diff --git a/doc/README.md b/doc/README.md
index 0b1d193..27662bc 100644
--- a/doc/README.md
+++ b/doc/README.md
@@ -8,3 +8,4 @@ properly running.
 
 * [The shell](shell.md)
 * [Load code in RAM and run it](load-run-code.md)
+* [Using block devices](blockdev.md)
diff --git a/doc/blockdev.md b/doc/blockdev.md
new file mode 100644
index 0000000..084e23e
--- /dev/null
+++ b/doc/blockdev.md
@@ -0,0 +1,91 @@
+# 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 and seek into (select at
+which offset we will read/write to next).
+
+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	aciaGetC, aciaPutC, 0
+    [...]
+
+That tells `blockdev` that we're going to set up one device, that its GetC and
+PutC are the ones defined by `acia.asm` and that it has no Seek.
+
+blockdev routines defined as zero are dummies (we don't actually call `0x0000`).
+
+## Routine definitions
+
+Parts that implement GetC, PutC and Seek do so in a loosely-coupled manner, but
+they should try to adhere to the convention, that is:
+
+**GetC**: Get a character at current position, advance the position by 1, then
+          return the fetched character in register `A`. If no input is
+          available, block until it is (in other words, we always get a valid
+          character).
+
+**PutC**: The opposite of GetC. Write the character in `A` at current position
+          and advance. If it can't write, block until it can.
+
+**Seek**: Set current position (word) to value in register `HL`.
+
+## Shell usage
+
+`blockdev.asm` supplies 2 shell commands that you can graft to your shell thus:
+
+    [...]
+    SHELL_EXTRA_CMD_COUNT	.equ	2
+    #include "shell.asm"
+    ; extra commands
+    .dw	blkBselCmd, blkSeekCmd
+    [...]
+
+### bsel
+
+`bsel` select the active block device. For now, this only affects `load`. 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.
+
+### 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
+    > load 4 [device 0 is selected initially]
+    [enter "abcd"]
+    > peek 4
+    61626364
+    > mptr c000
+    C000
+    > peek 4
+    [RAM garbage]
+    > bsel 1
+    > load 4
+    [returns immediately]
+    > peek 4
+    61626364
+    > seek 0002
+    > load 2
+    > peek 4
+    63646364
+
+Awesome, right?