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Put the transport on the 68000, and find PIO costs 87 clocks a byte

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ROADMAP P4b. src/player/xfer.i answers src/player/ring.i's XF_* mailbox with a
real READ(10) to a real MB89352 in place of tools/bench/stream.lua's modelled
transport: 120 records, 4,488,588 B, pixel-exact out of a 256 KB ring, with a
real mid-stream seek in a second pass. The tiling is the SAME 18 wraps and
14.7 KB mean hole that 49.4's host producer and 55.4's modelled transport
produced -- a third transport, same placement, which is the assertion that
ring.i could not tell which side of the seam answered it.

What it costs is the finding. tools/bench/xfer_cost.sh subtracts the same 120
frames run twice and gets 87.28 clocks per delivered byte, against the 68000's
own cycle table for the loop, which says 87.15 -- 0.2% apart, so the cost is the
instruction stream and not MAME's device model, and it is the first number this
rig has produced that survives leaving the emulator. That is 391.8% of a 12 fps
frame; the machine's own V-DISP clock agrees from the other end at 2.57 fps.
Against the ladder, W=5 held is 22.4% of a frame and W=19 is 85.3%, so P4a is
worth 4.6x the worst DMA configuration in this tree and 17.5x the best -- where
before this session it was worth 9 against 19. W itself did not move by a clock.

"UNDERRUNS: 0/120" is vacuous with a synchronous transport, and stream.lua now
prints that argument next to the zero: a frame cannot start before its record
has landed because the decoder IS the transport. The counter that means
something is NO IDLE, 119/120 with a worst overrun of 441 whole ticks. Same
class of error as 49.7.2's free-running ring passing at 48 KB.

58.3: a record is not a sector -- 117 of 120 start part way into one, and
reading whole sectors into the ring corrupts the neighbours rather than wasting
bytes (49.2, no bounds check). scsi.i reads the covering sectors and stores only
the window, which is free in PIO and stops being free the moment P4a succeeds.
tools/analysis/26_sector_align.py prices the three ways out and sector-aligned
records win on both axes: +0.43% wire and zero clocks, against +1.34% and a
bounce copy at +5 clk/B. ROADMAP now carries a four-item re-encode bundle and
P4a should be attempted against a sector-aligned container.

check.sh gains two stages and was ALL GREEN before and after. decode.bin is
unchanged at 1,296 B and the same MD5.

Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6
This commit is contained in:
prosolis
2026-08-24 23:15:53 -07:00
parent e935d8661c
commit 5921fab118
15 changed files with 1221 additions and 48 deletions
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tools/bench/check.sh
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@@ -433,6 +433,77 @@ else
echo " SKIPPED: no chdman (ships with mame-tools) -- cannot build the volume"
fi
echo "--- session 26: the ring is filled off a real SCSI volume (FINDINGS 58) ---"
# ROADMAP P4b. The stage above shows the 68000 can READ the disc. This shows it
# can RUN off it: src/player/xfer.i sits behind src/player/ring.i's XF_* mailbox
# in place of tools/bench/stream.lua's modelled transport, and the same 120
# frames are decoded out of the same 256 KB ring with NOTHING outside the
# machine in the transfer path -- no host file, no modelled rate, no synthesised
# ack.
#
# WHAT IS GATED, and it is correctness rather than rate on purpose:
# * pixel-exact, which is the only test that can see a wrong record: the
# window in scsi.i decides which of a sector's bytes reach the ring, and a
# window off by one byte desyncs the bitstream rather than faulting (49.2);
# * the SAME 18 wraps and 14.7 KB mean hole -- ring.i's placement policy must
# not be able to tell which transport answered it, and this is the assertion
# that says it could not;
# * every record accounted for: 120 READ(10)s, 4,488,588 B into the ring, and
# 4,548,608 B off the disc. The two byte counts differ by 1.34% because a
# record is not a sector, and that gap is a delivery cost (58.3) -- gating
# both numbers means neither can drift silently into the other;
# * a real mid-stream SEEK with the real transport, in the second pass. This
# is the one path that could not exist before: ring_seek waits for the
# channel to go quiet, and with the transport INSIDE the machine the only
# thing that can retire an outstanding request is that wait loop itself.
#
# NOTHING HERE IS GATED ON RATE and nothing here can be. What the run DOES cost
# is printed by tools/bench/xfer_cost.sh and recorded in FINDINGS 58.2; it is a
# measurement, and a change in it is a re-derivation there rather than a red
# light here. Skipped rather than failed when chdman is absent.
if command -v chdman > /dev/null; then
DLX_PACE=0 DLX_RINGOWN=1 DLX_QDEPTH=2 DLX_XFER=scsi \
bash tools/bench/pace_run.sh 256 0 > tmp/p4b_check.log 2>&1 || {
echo "FAIL: the 68000 could not run the ring off a real SCSI volume."
tail -12 tmp/p4b_check.log; exit 1; }
grep -aE "REAL TRANSPORT:|SECTOR OVERHEAD|ring: " tmp/p4b_check.log \
| sed "s/^ *//;s/^/ /"
grep -aq "TRANSPORT FAILED" tmp/p4b_check.log && {
echo "FAIL: a record's READ(10) reported an error."; exit 1; }
grep -aq "REAL TRANSPORT: 120 READ(10)s by the 68000, 4488588 B into the ring" \
tmp/p4b_check.log || {
echo "FAIL: the 68000 did not fetch all 120 records, or did not fetch"
echo " 4,488,588 B of them. A short record is a desync, not a shortfall."
exit 1; }
grep -aq "SECTOR OVERHEAD: 4548608 B off the disc" tmp/p4b_check.log || {
echo "FAIL: the bytes the DISC moved are no longer 4,548,608. A record is"
echo " not a sector; this is the covering-sector read, and if it moved"
echo " then either the layout or scsi.i's window did. See FINDINGS 58.3."
exit 1; }
grep -aq "ring: 18 wraps" tmp/p4b_check.log || {
echo "FAIL: the placement policy tiled this container differently with a"
echo " real transport behind it than with a modelled one. ring.i is"
echo " not supposed to be able to tell them apart."; exit 1; }
grep -aq "^OK" tmp/p4b_check.log || {
echo "FAIL: the pass off the SCSI volume was not pixel-exact."
tail -4 tmp/p4b_check.log; exit 1; }
DLX_PACE=2 DLX_RINGOWN=1 DLX_QDEPTH=2 DLX_ITER=2 DLX_XFER=scsi \
DLX_SECONDS=240 bash tools/bench/pace_run.sh 256 0 \
> tmp/p4b_seek_check.log 2>&1 || {
echo "FAIL: the seek pass off the SCSI volume did not complete."
tail -12 tmp/p4b_seek_check.log; exit 1; }
grep -aE "SEEK PASS|IS VACUOUS" tmp/p4b_seek_check.log | sed "s/^ *//;s/^/ /"
grep -aq "SEEK PASS 2" tmp/p4b_seek_check.log || {
echo "FAIL: no real seek -- the second pass never threw its ring away, so"
echo " ring_seek's quiet-wait was never asked to retire an outstanding"
echo " transfer and this gated nothing."; exit 1; }
grep -aq "^OK" tmp/p4b_seek_check.log || {
echo "FAIL: the decode after a seek off the SCSI volume was not pixel-exact."
tail -4 tmp/p4b_seek_check.log; exit 1; }
else
echo " SKIPPED: no chdman (ships with mame-tools) -- cannot build the volume"
fi
echo "--- session 24: the scene graph, and the gap between branch points (FINDINGS 56) ---"
# The arcade scene graph is not in this repo and is not redistributable from
# here. tools/import/scenegraph.py is the ONE file in the tree that knows the