# 🤖 uvm32

uvm32 is a minimalist, dependency-free virtual machine sandbox designed for microcontrollers and other resource-constrained devices. Single C file, no dynamic memory allocations, asynchronous design, pure C99.

## Features

* Bytecode example apps written in C, Zig and Rust
* Non-blocking design, preventing misbehaving bytecode from stalling the host
* No assumptions about host IO capabilities (no stdio)
* Simple, opinionated execution model
* Safe minimalistic FFI
* Small enough for "if this then that" scripts/plugins, capable enough for much more

Although based on a fully fledged CPU emulator, uvm32 is intended for executing custom script like logic, not for simulating hardware.

## Samples

 * [host](host) vm host which loads a binary and runs to completion, handling multiple ioreq types
 * [host-mini](host-mini) minimal vm host (shown above), with baked in bytecode
 * [host-parallel](host-parallel) parallel vm host running multiple vm instances concurrently, with baked in bytecode
 * [host-arduino](host-arduino) vm host as Arduino sketch (tested on Arduino Uno ATmega328P, uses 9950 bytes of flash/1254 bytes RAM)
 * [apps/helloworld](apps/helloworld) C hello world program
 * [apps/hello-asm](apps/hello-asm) Minimal hello world assembly
 * [apps/fib](apps/fib) C fibonacci series program (iterative and recursive)
 * [apps/sketch](apps/sketch) C Arduino/Wiring/Processing type program in `setup()` and `loop()` style
 * [apps/rust-hello](apps/rust-hello) Rust hello world program (note, the version of rust installed by brew on mac has issues, use the official rust installer from https://rust-lang.org/learn/get-started/)
 * [apps/zig-mandel](apps/zig-mandel) Zig ASCII mandelbrot generator program

## Quickstart

    make
    host/host precompiled/mandel.bin

Build one of the sample apps (requires docker for C, or Zig, or Rust)

	cd apps/helloworld && make
	
Run the app

	./host ../apps/helloworld/helloworld.bin

## Quickstart API

```c
uint8_t bytecode[] = { /* ... */ }; // some compiled bytecode
uvm32_state_t vmst; // execution state of the vm
uvm32_evt_t evt; // events passed from vm to host

uvm32_init(&vmst, NULL, 0); // setup vm and pass in handlers for host functions
uvm32_load(&vmst, bytecode, sizeof(bytecode)); // load the bytecode
uvm32_run(&vmst, &evt, 100); // run up to 100 instructions

switch(evt.typ) {
	// check why the vm stopped executing
}
```

## Operation

Once loaded with bytecode, uvm32's state is advanced by calling `uvm32_run()`.

	uint32_t uvm32_run(uvm32_state_t *vmst, uvm32_evt_t *evt, uint32_t instr_meter)
	
`uvm32_run()` will execute until the bytecode requests some IO activity from the host.
These IO activities are called "ioreqs" and are the only way for bytecode to communicate with the host.
If the bytecode attempts to execute more instructions than the the passed value of `instr_meter` it is assumed to have crashed and an error is reported.

(As with a watchdog on an embedded system, the `yield()` bytecode function tells the host that the code requires more time to complete and has not hung)

`uvm32_run()` always returns an event. There are four possible events:

* `UVM32_EVT_END` the program has ended
* `UVM32_EVT_ERR` the program has encountered an error
* `UVM32_EVT_YIELD` the program has called `yield()` signifying that it requires more instructions to be executed, but has not crashed/hung
* `UVM32_EVT_IOREQ` the program requests some IO via the host

## Internals

uvm32 emulates a RISC-V 32bit CPU using [mini-rv32ima](https://github.com/cnlohr/mini-rv32ima). All IO from vm bytecode to the host is performed using [CSRs](https://five-embeddev.com/riscv-priv-isa-manual/Priv-v1.12/priv-csrs.html). Each "function" provided by the host requires a unique CSR value. A CSR passes a single `uint32_t` from bytecode to the host.

uvm32 is always in one of 4 states, paused, running, ended or error.

```mermaid
stateDiagram
    [*] --> UVM32_STATUS_PAUSED : uvm32_init()
    UVM32_STATUS_PAUSED-->UVM32_STATUS_RUNNING : uvm32_run()
    UVM32_STATUS_RUNNING --> UVM32_STATUS_PAUSED : ioreq event
    UVM32_STATUS_RUNNING --> UVM32_STATUS_ENDED : halt()
    UVM32_STATUS_RUNNING --> UVM32_STATUS_ERROR
```

## Boot

At boot, the whole memory is zeroed. The user program is placed at the start, the CPU registers are stored at the end. The stack pointer is set to the start of the CPU registers and grows downwards.

## ioreqs

There are two system ioreqs used by uvm32, `halt()` and `yield()`.

`halt()` tells the host that the program has ended normally. `yield()` tells the host that the program requires more instructions to be executed.

New ioreqs can be added to the host via `uvm32_init()`.
Each ioreq maps a CSR number to a value understood by the host (`F_PRINTD` below) and has an associated type which tells the host how to interpret the data passed to the CSR.

Here is a full example of a working VM host from [apps/host-mini](apps/host-mini)

--

```c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "uvm32.h"
#include "../common/uvm32_common_custom.h"

// Precompiled binary program to print integers
// This code expects to print via CSR 0x13C (IOREQ_PRINTD in common/uvm32_common_custom.h)
uint8_t rom[] = {
  0x23, 0x26, 0x11, 0x00, 0xef, 0x00, 0x00, 0x01, 0x73, 0x50, 0x80, 0x13,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x93, 0x07, 0x00, 0x00,
  0x13, 0x07, 0xa0, 0x00, 0x73, 0x90, 0xc7, 0x13, 0x93, 0x87, 0x17, 0x00,
  0xe3, 0x9c, 0xe7, 0xfe, 0x67, 0x80, 0x00, 0x00
};

// Create an identifier for our host handler
typedef enum {
    F_PRINTD,
} f_code_t;

// Map VM ioreq IOREQ_PRINTD (0x13C) to F_PRINTD, tell VM to expect write of a U32
const uvm32_mapping_t env[] = {
    { .csr = IOREQ_PRINTD, .typ = IOREQ_TYP_U32_WR, .code = F_PRINTD },
};

int main(int argc, char *argv[]) {
    uvm32_state_t vmst;
    uvm32_evt_t evt;
    bool isrunning = true;

    uvm32_init(&vmst, env, sizeof(env) / sizeof(env[0]));
    uvm32_load(&vmst, rom, sizeof(rom));

    while(isrunning) {
        uvm32_run(&vmst, &evt, 100);   // num instructions before vm considered hung

        switch(evt.typ) {
            case UVM32_EVT_END:
                isrunning = false;
            break;
            case UVM32_EVT_IOREQ:    // vm has paused to handle IOREQ
                switch((f_code_t)evt.data.ioreq.code) {
                    case F_PRINTD:
                        // Type of F_PRINTD is IOREQ_TYP_U32_WR, so expect value in evt.data.ioreq.val.u32
                        printf("%d\n", evt.data.ioreq.val.u32);
                    break;
                }
            break;
            default:
            break;
        }
    }

    return 0;
}
```

## Configuration

The uvm32 memory size is set at compile time with `-DUVM32_MEMORY_SIZE=X` (in bytes). A memory of 512 bytes will be sufficient for trivial programs.

## License

This project is licensed under the MIT License. Feel free to use in research, products and embedded devices.