Work across two disks: copy between them, and run a program from one on

files from the other

Two things anybody expects of a second disk, and each needed something
different.

COPYING NEEDED TWO THINGS TO REMEMBER A DRIVE.

The write stream is the only thing here that lives across service calls, so
it is the only thing whose drive can change underneath it: every
osFileBlock names its source path again and goes back to the source drive,
and then osFileWrite has to come home. It records the drive it was opened
on and returns there.

And the file lookup CACHE. It keeps the last path resolved so a reader
walking a file does not re-walk the directory for every block - and
skipping the walk skipped the drive the path named, so block one of a
cross-drive copy read the source's block numbers off the DESTINATION disk.
It only showed on files of more than one block, because a file of one is
never looked up twice. One block worked and two did not, which is a
suspicious enough shape to have suspected sooner.

RUNNING A PROGRAM FROM ELSEWHERE NEEDED A THIRD PLACE TO LOOK, and two
restorations.

The shell tried where you are and /Apps on the disk you are on. It now
tries /Apps on drive 0 as well, which is what makes the system's programs
work from a disk of your own - one with your files on it and no system,
which is most of the point of having a second disk.

The drive goes back after the load, because by then the program is in
memory and the blocks it came from mean nothing; and again when it exits,
because a program that copies between disks moves the drive as its own
paths need to and being left wherever it finished is not what was asked
for. Copy 1:/a 0:/b now leaves you exactly where you were.

The fixture disk grew an /Apps, because it kept its programs at the root
and so could not exercise the third place at all.

Two hours of the debugging above were spent on a stale disk image. The
machine boots the system that is ON the image, so a rebuilt cosmos.bin
means nothing until the image is rebuilt too - and the trace said my new
code never ran, which was true. Third time this project has been misled by
one.

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

Programs/CosmOS/README.md

@@ -836,10 +836,28 @@ tells the two apart. `2things` is a directory; `2:` is a drive.
 A drive the machine cannot read makes the whole path unfindable, and says so as `no such
 file` - which it is, since there is nowhere for the rest of it to be.
 
-**Copying between two disks is still not one command.** Each path resolves on its own drive
-and the drive stays where the last path left it, so a copy would read from one and write to
-the other with only one of them selected. That wants the copy itself to change drives between
-blocks, which is a change to `Copy` rather than to paths.
+**Copying between two disks is one command**: `Copy 1:/notes.txt 0:/keep.txt`. Every
+`osFileBlock` names its path again and so goes back to the source drive; the write stream
+remembers the drive it was opened on and returns there for each block. Between them the copy
+walks back and forth without `Copy` itself knowing there is more than one disk.
+
+### Where A Program Is Looked For:
+
+Three places, tried in order:
+
+1. Where you are.
+2. `/Apps` on the disk you are on.
+3. `/Apps` on drive 0.
+
+The first makes a program you are working on the one that runs. The second makes `Snake` work
+from any directory. **The third makes the system's programs work from a disk of your own** -
+one with your files on it and no system - which is most of the point of having a second disk.
+
+**Fetching a program does not move you, and neither does running one.** The drive is put back
+after the load, because by then the program is in memory and the block numbers it came from
+mean nothing; and put back again when the program exits, because a program that copies between
+two disks moves the drive as its own paths need it to and being left wherever it finished is
+not what anybody asked for. So `Copy 1:/a 0:/b` leaves you exactly where you were.
 
 ### Bank Numbers Are One Namespace: