collapseos/doc/blockdev.md

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.

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>