GitHub – JanNeuendorf/SVC16: Simplest Virtual Computer

This is the specification for an extremely simple “virtual computer” that can be emulated.

The goal in one word is simplicity. It should be simple to understand every instruction, to write machine code that runs on it, and to write a compiler for it.

The instruction set and the design in general are in no way meant to resemble something that would make sense in real hardware.
It is also not intended to be as simple and elegant as it could possibly be.

This repo contains an emulator to run games or programs. It can be installed with cargo:

cargo install --git https://github.com/JanNeuendorf/SVC16

You can then run a program from the cli:

svc16 /path/to/my_rom.svc16

Use --help to see some basic options.

I do not want to provide an assembler, any kind of compiler or even any ideas about things like call conventions.
The idea is that you have to build that yourself. You can play a game from the example folder to get an idea of what can be built.

⚠️ Warning: Until there are specifications with version 1.0, there could be breaking changes.

There are no CPU registers, just one chunk of memory. Values can be loaded from every memory address and written to every memory address.

Everything is represented as a (le) unsigned 16-bit integer. That includes numbers, addresses, colors, the instruction pointer and the input.
Booleans are represented as u16 values as well: 0 for false and >0 for true. (1 when written as a boolean.)

All numerical operations that will appear in the instructions are wrapping operations. Division by zero crashes the program.

The main memory contains one valid address for every u16.
The screen-buffer is the same size as the memory and there is one pixel for every u16.

There are as few features as possible.
That means limited input, no sound, no variable display size etc.
It also means that there are no accelerators or tricks to help with performance.

Here is a sketch of all components of the virtual computer:

The instruction pointer represents an address in main memory. It starts as zero. Then, it is manipulated by the instructions. All operations performed on the instruction pointer are wrapping.

The screen has a resolution of $256*256=2^{16}$ pixels. The color of each pixel is represented with 16-bits using RGB565. The coordinate $(x,y)$ of the screen maps to the index $256y+x$ in the screen-buffer. The coordinate $(0,0)$ is in the upper left-hand corner. Changes to the screen-buffer are not reflected on the screen until the system is synchronized.

The only supported inputs are the mouse position and the left and right mouse keys.

On synchronization the input on the last frame is loaded into the input-buffer.

The position code is the index of the pixel, the mouse is currently on.
The key code is given by left_mouse+2*right_mouse. So it can have the values 0 1 2 or 3.

There is no guarantee that the inputs are synced on the next frame. Before the first synchronization, the input codes are zero.

When the console executes the Sync instruction, the screen-buffer is drawn to the screen.
It is not cleared. The input-buffer is updated.
The system will be put to sleep until the beginning of the next frame.
The targeted timing is 30fps. There is a hard limit of 3000000 instructions per frame.
This means that if the Sync command has not be called for 3000000 instructions, it will be performed automatically.

All instructions are 4 values long. A value is, of course, a u16.

The instructions have the form opcode arg1 arg2 arg3.

In the following table, all instructions are listed. @arg1 refers to the value at the memory address arg1.
If the opcode is greater than 15, the system will abort.
If one of the three arguments is not used, it can be set to any value, but it can not be omitted.

When the instruction pointer advances, it does so by four positions.

A program is really just the initial state of the main memory.
There is no distinction between memory that contains instructions and memory that contains some other asset.
The initial state is loaded from a binary file that is read as containing the (le) u16 values in order. The maximum size is $2*2^{16}=$ bytes ($\approx$ 131.1kB).
It can be shorter, in which case the end is padded with zeroes. The computer will begin by executing the instruction at index 0.

A simple example would be to print all $2^{16}$ possible colors to the screen.
We make our lives easier, by mapping each index of the screen-buffer to the color which is encoded with the index.
Here, we use the names of the opcodes instead of their numbers.

Set 501 1 0       // Write the value 1 to address 501
Set 502 65535 0   // Write the largest possible value to 502
Print 500 500 0   // Display color=@500 at screen-index=@500
Add 500 501 500   // Increment the color/screen-index
Cmp 500 502 503   // See if we are not at the max number
Xor 503 501 503   // Negate it
Skip 0 4 503      // Unless we are at the max number, go back 4 instructions
Sync 0 0 0        // Sync 
GoTo 0 0 0        // Repeat to keep the window open

We could rely on the fact that the value at index 500 starts at zero and we did not have to initialize it.

To build a program that we can execute, we could use python:

import struct

code = [
    0, 501, 1, 0, #Opcodes replaced with numbers
    0, 502, 65535, 0,
    11, 500, 500, 0,
    # ...
]
with open("all_colors.svc16", "wb") as f:
    for value in code:
        f.write(struct.pack(", value))

Inspecting the file, we should see:

➜ hexyl examples/all_colors.svc16 -pv --panels 1

  00 00 f5 01 01 00 00 00
  00 00 f6 01 ff ff 00 00
  0b 00 f4 01 f4 01 00 00
  03 00 f4 01 f5 01 f4 01
  07 00 f4 01 f6 01 f7 01
  0e 00 f7 01 f5 01 f7 01
  02 00 00 00 04 00 f7 01
  0f 00 00 00 00 00 00 00
  01 00 00 00 00 00 00 00

When we run this, we get the following output:

First of all, if you managed to build a cool game or program for the system, please share it!

If you find a discrepancy between this README and the behavior of the emulator or some other problem or bug,
feel free to open an issue.

Keep your files stored safely and securely with the SanDisk 2TB Extreme Portable SSD. With over 69,505 ratings and an impressive 4.6 out of 5 stars, this product has been purchased over 8K+ times in the past month. At only $129.99, this Amazon’s Choice product is a must-have for secure file storage.

Help keep private content private with the included password protection featuring 256-bit AES hardware encryption. Order now for just $129.99 on Amazon!