --debug --trace run examples/factorial + saving trace to 'last_run' + + $ ./browse_trace/browse_trace last_run # text-mode debugger UI + ``` + +You can write tests for your programs. The entire stack is thoroughly covered +by automated tests. SubX's tagline: tests before syntax. + + ```sh + $ ./subx test + $ ./subx run apps/factorial test + ``` + +You can package up SubX binaries with the minimal hobbyist OS [Soso](https://github.com/ozkl/soso) +and run them on Qemu. (Requires graphics and sudo access. Currently doesn't +work on a cloud server.) + + ```sh + # dependencies + $ sudo apt install util-linux nasm xorriso # maybe also dosfstools and mtools + # package up a "hello world" program with a third-party kernel into mu_soso.iso + # requires sudo + $ ./gen_soso_iso init.soso examples/ex6.subx + # try it out + $ qemu-system-i386 -cdrom mu_soso.iso + ``` + +You can also package up SubX binaries with a Linux kernel and run them on +either Qemu or [a cloud server that supports custom images](http://akkartik.name/post/iso-on-linode). +(Takes 12 minutes with 8GB RAM. Requires 12 million LoC of C for the Linux +kernel; that number will gradually go down.) + + ```sh + $ sudo apt install build-essential flex bison wget libelf-dev libssl-dev xorriso + $ ./gen_linux_iso init.linux examples/ex6.subx + $ qemu-system-x86_64 -m 256M -cdrom mu.iso -boot d + ``` + +## Conclusion + +The hypothesis of Mu and SubX is that designing the entire system to be +testable from day 1 and from the ground up would radically impact the culture +of the eco-system in a way that no bolted-on tool or service at higher levels +can replicate: + +* Tests would make it easier to write programs that can be easily understood + by newcomers. + +* More broad-based understanding would lead to more forks. + +* Tests would make it easy to share code across forks. Copy the tests over, + and then copy code over and polish it until the tests pass. Manual work, but + tractable and without major risks. + +* The community would gain a diversified portfolio of forks for each program, + a “wavefront” of possible combinations of features and alternative + implementations of features. Application writers who wrote thorough tests + for their apps (something they just can’t do today) would be able to bounce + around between forks more easily without getting locked in to a single one + as currently happens. + +* There would be a stronger culture of reviewing the code for programs you use + or libraries you depend on. [More eyeballs would make more bugs shallow.](https://en.wikipedia.org/wiki/Linus%27s_Law) + +To falsify these hypotheses, here's a roadmap of the next few planned features: + +* Testable, dependency-injected vocabulary of primitives + - Streams: `read()`, `write()`. (✓) + - `exit()` (✓) + - Client-like non-blocking socket/file primitives: `load`, `save` + - Concurrency, and a framework for testing blocking code + - Server-like blocking socket/file primitives + +* Higher-level notations. Like programming languages, but with thinner + implementations that you can -- and are expected to! -- modify. + - syntax for addressing modes: `%reg`, `*reg`, `*(reg+disp)`, + `*(reg+reg+disp)`, `*(reg+reg<<n + disp)` + - function calls in a single line, using addressing modes for arguments + - syntax for controlling a type checker, like [the mu1 prototype](https://github.com/akkartik/mu1). + - a register allocation _verifier_. Programmer provides registers for + variables; verifier checks that register reads are for the same type that + was last written -- across all control flow paths. + +* Gradually streamline the bundled kernel, stripping away code we don't need. + +--- + +If you're still reading, here are some more things to check out: + +a) Try running the tests: + + ```shell + $ ./test_apps + ``` + +b) Check out the online help. Try typing just `./subx`, and then `./subx +help`. + +c) Familiarize yourself with `./subx help opcodes`. You'll spend a lot of time +with it. (It's also [in this repo](https://github.com/akkartik/mu/blob/master/opcodes).) +[Here](https://lobste.rs/s/qglfdp/subx_minimalist_assembly_language_for#c_o9ddqk) +are some tips on my setup for quickly finding the right opcode for any +situation from within Vim. + +d) Try working on [the starter exercises](https://github.com/akkartik/mu/pulls) +(labelled `hello`). + +## Credits + +Mu builds on many ideas that have come before, especially: + +- [Peter Naur](http://akkartik.name/naur.pdf) for articulating the paramount + problem of programming: communicating a codebase to others; +- [Christopher Alexander](http://www.amazon.com/Notes-Synthesis-Form-Harvard-Paperbacks/dp/0674627512) + and [Richard Gabriel](http://dreamsongs.net/Files/PatternsOfSoftware.pdf) for + the intellectual tools for reasoning about the higher order design of a + codebase; +- Unix and C for showing us how to co-evolve language and OS, and for teaching + the (much maligned, misunderstood and underestimated) value of concise + *implementation* in addition to a clean interface; +- Donald Knuth's [literate programming](http://www.literateprogramming.com/knuthweb.pdf) + for liberating "code for humans to read" from the tyranny of compiler order; +- [David Parnas](http://www.cs.umd.edu/class/spring2003/cmsc838p/Design/criteria.pdf) + and others for highlighting the value of separating concerns and stepwise + refinement; +- [Lisp](http://www.paulgraham.com/rootsoflisp.html) for showing the power of + dynamic languages, late binding and providing the right primitives *a la + carte*, especially lisp macros; +- The folklore of debugging by print and the trace facility in many lisp + systems; +- Automated tests for showing the value of developing programs inside an + elaborate harness; +- [Python doctest](http://docs.python.org/2/library/doctest.html) for + exemplifying interactive documentation that doubles as tests; +- [ReStructuredText](https://en.wikipedia.org/wiki/ReStructuredText) + and [its antecedents](https://en.wikipedia.org/wiki/Setext) for showing that + markup can be clean; +- BDD for challenging us all to write tests at a higher level; +- JavaScript and CSS for demonstrating the power of a DOM for complex + structured documents; +- Rust for demonstrating that a system-programming language can be safe; +- Forth for demonstrating that ergonomics don't require grammar; and +- [Minimal Linux Live](http://minimal.linux-bg.org) for teaching how to create + a bootable disk image. +- [Soso](https://github.com/ozkl/soso), a tiny hackable OS. + +## Coda + +* [Some details on the unconventional organization of this project.](http://akkartik.name/post/four-repos) +* Previous prototypes: [mu0](https://github.com/akkartik/mu0), [mu1](https://github.com/akkartik/mu1). diff --git a/draft/SubX.md b/draft/SubX.md new file mode 100644 index 00000000..b2475ad2 --- /dev/null +++ b/draft/SubX.md @@ -0,0 +1,727 @@ +# SubX: a minimal notation for x86 machine code + +SubX is a notation for a subset of the x86 instruction set. + +Here's a program (`examples/ex1.subx`) that returns 42: + + ```sh + bb/copy-to-ebx 0x2a/imm32 # 42 in hex + b8/copy-to-eax 1/imm32/exit + cd/syscall 0x80/imm8 + ``` + +You can generate tiny zero-dependency ELF binaries with it that run on Linux. + + ```sh + $ ./ntranslate init.linux examples/ex1.subx -o examples/ex1 + $ ./examples/ex1 + $ echo $? + 42 + ``` + +You can run the generated binaries on an interpreter/VM for better error +messages. + + ```sh + $ ./subx run examples/ex1 # on Linux or BSD or Mac + $ echo $? + 42 + ``` + +Emulated runs can generate a trace that permits [time-travel debugging](https://github.com/akkartik/mu/blob/master/browse_trace/Readme.md). + + ```sh + $ ./subx --debug translate init.linux examples/factorial.subx -o examples/factorial + saving address->label information to 'labels' + saving address->source information to 'source_lines' + + $ ./subx --debug --trace run examples/factorial + saving trace to 'last_run' + + $ ./browse_trace/browse_trace last_run # text-mode debugger UI + ``` + +You can write tests for your programs. The entire stack is thoroughly covered +by automated tests. SubX's tagline: tests before syntax. + + ```sh + $ ./subx test + $ ./subx run apps/factorial test + ``` + +SubX is implemented in layers of syntax sugar over a tiny core. The core has +two translators that emit identical binaries. The first, `subx`, is in C++. As +a result it looks reasonable familiar but has a sprawling set of dependencies. +The second, `ntranslate` is self-hosted, so it takes some practice to read. +However, it has a miniscule set of dependencies. These complementary strengths +and weaknesses make it easy to audit and debug. + + ```sh + # generate translator phases using the C++ translator + $ ./subx translate init.linux 0*.subx apps/subx-params.subx apps/hex.subx -o hex + $ ./subx translate init.linux 0*.subx apps/subx-params.subx apps/survey.subx -o survey + $ ./subx translate init.linux 0*.subx apps/subx-params.subx apps/pack.subx -o pack + $ ./subx translate init.linux 0*.subx apps/subx-params.subx apps/assort.subx -o assort + $ ./subx translate init.linux 0*.subx apps/subx-params.subx apps/dquotes.subx -o dquotes + $ ./subx translate init.linux 0*.subx apps/subx-params.subx apps/tests.subx -o tests + $ chmod +x hex survey pack assort dquotes tests + + # use the generated translator phases to translate SubX programs + $ cat init.linux examples/ex1.subx |./tests |./dquotes |./assort |./pack |./survey |./hex > a.elf + $ chmod +x a.elf + $ ./a.elf + $ echo $? + 42 + + # or, automating the above steps + $ ./ntranslate init.linux ex1.subx + $ ./a.elf + $ echo $? + 42 + ``` + +Or, running in a VM on other platforms: + + ```sh + $ ./translate init.linux ex1.subx # generates identical a.elf to above + $ ./subx run a.elf + $ echo $? + 42 + ``` + +You can package up SubX binaries with the minimal hobbyist OS [Soso](https://github.com/ozkl/soso) +and run them on Qemu. (Requires graphics and sudo access. Currently doesn't +work on a cloud server.) + + ```sh + # dependencies + $ sudo apt install util-linux nasm xorriso # maybe also dosfstools and mtools + # package up a "hello world" program with a third-party kernel into mu_soso.iso + # requires sudo + $ ./gen_soso_iso init.soso examples/ex6.subx + # try it out + $ qemu-system-i386 -cdrom mu_soso.iso + ``` + +You can also package up SubX binaries with a Linux kernel and run them on +either Qemu or [a cloud server that supports custom images](http://akkartik.name/post/iso-on-linode). +(Takes 12 minutes with 8GB RAM. Requires 12 million LoC of C for the Linux +kernel; that number will gradually go down.) + + ```sh + $ sudo apt install build-essential flex bison wget libelf-dev libssl-dev xorriso + $ ./gen_linux_iso init.linux examples/ex6.subx + $ qemu-system-x86_64 -m 256M -cdrom mu.iso -boot d + ``` + +## What it looks like + +Here is the above example again: + + ```sh + bb/copy-to-ebx 0x2a/imm32 # 42 in hex + b8/copy-to-eax 1/imm32/exit + cd/syscall 0x80/imm8 + ``` + +Every line contains at most one instruction. Instructions consist of words +separated by whitespace. Words may be _opcodes_ (defining the operation being +performed) or _arguments_ (specifying the data the operation acts on). Any +word can have extra _metadata_ attached to it after `/`. Some metadata is +required (like the `/imm32` and `/imm8` above), but unrecognized metadata is +silently skipped so you can attach comments to words (like the instruction +name `/copy-to-eax` above, or the `/exit` operand). + +SubX doesn't provide much syntax (there aren't even the usual mnemonics for +opcodes), but it _does_ provide error-checking. If you miss an operand or +accidentally add an extra operand you'll get a nice error. SubX won't arbitrarily +interpret bytes of data as instructions or vice versa. + +So much for syntax. What do all these numbers actually _mean_? SubX supports a +small subset of the 32-bit x86 instruction set that likely runs on your +computer. (Think of the name as short for "sub-x86".) Instructions operate on +a few registers: + +* Six general-purpose 32-bit registers: `eax`, `ebx`, `ecx`, `edx`, `esi` and + `edi` +* Two additional 32-bit registers: `esp` and `ebp` (I suggest you only use + these to manage the call stack.) +* Four 1-bit _flag_ registers for conditional branching: + - zero/equal flag `ZF` + - sign flag `SF` + - overflow flag `OF` + - carry flag `CF` + +SubX programs consist of instructions like `89/copy`, `01/add`, `3d/compare` +and `51/push-ecx` which modify these registers as well as a byte-addressable +memory. For a complete list of supported instructions, run `subx help opcodes`. + +(SubX doesn't support floating-point registers yet. Intel processors support +an 8-bit mode, 16-bit mode and 64-bit mode. SubX will never support them. +There are other flags. SubX will never support them. There are also _many_ +more instructions that SubX will never support.) + +It's worth distinguishing between an instruction's _operands_ and its _arguments_. +Arguments are provided directly in instructions. Operands are pieces of data +in register or memory that are operated on by instructions. Intel processors +determine operands from arguments in fairly complex ways. + +## Lengthy interlude: How x86 instructions compute operands + +The [Intel processor manual](http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf) +is the final source of truth on the x86 instruction set, but it can be +forbidding to make sense of, so here's a quick orientation. You will need +familiarity with binary numbers, and maybe a few other things. Email [me](mailto:mu@akkartik.com) +any time if something isn't clear. I love explaining this stuff for as long as +it takes. The bad news is that it takes some getting used to. The good news is +that internalizing the next 500 words will give you a significantly deeper +understanding of your computer. + +Most instructions operate on an operand in register or memory ('reg/mem'), and +a second operand in a register. The register operand is specified fairly +directly using the 3-bit `/r32` argument: + + - 0 means register `eax` + - 1 means register `ecx` + - 2 means register `edx` + - 3 means register `ebx` + - 4 means register `esp` + - 5 means register `ebp` + - 6 means register `esi` + - 7 means register `edi` + +The reg/mem operand, however, gets complex. It can be specified by 1-7 +arguments, each ranging in size from 2 bits to 4 bytes. + +The key argument that's always present for reg/mem operands is `/mod`, the +_addressing mode_. This is a 2-bit argument that can take 4 possible values, +and it determines what other arguments are required, and how to interpret +them. + +* If `/mod` is `3`: the operand is in the register described by the 3-bit + `/rm32` argument similarly to `/r32` above. + +* If `/mod` is `0`: the operand is in the address provided in the register + described by `/rm32`. That's `*rm32` in C syntax. + +* If `/mod` is `1`: the operand is in the address provided by adding the + register in `/rm32` with the (1-byte) displacement. That's `*(rm32 + /disp8)` + in C syntax. + +* If `/mod` is `2`: the operand is in the address provided by adding the + register in `/rm32` with the (4-byte) displacement. That's `*(/rm32 + + /disp32)` in C syntax. + +In the last three cases, one exception occurs when the `/rm32` argument +contains `4`. Rather than encoding register `esp`, it means the address is +provided by three _whole new_ arguments (`/base`, `/index` and `/scale`) in a +_totally_ different way (where `<<` is the left-shift operator): + + ``` + reg/mem = *(base + (index << scale)) + ``` + +(There are a couple more exceptions ☹; see [Table 2-2](modrm.pdf) and [Table 2-3](sib.pdf) +of the Intel manual for the complete story.) + +Phew, that was a lot to take in. Some examples to work through as you reread +and digest it: + +1. To read directly from the `eax` register, `/mod` must be `3` (direct mode), + and `/rm32` must be `0`. There must be no `/base`, `/index` or `/scale` + arguments. + +1. To read from `*eax` (in C syntax), `/mod` must be `0` (indirect mode), and + the `/rm32` argument must be `0`. There must be no `/base`, `/index` or + `/scale` arguments (Intel calls the trio the 'SIB byte'.). + +1. To read from `*(eax+4)`, `/mod` must be `1` (indirect + disp8 mode), + `/rm32` must be `0`, there must be no SIB byte, and there must be a single + displacement byte containing `4`. + +1. To read from `*(eax+ecx+4)`, one approach would be to set `/mod` to `1` as + above, `/rm32` to `4` (SIB byte next), `/base` to `0`, `/index` to `1` + (`ecx`) and a single displacement byte to `4`. (What should the `scale` bits + be? Can you think of another approach?) + +1. To read from `*(eax+ecx+1000)`, one approach would be: + - `/mod`: `2` (indirect + disp32) + - `/rm32`: `4` (`/base`, `/index` and `/scale` arguments required) + - `/base`: `0` (eax) + - `/index`: `1` (ecx) + - `/disp32`: 4 bytes containing `1000` + +## Putting it all together + +Here's a more meaty example: + +<img alt='examples/ex3.subx' src='html/ex3.png'> + +This program sums the first 10 natural numbers. By convention I use horizontal +tabstops to help read instructions, dots to help follow the long lines, +comments before groups of instructions to describe their high-level purpose, +and comments at the end of complex instructions to state the low-level +operation they perform. Numbers are always in hexadecimal (base 16) and must +start with a digit ('0'..'9'); use the '0x' prefix when a number starts with a +letter ('a'..'f'). I tend to also include it as a reminder when numbers look +like decimal numbers. + +Try running this example now: + +```sh +$ ./subx translate init.linux examples/ex3.subx -o examples/ex3 +$ ./subx run examples/ex3 +$ echo $? +55 +``` + +If you're on Linux you can also run it natively: + +```sh +$ ./examples/ex3 +$ echo $? +55 +``` + +Use it now to follow along for a more complete tour of SubX syntax. + +## The syntax of SubX programs + +SubX programs map to the same ELF binaries that a conventional Linux system +uses. Linux ELF binaries consist of a series of _segments_. In particular, they +distinguish between code and data. Correspondingly, SubX programs consist of a +series of segments, each starting with a header line: `==` followed by a name +and approximate starting address. + +All code must lie in a segment called 'code'. + +Segments can be added to. + +```sh +== code 0x09000000 # first mention requires starting address +...A... + +== data 0x0a000000 +...B... + +== code # no address necessary when adding +...C... +``` + +The `code` segment now contains the instructions of `A` as well as `C`. + +Within the `code` segment, each line contains a comment, label or instruction. +Comments start with a `#` and are ignored. Labels should always be the first +word on a line, and they end with a `:`. + +Instruction arguments must specify their type, from: + - `/mod` + - `/rm32` + - `/r32` + - `/subop` (sometimes the `/r32` bits in an instruction are used as an extra opcode) + - displacement: `/disp8` or `/disp32` + - immediate: `/imm8` or `/imm32` + +Different instructions (opcodes) require different arguments. SubX will +validate each instruction in your programs, and raise an error anytime you +miss or spuriously add an argument. + +I recommend you order arguments consistently in your programs. SubX allows +arguments in any order, but only because that's simplest to explain/implement. +Switching order from instruction to instruction is likely to add to the +reader's burden. Here's the order I've been using after opcodes: + +``` + |<--------- reg/mem --------->| |<- reg/mem? ->| +/subop /mod /rm32 /base /index /scale /r32 /displacement /immediate +``` + +Instructions can refer to labels in displacement or immediate arguments, and +they'll obtain a value based on the address of the label: immediate arguments +will contain the address directly, while displacement arguments will contain +the difference between the address and the address of the current instruction. +The latter is mostly useful for `jump` and `call` instructions. + +Functions are defined using labels. By convention, labels internal to functions +(that must only be jumped to) start with a `$`. Any other labels must only be +called, never jumped to. All labels must be unique. + +A special label is `Entry`, which can be used to specify/override the entry +point of the program. It doesn't have to be unique, and the latest definition +will override earlier ones. + +(The `Entry` label, along with duplicate segment headers, allows programs to +be built up incrementally out of multiple [_layers_](http://akkartik.name/post/wart-layers).) + +The data segment consists of labels as before and byte values. Referring to +data labels in either `code` segment instructions or `data` segment values +yields their address. + +Automatic tests are an important part of SubX, and there's a simple mechanism +to provide a test harness: all functions that start with `test-` are called in +turn by a special, auto-generated function called `run-tests`. How you choose +to call it is up to you. + +I try to keep things simple so that there's less work to do when I eventually +implement SubX in SubX. But there _is_ one convenience: instructions can +provide a string literal surrounded by quotes (`"`) in an `imm32` argument. +SubX will transparently copy it to the `data` segment and replace it with its +address. Strings are the only place where a SubX word is allowed to contain +spaces. + +That should be enough information for writing SubX programs. The `examples/` +directory provides some fodder for practice, giving a more gradual introduction +to SubX features. This repo includes the binary for all examples. At any +commit, an example's binary should be identical bit for bit with the result of +translating the corresponding `.subx` file. The binary should also be natively +runnable on a Linux system running on Intel x86 processors, either 32- or +64-bit. If either of these invariants is broken it's a bug on my part. + +## Running + +`subx` currently has the following sub-commands: + +* `subx help`: some helpful documentation to have at your fingertips. + +* `subx test`: runs all automated tests. + +* `subx translate <input files> -o <output ELF binary>`: translates `.subx` + files into an executable ELF binary. + +* `subx run <ELF binary>`: simulates running the ELF binaries emitted by `subx + translate`. Useful for debugging, and also enables more thorough testing of + `translate`. + + Remember, not all 32-bit Linux binaries are guaranteed to run. I'm not + building general infrastructure here for all of the x86 instruction set. + SubX is about programming with a small, regular subset of 32-bit x86. + +## A few hints for debugging + +Writing programs in SubX is surprisingly pleasant and addictive. Reading +programs is a work in progress, and hopefully the extensive unit tests help. +However, _debugging_ programs is where one really faces up to the low-level +nature of SubX. Even the smallest modifications need testing to make sure they +work. In my experience, there is no modification so small that I get it working +on the first attempt. And when it doesn't work, there are no clear error +messages. Machine code is too simple-minded for that. You can't use a debugger, +since SubX's simplistic ELF binaries contain no debugging information. So +debugging requires returning to basics and practicing with a new, more +rudimentary but hopefully still workable toolkit: + +* Start by nailing down a concrete set of steps for reproducibly obtaining the + error or erroneous behavior. + +* If possible, turn the steps into a failing test. It's not always possible, + but SubX's primary goal is to keep improving the variety of tests one can + write. + +* Start running the single failing test alone. This involves modifying the top + of the program (or the final `.subx` file passed in to `subx translate`) by + replacing the call to `run-tests` with a call to the appropriate `test-` + function. + +* Generate a trace for the failing test while running your program in emulated + mode (`subx run`): + ``` + $ ./subx translate input.subx -o binary + $ ./subx --trace run binary arg1 arg2 2>trace + ``` + The ability to generate a trace is the essential reason for the existence of + `subx run` mode. It gives far better visibility into program internals than + running natively. + +* As a further refinement, it is possible to render label names in the trace + by adding a second flag to both the `translate` and `run` commands: + ``` + $ ./subx --debug translate input.subx -o binary + $ ./subx --debug --trace run binary arg1 arg2 2>trace + ``` + `subx --debug translate` emits a mapping from label to address in a file + called `labels`. `subx --debug --trace run` reads in the `labels` file at + the start and prints out any matching label name as it traces each instruction + executed. + + Here's a sample of what a trace looks like, with a few boxes highlighted: + + <img alt='trace example' src='html/trace.png'> + + Each of the green boxes shows the trace emitted for a single instruction. + It starts with a line of the form `run: inst: ___` followed by the opcode + for the instruction, the state of registers before the instruction executes, + and various other facts deduced during execution. Some instructions first + print a matching label. In the above screenshot, the red boxes show that + address `0x0900005e` maps to label `$loop` and presumably marks the start of + some loop. Function names get similar `run: == label` lines. + +* One trick when emitting traces with labels: + ``` + $ grep label trace + ``` + This is useful for quickly showing you the control flow for the run, and the + function executing when the error occurred. I find it useful to start with + this information, only looking at the complete trace after I've gotten + oriented on the control flow. Did it get to the loop I just modified? How + many times did it go through the loop? + +* Once you have SubX displaying labels in traces, it's a short step to modify + the program to insert more labels just to gain more insight. For example, + consider the following function: + + <img alt='control example -- before' src='html/control0.png'> + + This function contains a series of jump instructions. If a trace shows + `is-hex-lowercase-byte?` being encountered, and then `$is-hex-lowercase-byte?:end` + being encountered, it's still ambiguous what happened. Did we hit an early + exit, or did we execute all the way through? To clarify this, add temporary + labels after each jump: + + <img alt='control example -- after' src='html/control1.png'> + + Now the trace should have a lot more detail on which of these labels was + reached, and precisely when the exit was taken. + +* If you find yourself wondering, "when did the contents of this memory + address change?", `subx run` has some rudimentary support for _watch + points_. Just insert a label starting with `$watch-` before an instruction + that writes to the address, and its value will start getting dumped to the + trace after every instruction thereafter. + +* Once we have a sense for precisely which instructions we want to look at, + it's time to look at the trace as a whole. Key is the state of registers + before each instruction. If a function is receiving bad arguments it becomes + natural to inspect what values were pushed on the stack before calling it, + tracing back further from there, and so on. + + I occasionally want to see the precise state of the stack segment, in which + case I uncomment a commented-out call to `dump_stack()` in the `vm.cc` + layer. It makes the trace a lot more verbose and a lot less dense, necessitating + a lot more scrolling around, so I keep it turned off most of the time. + +* If the trace seems overwhelming, try [browsing it](https://github.com/akkartik/mu/blob/master/browse_trace/Readme.md) + in the 'time-travel debugger'. + +Hopefully these hints are enough to get you started. The main thing to +remember is to not be afraid of modifying the sources. A good debugging +session gets into a nice rhythm of generating a trace, staring at it for a +while, modifying the sources, regenerating the trace, and so on. Email +[me](mailto:mu@akkartik.com) if you'd like another pair of eyes to stare at a +trace, or if you have questions or complaints. + +## Reference documentation on available primitives + +### Data Structures + +* Kernel strings: null-terminated arrays of bytes. Unsafe and to be avoided, + but needed for interacting with the kernel. + +* Strings: length-prefixed arrays of bytes. String contents are preceded by + 4 bytes (32 bytes) containing the `length` of the array. + +* Slices: a pair of 32-bit addresses denoting a [half-open](https://en.wikipedia.org/wiki/Interval_(mathematics)) + \[`start`, `end`) interval to live memory with a consistent lifetime. + + Invariant: `start` <= `end` + +* Streams: strings prefixed by 32-bit `write` and `read` indexes that the next + write or read goes to, respectively. + + * offset 0: write index + * offset 4: read index + * offset 8: length of array (in bytes) + * offset 12: start of array data + + Invariant: 0 <= `read` <= `write` <= `length` + +* File descriptors (fd): Low-level 32-bit integers that the kernel uses to + track files opened by the program. + +* File: 32-bit value containing either a fd or an address to a stream (fake + file). + +* Buffered files (buffered-file): Contain a file descriptor and a stream for + buffering reads/writes. Each `buffered-file` must exclusively perform either + reads or writes. + +### 'system calls' + +As I said at the top, a primary design goal of SubX (and Mu more broadly) is +to explore ways to turn arbitrary manual tests into reproducible automated +tests. SubX aims for this goal by baking testable interfaces deep into the +stack, at the OS syscall level. The idea is that every syscall that interacts +with hardware (and so the environment) should be *dependency injected* so that +it's possible to insert fake hardware in tests. + +But those are big goals. Here are the syscalls I have so far: + +* `write`: takes two arguments, a file `f` and an address to array `s`. + + Comparing this interface with the Unix `write()` syscall shows two benefits: + + 1. SubX can handle 'fake' file descriptors in tests. + + 1. `write()` accepts buffer and its length in separate arguments, which + requires callers to manage the two separately and so can be error-prone. + SubX's wrapper keeps the two together to increase the chances that we + never accidentally go out of array bounds. + +* `read`: takes two arguments, a file `f` and an address to stream `s`. Reads + as much data from `f` as can fit in (the free space of) `s`. + + Like with `write()`, this wrapper around the Unix `read()` syscall adds the + ability to handle 'fake' file descriptors in tests, and reduces the chances + of clobbering outside array bounds. + + One bit of weirdness here: in tests we do a redundant copy from one stream + to another. See [the comments before the implementation](http://akkartik.github.io/mu/html/060read.subx.html) + for a discussion of alternative interfaces. + +* `stop`: takes two arguments: + - `ed` is an address to an _exit descriptor_. Exit descriptors allow us to + `exit()` the program in production, but return to the test harness within + tests. That allows tests to make assertions about when `exit()` is called. + - `value` is the status code to `exit()` with. + + For more details on exit descriptors and how to create one, see [the + comments before the implementation](http://akkartik.github.io/mu/html/059stop.subx.html). + +* `new-segment` + + Allocates a whole new segment of memory for the program, discontiguous with + both existing code and data (heap) segments. Just a more opinionated form of + [`mmap`](http://man7.org/linux/man-pages/man2/mmap.2.html). + +* `allocate`: takes two arguments, an address to allocation-descriptor `ad` + and an integer `n` + + Allocates a contiguous range of memory that is guaranteed to be exclusively + available to the caller. Returns the starting address to the range in `eax`. + + An allocation descriptor tracks allocated vs available addresses in some + contiguous range of memory. The int specifies the number of bytes to allocate. + + Explicitly passing in an allocation descriptor allows for nested memory + management, where a sub-system gets a chunk of memory and further parcels it + out to individual allocations. Particularly helpful for (surprise) tests. + +* ... _(to be continued)_ + +I will continue to import syscalls over time from [the old Mu VM in the parent +directory](https://github.com/akkartik/mu), which has experimented with +interfaces for the screen, keyboard, mouse, disk and network. + +### primitives built atop system calls + +_(Compound arguments are usually passed in by reference. Where the results are +compound objects that don't fit in a register, the caller usually passes in +allocated memory for it.)_ + +#### assertions for tests +* `check-ints-equal`: fails current test if given ints aren't equal +* `check-stream-equal`: fails current test if stream doesn't match string +* `check-next-stream-line-equal`: fails current test if next line of stream + until newline doesn't match string + +#### error handling +* `error`: takes three arguments, an exit-descriptor, a file and a string (message) + + Prints out the message to the file and then exits using the provided + exit-descriptor. + +* `error-byte`: like `error` but takes an extra byte value that it prints out + at the end of the message. + +#### predicates +* `kernel-string-equal?`: compares a kernel string with a string +* `string-equal?`: compares two strings +* `stream-data-equal?`: compares a stream with a string +* `next-stream-line-equal?`: compares with string the next line in a stream, from + `read` index to newline + +* `slice-empty?`: checks if the `start` and `end` of a slice are equal +* `slice-equal?`: compares a slice with a string +* `slice-starts-with?`: compares the start of a slice with a string +* `slice-ends-with?`: compares the end of a slice with a string + +#### writing to disk +* `write`: string -> file + - Can also be used to cat a string into a stream. + - Will abort the entire program if destination is a stream and doesn't have + enough room. +* `write-stream`: stream -> file + - Can also be used to cat one stream into another. + - Will abort the entire program if destination is a stream and doesn't have + enough room. +* `write-slice`: slice -> stream + - Will abort the entire program if there isn't enough room in the + destination stream. +* `append-byte`: int -> stream + - Will abort the entire program if there isn't enough room in the + destination stream. +* `append-byte-hex`: int -> stream + - textual representation in hex, no '0x' prefix + - Will abort the entire program if there isn't enough room in the + destination stream. +* `print-int32`: int -> stream + - textual representation in hex, including '0x' prefix + - Will abort the entire program if there isn't enough room in the + destination stream. +* `write-buffered`: string -> buffered-file +* `write-slice-buffered`: slice -> buffered-file +* `flush`: buffered-file +* `write-byte-buffered`: int -> buffered-file +* `print-byte-buffered`: int -> buffered-file + - textual representation in hex, no '0x' prefix +* `print-int32-buffered`: int -> buffered-file + - textual representation in hex, including '0x' prefix + +#### reading from disk +* `read`: file -> stream + - Can also be used to cat one stream into another. + - Will silently stop reading when destination runs out of space. +* `read-byte-buffered`: buffered-file -> byte +* `read-line-buffered`: buffered-file -> stream + - Will abort the entire program if there isn't enough room. + +#### non-IO operations on streams +* `new-stream`: allocates space for a stream of `n` elements, each occupying + `b` bytes. + - Will abort the entire program if `n*b` requires more than 32 bits. +* `clear-stream`: resets everything in the stream to `0` (except its `length`). +* `rewind-stream`: resets the read index of the stream to `0` without modifying + its contents. + +#### reading/writing hex representations of integers +* `is-hex-int?`: takes a slice argument, returns boolean result in `eax` +* `parse-hex-int`: takes a slice argument, returns int result in `eax` +* `is-hex-digit?`: takes a 32-bit word containing a single byte, returns + boolean result in `eax`. +* `from-hex-char`: takes a hexadecimal digit character in `eax`, returns its + numeric value in `eax` +* `to-hex-char`: takes a single-digit numeric value in `eax`, returns its + corresponding hexadecimal character in `eax` + +#### tokenization + +from a stream: +* `next-token`: stream, delimiter byte -> slice +* `skip-chars-matching`: stream, delimiter byte +* `skip-chars-not-matching`: stream, delimiter byte + +from a slice: +* `next-token-from-slice`: start, end, delimiter byte -> slice + - Given a slice and a delimiter byte, returns a new slice inside the input + that ends at the delimiter byte. + +* `skip-chars-matching-in-slice`: curr, end, delimiter byte -> new-curr (in `eax`) +* `skip-chars-not-matching-in-slice`: curr, end, delimiter byte -> new-curr (in `eax`) + +## Resources + +* [Single-page cheatsheet for the x86 ISA](https://net.cs.uni-bonn.de/fileadmin/user_upload/plohmann/x86_opcode_structure_and_instruction_overview.pdf) + (pdf; [cached local copy](https://github.com/akkartik/mu/blob/master/cheatsheet.pdf)) +* [Concise reference for the x86 ISA](https://c9x.me/x86) +* [Intel processor manual](http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf) (pdf) +- [“Bootstrapping a compiler from nothing”](http://web.archive.org/web/20061108010907/http://www.rano.org/bcompiler.html) by Edmund Grumley-Evans. +- [“Creating tiny ELF executables”](https://www.muppetlabs.com/~breadbox/software/tiny/teensy.html) by Brian Raiter. +- [StoneKnifeForth](https://github.com/kragen/stoneknifeforth) by [Kragen Sitaker](http://canonical.org/~kragen). |