mirror of https://github.com/hsoft/collapseos.git
113 lines
3.5 KiB
Markdown
113 lines
3.5 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, optionally at arbitrary
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offsets.
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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 sdcGetB, sdcPutB
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[...]
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That tells `blockdev` that we're going to set up one device, that its GetB and
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PutB are the ones defined by `sdc.asm`.
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If your block device is read-only or write-only, use dummy routines. `unsetZ`
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is a good choice since it will return with the `Z` flag unset, indicating an
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error (dummy methods aren't supposed to be called).
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Each defined block device, in addition to its routine definition, holds a
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seek pointer. This seek pointer is used in shell commands described below.
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## Routine definitions
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Parts that implement GetB and PutB 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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**GetB**: Get the byte at position specified by `HL`. If it supports 32-bit
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addressing, `DE` contains the high-order bytes. Return the result in
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`A`. If there's an error (for example, address out of range), unset
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`Z`. This routine is not expected to block. We expect the result to be
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immediate.
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**PutB**: The opposite of GetB. Write the character in `A` at specified
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position. `Z` unset on error.
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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. This specify a target for `load` and
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`save`. Some applications also use the active blockdev. It receives one
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argument, the device index. `bsel 0` selects the first defined device, `bsel 1`,
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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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It moves the device's position to the byte after the last loaded 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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It moves the device's position to the byte after the last written byte.
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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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