# I/O Devices

I/O devices on Nga are exposed via three instructions:

    ie  enumerate i/o devices
    iq  query i/o device for class and version
    ii  invoke i/o interaction

All devices are registered with the VM. How this occurs is
implementation dependent.

## Counting Devices

Use the `ie` instruction to return the number of attached devices.

    i ie......

Upon running, the stack will contain the number of devices. You
can then query these by passing the device number to `iq`.

## Query Devices

Use `iq` to query an attached device. This will return two values,
a device class and a revision number.

The device class will be the top value on the stack.

## Invoking a Device

You can trigger an I/O operation by passing the device handle to
the `ii` instruction.

E.g., to display a character (ASCII code 98 in this case):

    i liliii..
    d 98
    d 0

Be sure to pass the device handle, not the device class.

## Device Class

Ultimately devices are implementation-specific, but the
standard system provides or reserves the following:

     ID  | Device Class     | Notes                      |
    -----+------------------+----------------------------+
    0000 | Generic Output   | Always present as handle 0 |
    0001 | Keyboard         |                            |
    0002 | Floating Point   |                            |
    0003 | Block Storage    | Raw, 1024 cell blocks      |
    0004 | Filesystem       | Unix-style Files           |
    0005 | Clock            |                            |
    0006 |                  |                            |
    0007 | Network: Sockets |                            |
    0008 | Syscalls: Unix   |                            |
    0009 | Scripting Hooks  |                            |
    0010 | Random Number    |                            |
    1000 | Image Saving     |                            |

It must be noted here that nothing forces devices to use these
class identifiers, and one must take care to use an Nga
implementation that provides the devices they need.

## Device Revisions

Over time, the functionality a device provides may change. To allow
detection of this, the query functionality provides a revision number.
Your code can use this to ensure that the device provided supports
the level of functionality you need.

## Device Class Details

### 0000: Generic Output

Supported by all Nga implementations. This is required to be the
first device, and is the only one guaranteed to be provided. It
consumes a value from the stack, writing to to the host-specific
output. (This does not need to be a screen).

### 0001: Keyboard

Read and return a keypress.

Consumes no data, returns a single value representing the
character that was read.

No subcommands are defined.

### 0002: Floating Point

The current revision is 1.

It currently provides:

    n:to-float  (n-_f:-n)
    s:to-float  (s-_f:-n)
    f:to-number (f:a-__-n)
    f:to-string (f:n-__-s)
    f:+     (f:ab-c)
    f:-     (f:ab-c)
    f:*     (f:ab-c)
    f:/     (f:ab-c)
    f:floor (f:ab-c)
    f:ceiling (f:f-f)
    f:sqrt  (f:f-f)
    f:eq?   (f:ab-c)
    f:-eq?  (f:ab-c)
    f:lt?   (f:ab-c)
    f:gt?   (f:ab-c)
    f:depth (-n)
    f:dup   (f:a-aa)
    f:drop  (f:a-)
    f:swap  (f:ab-ba)
    f:log   (f:ab-c)
    f:power (f:ab-c)
    f:sin   (f:f-f)
    f:cos   (f:f-f)
    f:tan   (f:f-f)
    f:asin  (f:f-f)
    f:acos  (f:f-f)
    f:atan  (f:f-f)
    f:push  (f:f-)
    f:pop   (f:-f)
    f:adepth  (-n)

### 0003: Block Storage

Reserved for future use.

### 0004: Filesystem

Currently at revision 0.

This implements a device providing traditional Unix-like files.

Takes a value indicating an operation, and each operation takes
additional values.

    | Operation | Stack | Action                           |
    | --------- | ----- | -------------------------------- |
    | 0         | sm-h  | Open a file                      |
    | 1         | h-    | Close a file                     |
    | 2         | h-c   | Read a byte from a file          |
    | 3         | ch-   | Write a byte to a file           |
    | 4         | h-n   | Return current pointer into file |
    | 5         | nh-   | Move pointer in a file           |
    | 6         | h-n   | Return the size of a file        |
    | 7         | s-    | Delete a file                    |
    | 8         | h-    | Flush pending writes             |

### 0010: Random Number Generator

This is currently at revision 0.

On invocation, this returns a random number.

## Implementation Details (C)

On the C implementation, each I/O device has the needed support
functions defined, then a query function and invocation function
defined.

As an example, to add a device that has two functions, I might do:

    void one() {
      stack_push(100);
    }

    void two() {
      stack_push(200);
    }

    Handler device_actions[] = {
      one, two
    }

    void io_device() {
      device_actions[stack_pop()]();
    }

    void query_device() {
      stack_push(0);    /* Revision     */
      stack_push(1234); /* Device Class */
    }

Then add pointers to `io_device` to `IO_deviceHandlers` and
`query_device` to `IO_queryHandlers` and increase the `NUM_DEVICES`
by one.

You will then need to write a set of Retro words to use the new
device.

    :device:one #1 #1234 io:scan-for io:invoke ;
    :device:two #2 #1234 io:scan-for io:invoke ;

Rebuild the VM, adding these to image.