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104 lines
3.1 KiB
Markdown
104 lines
3.1 KiB
Markdown
# Using block devices
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The `blockdev.asm` part manage what we call "block devices", an abstraction over
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something that we can read a byte to, write a byte to and seek into (select at
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which offset we will read/write to next).
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A Collapse OS system can define up to `0xff` devices. Those definitions are made
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in the glue code, so they are static.
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Definition of block devices happen at include time. It would look like:
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[...]
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BLOCKDEV_COUNT .equ 1
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#include "blockdev.asm"
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; List of devices
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.dw aciaGetC, aciaPutC, 0
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[...]
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That tells `blockdev` that we're going to set up one device, that its GetC and
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PutC are the ones defined by `acia.asm` and that it has no Seek.
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blockdev routines defined as zero are dummies (we don't actually call `0x0000`).
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## Routine definitions
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Parts that implement GetC, PutC and Seek do so in a loosely-coupled manner, but
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they should try to adhere to the convention, that is:
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**GetC**: Get a character at current position, advance the position by 1, then
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return the fetched character in register `A`. If no input is
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available, block until it is (in other words, we always get a valid
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character).
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**PutC**: The opposite of GetC. Write the character in `A` at current position
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and advance. If it can't write, block until it can.
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**Seek**: Set current position (word) to value in register `HL`.
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## Shell usage
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`blockdev.asm` supplies 4 shell commands that you can graft to your shell thus:
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[...]
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SHELL_EXTRA_CMD_COUNT .equ 4
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#include "shell.asm"
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; extra commands
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.dw blkBselCmd, blkSeekCmd, blkLoadCmd, blkSaveCmd
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[...]
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### bsel
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`bsel` select the active block device. For now, this only affects `load`. It
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receives one argument, the device index. `bsel 0` selects the first defined
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device, `bsel 1`, the second, etc. Error `0x04` when argument is out of bounds.
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### seek
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`seek` receives one word argument and sets the pointer for the currently active
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device to the specified address. Example: `seek 1234`.
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The device position is device-specific: if you seek on a device, then switch
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to another device and seek again, your previous position isn't lost. You will
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still be on the same position when you come back.
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### load
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`load` works a bit like `poke` except that it reads its data from the currently
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active blockdev at its current position. If it hits the end of the blockdev
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before it could load its specified number of bytes, it stops. It only raises an
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error if it couldn't load any byte.
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### save
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`save` is the opposite of `load`. It writes the specified number of bytes from
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memory to the active blockdev at its current position.
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### Example
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Let's try an example: You glue yourself a Collapse OS with ACIA as its first
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device and a mmap starting at `0xd000` as your second device. Here's what you
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could do to copy memory around:
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> mptr d000
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D000
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> poke 4
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[enter "abcd"]
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> peek 4
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61626364
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> mptr c000
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C000
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> peek 4
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[RAM garbage]
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> bsel 1
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> load 4
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[returns immediately]
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> peek 4
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61626364
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> seek 00 0002
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> load 2
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> peek 4
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63646364
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Awesome, right?
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