5748

A new draft of docs as I build out mu.subx. Once we have the second Mu
language, it makes sense to relegate most of the underlying SubX docs to
a separate doc.

I want each layer to be learned bottom-up as I've been organizing SubX
so far. But it's counter-productive to require people to learn SubX before
Mu.

1 files changed, 727 insertions, 0 deletions

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)
+- [&ldquo;Bootstrapping a compiler from nothing&rdquo;](http://web.archive.org/web/20061108010907/http://www.rano.org/bcompiler.html) by Edmund Grumley-Evans.
+- [&ldquo;Creating tiny ELF executables&rdquo;](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).