CosmOS knows about all four drives

A mounted disk is EIGHT BYTES - where its directory starts, how many
blocks it is, how big the disk is, and where you are on it. They now sit
together in the data segment, and changing drives is one copy out and one
copy in. The other three thousand lines of filesystem go on reading the
same four names they always have and never learn there is more than one
disk, which is the whole reason this was affordable.

The version is not in the record. It is checked at mount and thrown away,
because a version one disk's zero parent already reads as "in the root".

Every drive is mounted at boot: the controller says how many are plugged
in and each is tried in turn. One with nothing in it, or a disk this
cannot read, is left unmounted rather than stopping the others, so a
machine with a good disk in drive 0 and a blank in drive 1 starts.

'drive' says which one, 'drive 1' goes to another, and the working
directory goes with it - where you are on a disk is part of which disk you
are on. A drive the machine has not got is refused, and refused
differently from one that is there with nothing readable in it.

Three things the assembly caught me on, all the same misunderstanding of
what survives a call:

  - OR reads A and B, and the bit came back from sbfsDriveBit in Q, which
    RET does not disturb - but RET does put A back. The mounted mask never
    got set and drive 0 was reported unmountable.
  - MVQA then RSTA throws away the copy it just made, so doubling a bit
    doubled nothing. SHL does it in one instruction, because A and B are
    one register to it.
  - There is no move from A to B. INB reads a port straight into B, which
    is what the drive count comparison wanted.

run.sh takes more than one image now, separated by a plus, since the
machine has four drives and a test that could only name one could not
check any of this.

The buffer note is forgotten on a drive change and that is DELIBERATELY
kept although nothing can currently reach it: only the file read-ahead
consults it, a directory scan does not, and finding a file requires a
scan which overwrites the note on the way past. Two disks were built with
the same file at the same block to try to catch it and the answer was
right either way. Three instructions to hold an invariant rather than a
story about a bug - and the comment says so instead of claiming a fix.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Programs/CosmOS/README.md

@@ -234,6 +234,7 @@ CosmOS currently provides these built-in commands:
 | `do <script>` | Run the lines in a file as though they had been typed. See Scripts. |
 | `echo [words]` | Say the rest of the line, or a blank line with nothing after it. |
 | `clear` | Empty the screen. |
+| `drive [n]` | Say which disk the shell is on, or go to another. See Several Disks. |
 | `cd [path]` | Go to a directory, or to the root with nothing after it. |
 | `mkdir <path>` | Make a directory. |
 | `rmdir <path>` | Remove one, if it is empty. |
@@ -797,6 +798,33 @@ An application may also include its own libraries or access hardware ports direc
 The services are an interface offered by the system, not the only way software is allowed
 to use the computer.
 
+### Several Disks:
+
+The machine has four drives behind one controller, and `drive` says which one the shell is
+standing on. `drive 1` goes to another, and **the working directory goes with it** - where you
+are on a disk is part of which disk you are on, not something the shell keeps on the side. Go
+back and you are where you were.
+
+Every drive is mounted at boot: the controller says how many are plugged in and each is tried
+in turn. A drive with nothing in it, or a disk this cannot read, is left unmounted rather than
+stopping the others, so a machine with a good disk in drive 0 and a blank in drive 1 starts
+normally. `drive 1` then says there is nothing readable there, which is a different answer
+from there being no such drive.
+
+**What a mounted disk is, is eight bytes**: where its directory starts, how many blocks it is,
+how big the disk is, and where you are on it. They sit together in the data segment on purpose,
+because changing drives is one copy out and one copy in - and the other three thousand lines
+of filesystem go on reading the same four names they always have and never learn that more
+than one disk exists. That is the whole reason this was affordable.
+
+The version is not among them. It is checked at mount and thrown away, because a version one
+disk's zero parent already reads as "in the root", which is where all of its files are.
+
+**Paths do not name a drive yet.** `drive 1` then a path is how you reach the other disk, so
+copying between two of them is not possible in one command. A prefix like `1:/notes` is the
+obvious next thing and it touches the path walker, which is why it is not in the same change
+as the record.
+
 ### Bank Numbers Are One Namespace:
 
 A program that wants a device's memory registers it as a bank, and **bank numbers belong to