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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, optionally at arbitrary
- offsets.
-
- 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 sdcGetB, sdcPutB
- [...]
-
- That tells `blockdev` that we're going to set up one device, that its GetB and
- PutB are the ones defined by `sdc.asm`.
-
- If your block device is read-only or write-only, use dummy routines. `unsetZ`
- is a good choice since it will return with the `Z` flag unset, indicating an
- error (dummy methods aren't supposed to be called).
-
- Each defined block device, in addition to its routine definition, holds a
- seek pointer. This seek pointer is used in shell commands described below.
-
- ## Routine definitions
-
- Parts that implement GetB and PutB do so in a loosely-coupled manner, but
- they should try to adhere to the convention, that is:
-
- **GetB**: Get the byte at position specified by `HL`. If it supports 32-bit
- addressing, `DE` contains the high-order bytes. Return the result in
- `A`. If there's an error (for example, address out of range), unset
- `Z`. This routine is not expected to block. We expect the result to be
- immediate.
-
- **PutB**: The opposite of GetB. Write the character in `A` at specified
- position. `Z` unset on error.
-
- ## Shell usage
-
- `apps/basic/blk.asm` supplies 4 shell commands that you can add to your shell.
- See "Optional Modules/blk" in [the shell doc](../apps/basic/README.md).
-
- ### Example
-
- Let's try an example: You glue yourself a Collapse OS with a mmap starting at
- `0xe000` as your 4th device (like it is in the shell emulator). Here's what you
- could do to copy memory around:
-
- > m=0xe000
- > while m<0xe004 getc:poke m a:m=m+1
- [enter "abcd"]
- > bsel 3
- > i=0
- > while i<4 getb:puth a:i=i+1
- 61626364> bseek 2
- > getb:puth a
- 63> getb:puth a
- 64>
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