Crate avr_progmem

Progmem utilities for the AVR architectures.

This crate provides unsafe utilities for working with data stored in the program memory of an AVR micro-controller. Additionally, it defines a 'best-effort' safe wrapper struct ProgMem to simplify working with it.

This crate is implemented only in Rust and some short assembly, it does NOT depend on the avr-libc or any other C-library. However, due to the use of inline assembly, this crate may only be compiled using a nightly Rust compiler.

AVR Memory

This crate is specifically for AVR-base micro-controllers such as the Arduino Uno (and some other Arduino boards, but not all), which have a modified Harvard architecture which implies the strict separation of program code and data while having special instructions to read and write to program memory.

While, of course, all ordinary data is stored in the data domain where it is perfectly usable, the harsh constraints of most AVR processors make it very appealing to use the program memory (also referred to as progmem) for storing constant values. However, due to the Harvard design, those values are not usable with normal instructions (i.e. those emitted from normal Rust code). Instead, special instructions are required to load data from the program code domain, i.e. the lpm (load from program memory) instruction. And because there is no way to emit it from Rust code, this crate uses inline assembly to emit that instruction.

However, since a pointer into program code cannot be differentiated from a normal data pointer, it is entirely up to the programmer to ensure that these different 'pointer-types' are not accidentally mixed. In other words, this is unsafe in the context of Rust.

Loading Data from Program Memory

The first part of this crate simply provides a few functions (e.g. read_byte) to load constant data (i.e. a Rust static that is immutable) from the program memory into the data domain, so that sub-sequentially it is normal usable data, i.e. as owned data on the stack.

Because, as aforementioned, a simple *const u8 in Rust does not specify whether is lives in the program code domain or the data domain, all functions which simply load a given pointer from the program memory are inherently unsafe.

Notice that using a &u8 reference might make things rather worse than safe. Because keeping a pointer/reference/address into the program memory as Rust reference might easily cause it to be dereferenced, even in safe code. But since that address is only valid in the program code domain (and Rust doesn't know about it) it would illegally load the address from the data memory, causing undefined behavior!

Example

use avr_progmem::read_byte;

// This `static` must never be directly dereferenced/accessed!
// So a `let data: u8 = P_BYTE;` is **undefined behavior**!!!
/// Static byte stored in progmem!
#[link_section = ".progmem"]
static P_BYTE: u8 = b'A';

// Load the byte from progmem
// Here, it is sound, because due to the link_section it is indeed in the
// program code memory.
let data: u8 = unsafe { read_byte(&P_BYTE) };
assert_eq!(b'A', data);

The best-effort Wrapper

Since working with progmem data is inherently unsafe and rather difficult to do correctly, this crate introduces the best-effort 'safe' wrapper ProgMem, that is supposed to only wrap data in progmem, thus offering only functions to load its content using the progmem loading function. The latter are fine and safe, given that the wrapper type really contains data in the program memory. Therefore, to keep that invariant up, the constructor is unsafe.

Yet to make that also easier, this crate provides the progmem! macro (it has to be a macro), which will create a static variable in program memory for you and wrap it in the ProgMem struct. It will ensure that the static will be stored in the program memory by defining the #[link_section = ".progmem"] attribute on it. This makes the load functions on that struct sound and additionally prevents users to accidentally access that static directly, which, since it is in progmem, would be fundamentally unsound.

Example

use avr_progmem::progmem;

// It will be wrapped in the ProgMem struct and expand to:
// ```
// #[link_section = ".progmem"]
// static P_BYTE: ProgMem<u8> = unsafe { ProgMem::new(b'A') };
// ```
// Thus it is impossible for safe Rust to directly dereference/access it!
progmem! {
    /// Static byte stored in progmem!
    static progmem P_BYTE: u8 = b'A';
}

// Load the byte from progmem
// It is still sound, because the `ProgMem` guarantees us that it comes
// from the program code memory.
let data: u8 = P_BYTE.load();
assert_eq!(b'A', data);

Other Architectures

As mentioned before, this crate is specifically designed to be use with AVR-base micro-controllers. But since most of us don't write their programs on an AVR system but e.g. on x86 systems, and might want to test them there (well as far as it is possible), this crate also has a fallback implementation for all other architectures that are not AVR, falling back to a simple Rust static in the default data segment. And all the data loading functions will just dereference the pointed-to data, assuming that they just live in the default location.

This fallback is perfectly safe on x86 and friend, and should also be fine on all further architectures, otherwise normal Rust statics would be broken. However, it is an important point to know when for instance writing a library that is not limited to AVR.

Implementation Limitations

Aside from what has been already been covered, the current implementation has two further limitations.

First, since this crate uses an inline assembly loop on a 8-bit architecture, the loop counter only allows values up to 255. This means that not more that 255 bytes can be loaded at once with any of the methods of this crate. However, this only applies to a single continuous load operation, so for instance ProgMem<[u8;1024]>::load() will panic, but accessing such a big type in smaller chunks e.g. ProgMem<[u8;1024]>::load_sub_array::<[u8;128]>(512) is perfectly fine because the to be loaded type [u8;128] is only 128 bytes in size.

Second, since this crate only uses the lpm instruction, which is limited by a 16-bit pointer, this crate may only be used with data stored in the lower 64 kiB of program memory. Since this property has not be tested it is unclear whether it will cause a panic or right-up undefined behavior, so be very wary when working with AVR chips having more then 64 kiB of program memory. This second restriction, of course, dose not apply to non-AVR architectures.

Macros

progmem

Define a static in progmem.

Structs

ProgMem

Best-effort safe wrapper around a value in program memory.

Functions

read_byte⚠	Read a single byte from the progmem.
read_slice⚠	Read a slice of type T from progmem into given slice in data memory.
read_value⚠	Read a single T from progmem and return it by value.