ModVM-Rust

ModVM provides a simple framework for building Virtual Machines in Rust. VMs in ModVM are modular, and are to this end made up of two different types of components: Processors and Peripherals.

Execution Model

The VM is initialized with a vector of Processors and a vector of Peripherals, and automatically constructs the communication channels between each of the processors and the peripherals. The processor then makes requests down these channels in its exe_ins() function, which is called in a loop. When any peripheral receives data down a channel, its handle(request) function is called, and this returns data for the processor to use.

After initialization, the VM returns a MachineHandle struct, which can be used to join to the threads of the VM.

Useage

The TwoWayChannel Struct

The modVM crate implements the TwoWayChannel<I, O> struct for easy communication between VM components.

TwoWayChannel is always instantiated in pairs, and each pair ('end') contains two channels for communicating in both directions down the channel. It accepts the following methods:

• send - sends data down the channel.
• recv - waits for next item, then returns it.
• try_recv - tests to find an item in the channel's queue; if it finds it, it returns it.
• iter - looped version of recv.
• try_iter - looped version of try_recv.
• query - sends data down the channel, then waits until it gets a reply, then returns it.

Internally, Processors make requests with type modVM::Query and Peripherals respond with modVM::Response.

The Processor Trait

The Processor<BASE> trait requires the exe_ins() function to be implemented on the struct. This function must take in a Vector of FrontEnd<BASE> (a wrapper for TwoWayChannel<Query<BASE>>) and &mut self, and return Result<(), BASE>. The processor terminates if any of its functions return an Err.

A simple implementation would be:

impl Processor<u8> for ProcessorEightBit {
    fn exe_ins(&mut self, channels: &Vec<FrontEnd<u16>>) -> Result<(), u8> {
        // get instruction:
        let ins = channels[0].query(LoadRequest(self.counter)).unwrap();

        match ins {
            
            /* snip */

            // instruction didn't match:
            _ => Err(0),
        }
    }
}

The Peripheral Trait

The Peripheral<BASE> trait works in a similar fashion to a webserver: its primary purpose is to send and receive requests for data. This is achived through the handle() function, which takes in a Query enum request and outputs a Result<Response<BASE>, BASE>, which, if Ok is unwrapped and sent back down the channel. This versatility gives ModVM amazing scope for quickly creating different components for VMs. The handle() function is called every time the peripheral receives a request.

A simple implementation would be:

impl Peripheral<u8> for MemEightBit {
    fn handle(&mut self, incoming: Query<u16>) -> Result<Response<u16>, u16> {
        Ok(match incoming {
            LoadRequest(x) => {
                match self.mem.get(x as usize) {
                    Some(y) => {
                        Data(*y)
                    },
                    None => Fail(0),
                }
            },
            SaveRequest(x, y) => {
                match self.mem.get_mut(x as usize) {
                    Some(z) => {
                        *z = y;
                        Good
                    },
                    None => Fail(0),
                }
            },
        })
    }
}

However Peripherals are not only limited to memory. For example, you could configure one to act as a GPU, or handle I/O. To aid the user in this regard, the optional cycle function can be used to run processes that need to be active each cycle. This function can only mutate the component's state, however.

Shared Functions

All the traits listed above share certain functions: metadata, boot and halt. Metadametadata specifies data about the VM, and returns a Metadata struct. This is used for identification of threads. boot and halt are fairly self-explanitory; They run on startup and after termination, and in a Processor the functions have access to the channels, so they can make requests for data. However, of these, only the metadata function is required for compilation.

Here is an implementation for all these functions on a Processor:

impl Processor<u8> for ProcessorEightBit {
    fn boot(&mut self, channels: &Vec<FrontEnd<u8>>) -> Result<(), BASE> {
        channels[0].query(SaveRequest(PRINT, self.bootcode)).unwrap(); // print the bootcode
        Ok(())
    }

    fn halt(&mut self, channels: &Vec<FrontEnd<u8>>) -> Result<(), BASE> {
        channels[0].query(SaveRequest(PRINT, self.error)).unwrap(); // print the error message

        for i in channels {
            i.send(LoadRequest(HALT)); // halt all peripherals
        }
    }
}

And for a peripheral:

impl Peripheral<u8> for MemEightBit {
    fn boot(&mut self) -> Result<(), BASE> {
        /* initialise things */
        Ok(())
    }

    fn halt(&mut self) -> Result<(), BASE> {
        /* halt processes */
        Ok(())
    }
}