collapseos/doc/blockdev.md

# Using block devices

The `blockdev.asm` part manage what we call "block devices", an abstraction over
something that we can read a byte to, write a byte to and seek into (select at
which offset we will read/write to next).

A Collapse OS system can define up to `0xff` devices. Those definitions are made
in the glue code, so they are static.

Definition of block devices happen at include time. It would look like:

    [...]
    BLOCKDEV_COUNT .equ 1
    #include "blockdev.asm"
    ; List of devices
    .dw	aciaGetC, aciaPutC, 0
    [...]

That tells `blockdev` that we're going to set up one device, that its GetC and
PutC are the ones defined by `acia.asm` and that it has no Seek.

blockdev routines defined as zero are dummies (we don't actually call `0x0000`).

## Routine definitions

Parts that implement GetC, PutC and Seek do so in a loosely-coupled manner, but
they should try to adhere to the convention, that is:

**GetC**: Get a character at current position, advance the position by 1, then
          return the fetched character in register `A`. If no input is
          available, block until it is (in other words, we always get a valid
          character).

**PutC**: The opposite of GetC. Write the character in `A` at current position
          and advance. If it can't write, block until it can.

**Seek**: Set current position (word) to value in register `HL`.

## Shell usage

`blockdev.asm` supplies 4 shell commands that you can graft to your shell thus:

    [...]
    SHELL_EXTRA_CMD_COUNT	.equ	4
    #include "shell.asm"
    ; extra commands
    .dw	blkBselCmd, blkSeekCmd, blkLoadCmd, blkSaveCmd
    [...]

### bsel

`bsel` select the active block device. For now, this only affects `load`. It
receives one argument, the device index. `bsel 0` selects the first defined
device, `bsel 1`, the second, etc. Error `0x04` when argument is out of bounds.

### seek

`seek` receives one word argument and sets the pointer for the currently active
device to the specified address. Example: `seek 1234`.

The device position is device-specific: if you seek on a device, then switch
to another device and seek again, your previous position isn't lost. You will
still be on the same position when you come back.

### load

`load` works a bit like `poke` except that it reads its data from the currently
active blockdev at its current position. If it hits the end of the blockdev
before it could load its specified number of bytes, it stops. It only raises an
error if it couldn't load any byte.

### save

`save` is the opposite of `load`. It writes the specified number of bytes from
memory to the active blockdev at its current position.

### Example

Let's try an example: You glue yourself a Collapse OS with ACIA as its first
device and a mmap starting at `0xd000` as your second device. Here's what you
could do to copy memory around:

    > mptr d000
    D000
    > poke 4
    [enter "abcd"]
    > peek 4
    61626364
    > mptr c000
    C000
    > peek 4
    [RAM garbage]
    > bsel 1
    > load 4
    [returns immediately]
    > peek 4
    61626364
    > seek 00 0002
    > load 2
    > peek 4
    63646364

Awesome, right?