mu/vocabulary.md

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

  `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.

  2. 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 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.

• 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.

• 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, 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)