## Reference documentation on available primitives

### Data Structures

- Kernel strings: null-terminated regions of memory. Unsafe and to be avoided,
  but needed for interacting with the kernel.

- Arrays: size-prefixed regions of memory containing multiple elements of a
  single type. Contents are preceded by 4 bytes (32 bits) containing the
  `size` of the array in bytes.

- 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: size of array (in bytes)
  - offset 12: start of array data

  Invariant: 0 <= `read` <= `write` <= `size`

- 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 size 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 `size`).
- `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`)