A beat a program sets for itself

The only regular thing on this machine was the screen finishing a frame,
sixty times a second and not negotiable - a clock a program BORROWS rather
than one it sets. Every duration became a multiple of 16,667 cycles, so a
sixteenth note at 120 beats a minute, which is 125,000, is seven and a half
frames and cannot be asked for at all. The way round it was to choose a
tempo whose subdivisions happen to land on whole frames, which is making
the music fit the machine. Examples/tune.asm says so in its own header.

  0x50  Status: a period went by, it is running, it will interrupt
  0x51  Control: run, repeat, interrupt
  0x52-0x54  The period, in cycles, most significant first

THE PERIOD IS IN CYCLES because that is what everything else here is
counted in - the cost model counts them and a frame is measured in them -
so a timer counting anything else would be a second unit to remember.
Twenty four bits reaches from one cycle to sixteen and a half seconds, with
120 beats a minute at 500,000 in the middle, and there is no range left for
a prescaler to buy.

Starting loads the period; asking it to run while it already is does not,
so turning interrupts on half way through a period does not silently move
the beat being kept. What is left over carries into the next period, so a
period of 1,000 ticks every 1,000 and not every 1,000 plus however late
anybody looked. Reading the status takes the tick down and the line with
it, which is the rule this machine settled two days ago about every status
port.

The timing check is in terminal.sh and not the manifest, and the reason is
worth keeping: settle() strips cycle counts from recordings, which is right
for every other program and useless for a clock. "It printed eight dots"
would pass on a timer that fired them all at once. terminal.sh measures
that eight periods of 125,000 come to a million within a couple of hundred
cycles, and that 99.97% of them were spent asleep.

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

Tests/manifest

@@ -838,6 +838,17 @@ cosmosDrivePath      | CosmOS/Source/cosmos.asm           | run      | cosmosDri
 # place the shell looks. The drive it says afterwards is the check that fetching a program
 # did not move the person who ran it.
 cosmosCrossDisk      | CosmOS/Source/cosmos.asm           | run      | cosmosCrossDisk.in | 90000000 | disks/cosmos.img+disks/other.img
+# ---- A beat a program sets for itself ----
+#
+# That reading the status is what takes the tick down, and that without the repeat bit it
+# runs its period out and stops. NOT that the period is the length it says: settle() strips
+# cycle counts from these recordings, so how long anything took cannot be checked here.
+# Tests/terminal.sh measures that, which is where anything about cycles belongs.
+timerTest            | testPrograms/timerTest.asm         | run      | -                  | 5000000 | -
+# And interrupting, which is what a music routine wants. 125,000 cycles is a sixteenth note
+# at 120 beats a minute - seven and a half frames, so the screen cannot express it at all.
+# Eight of them is a second, and nearly all of that second is spent asleep.
+timerBeatTest        | testPrograms/timerBeatTest.asm     | run      | -                  | 5000000 | -
 printDecimalTest     | testPrograms/printDecimalTest.asm  | xfail    | -                  | -
 printDigitTest       | testPrograms/printDigitTest.asm    | xfail    | -                  | -
 printHexTest         | testPrograms/printHexTest.asm      | xfail    | -                  | -