CosmOS: a service interface for the disk and console, and the monitor in the shell

Two changes that arrived together because both live in cosmos.asm.

THE SERVICES. A loaded program that wanted a file had to include the whole
filesystem, carrying two and a half kilobytes of a private copy of code the
system already had running, and then mount a disk that was already mounted.
Five services are added at pinned numbers 20 to 24: osFileRead, osFileSave,
osFileDelete, osFileRename and osPrintNumber.

The sizes fit the registers exactly in both directions. A file that can be
read into Data Memory is under 64K by definition, so its length is sixteen
bits: coming back it is DP3, going out it is A and B together, and neither
direction needs a record in memory whose shape both sides must agree on.

There is deliberately no service to mount a disk. The system mounts one
before its first prompt, and a program mounting it again was only ever a
consequence of owning a second copy of the library, so that call disappears
rather than moving. Apps/Files.asm writes, reads, renames and deletes a file
in 645 bytes and includes nothing but the service names.

THE MONITOR. Previously an application, now part of the shell, because an
application occupies the one region a loaded application is given: a monitor
that was an application could never examine another one, since loading the
thing to be inspected would replace the thing doing the inspecting.

"monitor" turns it on and the prompt becomes "*". It is a mode rather than a
sub-prompt, and it persists: because the mode is a variable the prompt reads
rather than a second loop, and every path back to the prompt goes through one
place including osExit, a program started with "g" that gives the machine back
arrives at the monitor prompt it was started from. Examining a program and
running it therefore do not interrupt each other. "exit" leaves whatever you
are in.

It supersedes dump, and adds disassembly, writing bytes, and jumping to an
address. Its instruction table is generated from the assembler's own list by
Tests/instructiontable.py rather than typed again, and Tests/docs.sh checks
both that the system's copy matches the generator and that the lengths that
table implies are the ones the manual's Bytes column prints. A disassembler
that disagreed about a length would not print one line wrong, it would lose
its place and print everything after it wrong.

Also here: b refuses a bank that is not registered, since asking the
controller for one is refused and a refusal nobody catches stops the machine;
g records the Stack the way run does, without which a program returning
through osExit restored whatever the last run had left; and make cosmos-disk
now depends on the system as well as the image.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Programs/CosmOS/Source/services.asm

@@ -25,3 +25,29 @@
   osReadLine     0d17   ; DP0 names somewhere to put a line read from the console.
   osExit         0d18   ; Give the machine back to the system.
   osArgument     0d19   ; DP0 names somewhere to put the rest of the run command.
+
+; ---- What the system does with the disk on a program's behalf ----
+;
+; A loaded program that wanted a file used to include the whole filesystem, which is two
+; and a half kilobytes of it carrying a private copy of code the system already has
+; running. These are that code, reachable.
+;
+; NOTHING HERE MOUNTS ANYTHING. The system mounted the disk before it read the prompt, and
+; there is one disk with one buffer registered as one bank; a program mounting it again was
+; only ever an artefact of having its own copy of the library.
+;
+; Sizes are in bytes and fit the registers exactly. A file that can be read into Data
+; Memory is under 64K by definition, so its length is sixteen bits: coming back it is DP3,
+; going out it is A and B together, and neither direction needs a record in memory that
+; both sides have to agree on the shape of.
+  osFileRead     0d20   ; DP0 names it, DP1 says where. Q is zero if it read, DP3 is how many bytes.
+  osFileSave     0d21   ; DP0 names it, DP1 is the bytes, A and B are how many. Q is zero if it saved.
+  osFileDelete   0d22   ; DP0 names it. Q is zero if it went.
+  osFileRename   0d23   ; DP0 is the name it has, DP1 the name it should have. Q is zero if it moved.
+
+; ---- And with the console ----
+;
+; printString is already up there. This is the other half of what a program prints: a
+; number, in decimal, without leading zeroes. A and B together, so one service covers both
+; a line number and a byte count and there is no need for two.
+  osPrintNumber  0d24