Sequences, order lists, and a command that changes the instrument

The rung between M3 and M4, and the point of doing it before the format:
the engine learns the tracker's model with the tune still assembled in,
so M4 becomes serialising a thing that exists rather than designing a
thing that does not.

A voice no longer walks one long track. 0xFF now means THIS SEQUENCE
ended, and the voice takes the next address from an order list of its
own. That is where repetition comes from, and it costs no notation: the
bass plays the same sequence in the first bar and the last and it is
written once. Per voice rather than one shared table of four-column rows,
because a voice's order cursor is then a pointer it advances by itself -
the same LDD and STD move everything else here makes. Four columns is how
it reads, not how it is stored.

Sequences also carry COMMANDS, which take no tick: the reader acts and
reads the next event on the same boundary. One is defined, 0x80, which
plays the rest of that voice on another patch, and the other 125 values
are left alone. A patch change reshapes whatever is still ringing on the
voice and nothing can be done about that - a channel has one set of
parameters and a note in its release is using them - so it is a fact
about the hardware and the cure is a rest, which is the composer's.

The engine moved to Libraries/player.asm rather than being copied into
the test a second time, now that it is big enough to drift. Play supplies
the tune and the beat; the library supplies the scheduler, the patch
loader and a voice's state. Verified by rendering: bar for bar identical
across the move.

AND THE TEST FOUND A REAL FLAW IN THE FORMAT, which is the whole argument
for building the reader first. The order list ended with 0x0000, on the
reasoning that no sequence could live below the 0x3000 this program is
based at. True of a loaded program, false of a boot image whose data
starts at zero - so the first test written against it read its own first
sequence as the end of the list and played nothing at all. It is 0xFFFF
now, which mirrors the 0xFF ending a sequence and is impossible
everywhere: a sequence at 0xFF00 or above has fewer bytes left than it
needs. An address is a poor place to hide a flag unless the address is
impossible in every program, not just this one.

One order list in the test now really ends, because otherwise nothing
reached the terminator at all: every voice sat on a long rest and the
break went unnoticed. With it, breaking the test reads garbage past the
end and the counter sums two notes at 781 hertz.

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

Tests/makedisks.sh

@@ -182,7 +182,8 @@ printf '#! script\nfor a in 1 2\n  for b in x y\n    echo $a$b\n  end\nend\nset
 "$TOOL" put "$DISKS/cosmos.img" "$WORK/tune.sbx" >/dev/null
 # And Play, which is the same beat carrying four voices instead of one - each with an
 # instrument of its own out of Sounds, which is what four patches at once looks like.
-"$ROOT/Assembler" -I "$ROOT/Programs/Sounds" -I "$ROOT/Programs/CosmOS/Source" \
+"$ROOT/Assembler" -I "$ROOT/Programs/Sounds" -I "$ROOT/Programs/Libraries" \
+    -I "$ROOT/Programs/CosmOS/Source" \
     "$ROOT/Programs/CosmOS/Apps/Play.asm" -o "$WORK/Play.sbx" >/dev/null
 "$TOOL" put "$DISKS/cosmos.img" "$WORK/Play.sbx" >/dev/null
 printf 'this is not a program' > notes.txt