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64 lines
2.1 KiB
Markdown
64 lines
2.1 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, 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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`apps/basic/blk.asm` supplies 4 shell commands that you can add to your shell.
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See "Optional Modules/blk" in [the shell doc](../apps/basic/README.md).
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### Example
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Let's try an example: You glue yourself a Collapse OS with a mmap starting at
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`0xe000` as your 4th device (like it is in the shell emulator). Here's what you
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could do to copy memory around:
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> m=0xe000
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> while m<0xe004 getc:poke m a:m=m+1
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[enter "abcd"]
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> bsel 3
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> i=0
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> while i<4 getb:puth a:i=i+1
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61626364> bseek 2
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> getb:puth a
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63> getb:puth a
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64>
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