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

blockdev.asm supplies 4 shell commands that you can graft to your shell thus:

[...]
SHELL_EXTRA_CMD_COUNT	.equ	4
#include "shell.asm"
; extra commands
.dw	blkBselCmd, blkSeekCmd, blkLoadCmd, blkSaveCmd
[...]

bsel

bsel select the active block device. This specify a target for load and save. Some applications also use the active blockdev. It receives one argument, the device index. bsel 0 selects the first defined device, bsel 1, the second, etc. Error 0x04 when argument is out of bounds.

seek

seek receives one word argument and sets the pointer for the currently active device to the specified address. Example: seek 1234.

The device position is device-specific: if you seek on a device, then switch to another device and seek again, your previous position isn't lost. You will still be on the same position when you come back.

load

load works a bit like poke except that it reads its data from the currently active blockdev at its current position. If it hits the end of the blockdev before it could load its specified number of bytes, it stops. It only raises an error if it couldn't load any byte.

It moves the device's position to the byte after the last loaded byte.

save

save is the opposite of load. It writes the specified number of bytes from memory to the active blockdev at its current position.

It moves the device's position to the byte after the last written byte.

Example

Let's try an example: You glue yourself a Collapse OS with ACIA as its first device and a mmap starting at 0xd000 as your second device. Here's what you could do to copy memory around:

> mptr d000
D000
> poke 4
[enter "abcd"]
> peek 4
61626364
> mptr c000
C000
> peek 4
[RAM garbage]
> bsel 1
> load 4
[returns immediately]
> peek 4
61626364
> seek 00 0002
> load 2
> peek 4
63646364

Awesome, right?

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