# Internals: Nga Virtual Machine

## Overview

At the heart of RETRO is a simple MISC (minimal instruction
set computer) processor for a dual stack architecture.

This is a very simple and straightforward system. There are
30 instructions. The memory is a linear array of signed 32
bit values. And there are two stacks: one for data and one
for return addresses.

## Instruction Table

| Opcode | Muri | Full Name          | Data Stack | Address Stack |
| ------ | ---- | ------------------ | ---------- | ------------- |
|  0     | ..   | nop                |   -        |   -           |
|  1     | li   | lit                |   -n       |   -           |
|  2     | du   | dup                |  n-nn      |   -           |
|  3     | dr   | drop               |  n-        |   -           |
|  4     | sw   | swap               | xy-yx      |   -           |
|  5     | pu   | push               |  n-        |   -n          |
|  6     | po   | pop                |   -n       |  n-           |
|  7     | ju   | jump               |  a-        |   -           |
|  8     | ca   | call               |  a-        |   -A          |
|  9     | cc   | conditional call   | af-        |   -A          |
| 10     | re   | return             |   -        |  A-           |
| 11     | eq   | equality           | xy-f       |   -           |
| 12     | ne   | inequality         | xy-f       |   -           |
| 13     | lt   | less than          | xy-f       |   -           |
| 14     | gt   | greater than       | xy-f       |   -           |
| 15     | fe   | fetch              |  a-n       |   -           |
| 16     | st   | store              | na-        |   -           |
| 17     | ad   | addition           | xy-n       |   -           |
| 18     | su   | subtraction        | xy-n       |   -           |
| 19     | mu   | multiplication     | xy-n       |   -           |
| 20     | di   | divide & remainder | xy-rq      |   -           |
| 21     | an   | bitwise and        | xy-n       |   -           |
| 22     | or   | bitwise or         | xy-n       |   -           |
| 23     | xo   | bitwise xor        | xy-n       |   -           |
| 24     | sh   | shift              | xy-n       |   -           |
| 25     | zr   | zero return        |  n-?       |   -           |
| 26     | ha   | halt               |   -        |   -           |
| 27     | ie   | i/o enumerate      |   -n       |   -           |
| 28     | iq   | i/o query          |  n-xy      |   -           |
| 29     | ii   | i/o invoke         | ...n-      |   -           |

## Encoding

Up to four instructions can be packed into each memory cell.

As an example,

    Opcode 4 Opcode 3 Opcode 2 Opcode 1
    00000000:00000000:00000000:00000000

If we have a bundle of `duliswst`, it would look like:

    st       sw       li       du
    00010000:00000100:00000001:00000010

Each `li` should have a value in the following cell(s). These
values will be pushed to the stack. E.g., `lili....` and
1, 2:

    00000000:00000000:00000001:00000001
    00000000 00000000 00000000 00000001 (1)
    00000000 00000000 00000000 00000010 (2)

## Shifts

`sh` performs a bitwise arithmetic shift operation.

This takes two values:

    xy

And returns a single one:

    z

If y is positive, this shifts `x` right by `y` bits. If negative,
it shifts left.

## Queries: Memory, Stacks

The `fe` instruction allows queries of some data related to
the Nga VM state. These are returned by reading from negative
addresses:

| Address | Returns                |
| ------- | ---------------------- |
| -1      | Data stack depth       |
| -2      | Address stack depth    |
| -3      | Maximum Image Size     |
| -4      | Minimum Integer Value  |
| -5      | Maximum Integer Value  |

## I/O Devices

Nga provides three instructions for interacting with I/O devices.
These are:

    ie   i/o enumerate    returns the number of attached devices
    iq   i/o query        returns information about a device
    ii   i/o interact     invokes an interaction with a device

As an example, with an implementation providing an output source,
a block storage system, and keyboard:

    ie    will return `3` since there are three i/o devices
    0 iq  will return 0 0, since the first device is a screen (0)
          with a version of 0
    1 iq  will return 1 3, since the second device is a block
          storage (3), with a version of 1
    2 iq  will return 0 1, since the third device is a keyboard
          (1), with a version of 0

In this case, some interactions can be defined:

    : c:put
    i liiire..
    d 0
    
    : c:get
    i liiire..
    d 2

Setup the stack, push the device ID to the stack, and then use
`ii` to invoke the interaction.

A RETRO system requires one I/O device (a generic output for a
single character). This must be the first device, and must have
a device ID of 0.

All other devices are optional and can be specified in any order.

The currently supported and reserved device identifiers are:

| ID   | Device Type      | Notes                      |
| ---- | ---------------- | -------------------------- |
| 0000 | Generic Output   | Always present as device 0 |
| 0001 | Keyboard         |                            |
| 0002 | Floating Point   |                            |
| 0003 | Block Storage    | Raw, 1024 cell blocks      |
| 0004 | Filesystem       | Unix-style Files           |
| 0005 | Network: Gopher  | Make gopher requests       |
| 0006 | Network: HTTP    | Make HTTP requests         |
| 0007 | Network: Sockets |                            |
| 0008 | Syscalls: Unix   |                            |
| 0009 | Scripting Hooks  |                            |
| 0010 | Random Number    |                            |

This list may be revised in the future. The only guaranteed
stable indentifier is 0000 for generic output.