Trait Memory

Source

pub trait Memory {
    // Required methods
    fn r8(&mut self, addr: u32) -> u8;
    fn r16(&mut self, addr: u32) -> u16;
    fn r32(&mut self, addr: u32) -> u32;
    fn w8(&mut self, addr: u32, val: u8);
    fn w16(&mut self, addr: u32, val: u16);
    fn w32(&mut self, addr: u32, val: u32);

    // Provided methods
    fn x16(&mut self, addr: u32) -> u16 { ... }
    fn x32(&mut self, addr: u32) -> u32 { ... }
}

Expand description

Encodes how the Cpu accesses external memory / memory-mapped devices.

§Handling Memory Access Errors

At the moment, the Memory trait assumes that all memory operations are infallible, and as such, doesn’t support returning any sort of Result from reads / writes. This isn’t correct, as is a known-blocker for implementing proper Data / Prefetch Abort support (see issue #7)

Nonetheless, there are plenty of scenarios where a memory access might result in an application error. For example, what if while writing to an emulated UART device, a std::io::Error occurs?

Unfortunately, this library doesn’t provide an easy solution to these scenarios (yet?), but here are some possible approaches:

Write a application-specific, fallible Memory trait + an adapter to converts said trait into this crate’s Memory trait.
Use an “out-of-band” error signaling mechanism (e.g: a mpsc channel, or a shared queue behind a mutex)

e.g: an error occurs during a read operation. The failing device signals an error using a mpsc::channel, and returns a dummy value (e.g: 0x00). Cpu::step finishes executing the instruction, and returns back to user code. The user code then checks the channel to see if an error had just occurred, and takes an appropriate action.