collapseos/doc/blockdev.md
2019-04-16 10:40:51 -04:00

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# 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 2 shell commands that you can graft to your shell thus:
[...]
SHELL_EXTRA_CMD_COUNT .equ 2
#include "shell.asm"
; extra commands
.dw blkBselCmd, blkSeekCmd
[...]
### 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.
### 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
> load 4 [device 0 is selected initially]
[enter "abcd"]
> peek 4
61626364
> mptr c000
C000
> peek 4
[RAM garbage]
> bsel 1
> load 4
[returns immediately]
> peek 4
61626364
> seek 0002
> load 2
> peek 4
63646364
Awesome, right?